Tactics of Passing Chemistry

Learning chemistry made easier The Top Secret Tactics of Chemistry Smart Stars Enterprise. Kamiri Samuel

Tactics of Chemistry The Top Secret

Learning chemistry made easier The Top Secret Tactics of Chemistry Tactics to Understand and Excel Smart Stars Enterprise. Kamiri Samuel iii

Tactics of Chemistry The author asserts the moral to be identified as the author of this work. © Design, layout and illustrations Smart Stars Enterprise P.O Box 337 - 00232 Tel: +254 79240 390942 792976 Ruiru, Kenya Printed by Mirron printers & Publishers P.O Box 2010 - 01000 Thika Tel: +254 712 657 434 Thika, Kenya First published 2016 by Lapon Sounds Investments Ltd P.O Box 2010 - 01000 Thika Tel: +254 712 657 434 Thika, Kenya ISBN: 978 9966 097 65 1 All rights reserved. No part of this book may be reproduced in any form or by any means, electronic, mechanical, photocopying, recording or otherwise including information storage and retrieval systems without permission in writing from the publisher, except by reviewer who may quote brief passage for review. Most questions are sourced from past KCSE exams, KCSE standard exams and others are owners set. First published 2016 Revised 2017/18/19 © 201260 by Smart Stars Enterprise iv

Learning chemistry made easier APPRECIATION Smart Star Enterprise management would like to give thanks to the following. »» Mr. KKaKm..NNir.i Samuel - The Principal Rockfields senior school, Nairobi. Former: Principal (The Corner Brook School) and HOD science and Mathematics (Juja Preparatory & Senior School) »» Mr. Ambrose Maina - The Principal Agha Khan high school, Nairobi »» Mr. Mwaniki Nderui - Head of Chemistry department, Kiambu high School »» Ms. Florence Muthoni - Teacher (Language editor) »» Mr. John Kuria - Mirron Publishers »» Distinguished examiners »» All students who read the draft during its assessment. Chief Executive Officer Smart Stars Enterprise v

Tactics of Chemistry PROLOGUE The secret of passing chemistry is based on simple concepts that is incumbent to any student understanding. In chemistry, testing is mostly based on your knowledge of the properties of elements and the compounds only. Students are advised to study and understand all the chemical and physical properties of common chemicals used in the school chemistry laboratory. This way be assured of a good pass in exams and the most awaited final exam; KCSE. This book has 85 different sample questions covered from past KCSE exams and KCSE standard exams. These questions are carefully selected to indicate the common mistakes done by students and how the mistakes are corrected. It also have a tactical practise for a student to evaluate his or her concept grasp. This book is best suited for secondary school curriculum. Senior level students can use it with little or no subject teacher`s directives. Junior level students can use it under guidance of the subject teacher. vi

Learning chemistry made easier FOREWORD This book has been designed to assist all learners capture the concept required in the right way when handling chemistry as a subject and in both theory and practical examinations. Learners are advised to read this book keenly so as to avoid errors encountered during chemistry lessons and exams. This book will help all learners in retention of chemistry knowledge by application of laws and principles governing the subject now and in future. Mwaniki Nderui, Kiambu High School. Chemistry examiner and The author of chemistry students practical manual for secondary schools. vii

Tactics of Chemistry CONTENTS Page Appreciation ............................................................................................... v Prologue........................................................................................................ vi Fore ward...................................................................................................... vii Introduction................................................................................................. 1 Periodic table.............................................................................................. 4 SECTION A. {Paper I & II} 6 13 1. Chemical formulae and equations 14 I. Chemical representation and casing.................................... 15 II. Contacts............................................................................................ 17 III. Spacing.............................................................................................. 18 IV. Balancing ........................................................................................ 21 V. State symbols................................................................................. 22 VI. Naming.............................................................................................. VII. Calculation using balanced equations................................. 25 »» Tactical Practise A........................................................... 35 38 2. Key words used in questions 40 I. State and explain observations.............................................. 42 »» T actical Practise B........................................................... 45 II. Law statement............................................................................... 47 »» Tactical Practise C............................................................ 49 III. Describe........................................................................................... 51 »» Tactical Practise D........................................................... 52 IV. Uses of substances....................................................................... 55 »» Tactical Practise E........................................................... 61 V. Identify, name & give.................................................................. 65 »» Tactical Practise F............................................................ 67 VI. Flow charts..................................................................................... 70 »» Tactical Practise G........................................................... 72 VII. Compare........................................................................................... 73 »» Tactical Practise H........................................................... 78 VII. Definitions....................................................................................... 79 »» Tactical Practise I............................................................. 90 VIII. Specificity and tricky terms..................................................... 92 »» Tactical Practise J............................................................. 3. Diagrams...................................................................................................... »» Tactical Practise K........................................................... 4. Calculations and determination........................................................ viii

Learning chemistry made easier »» Tactical Practise L.............................................................. 99 5. Graphs............................................................................................................. 101 111 »» Tactical Practise M............................................................ 114 6. Scientific data representations............................................................ SECTION B {Paper III} 1. Titration.......................................................................................................... i. Filling of tables................................................................................ 118 ii. Uses of decimals.............................................................................. 122 iii. Principles of averaging................................................................. 124 124 iv. Accuracy and precision................................................................ 124 v. Final answer...................................................................................... 125 vi. Titration calculation...................................................................... »» Tactical Practise N.............................................................. 126 133 2. Qualitative analysis.................................................................................... i. Analyzing flames............................................................................. 139 ii. Test for cations................................................................................ 410 iii. Test for anions................................................................................. 144 iv. Test for saturation.......................................................................... 148 v. Does and don’t`s of qualitative analysis................................ 152 vi. Statements rejected in observations...................................... 156 vii. Other areas of great importance.............................................. 157 »» Tactical Practise O.............................................................. 159 3. Physical Chemistry 166 i. Thermal chemistry......................................................................... 172 »» Tactical Practise P............................................................... 175 ii. Solubility............................................................................................. 178 »» Tactical Practise Q.............................................................. 181 183 iii. Rates of reaction............................................................................. 183 »» Tactical Practise R.............................................................. 4. Accuracy of chemistry instruments i. Temperature..................................................................................... 185 ii. Time .................................................................................................... 188 iii. Volume ............................................................................................... 193 iv. Length.................................................................................................. 195 v. Mass .................................................................................................... 195 Units of measurements in chemistry.......................................................... 196 Hand writing, grammatical errors and spelling..................................... 198 »» Tactical Paper I.................................................................... 200 »» Tactical Paper II................................................................... 216 ix

Tactics of Chemistry

Learning chemistry made easier Tactics of Chemistry Passing chemistry now made easy xi

Tactics of Chemistry

Learning chemistry made easier INTRODUCTION Imagine, 70% of students don’t achieve their expectation marks or grade in chemistry after they have read thoroughly. Why? They think that they have answered questions as expected, only to be surprised when the results come out. This is due to their failure to know the tactics which an examiner checks and expect from any question he/she sets. We are determined to equip you with tactics which will boast your chemistry performance to a higher mark from your current grade. KCSE Chemistry exam has three papers. Paper 1: It is a theory paper with a total marks of 80. It has about 27 to 31 questions each of not more than 3 marks. Questions are usually short and objective covering form I - IV work. Paper I tests your general knowledge and understanding in chemistry as a whole. Paper 2: It is also a theory paper with a total marks of 80 spread almost equally to about 7 questions. These questions are structured. Each question covers a topic in a wider perspective and they test on your comprehensiveness and understanding of that topic. These questions usually comes from the following areas. 1. General Chemistry 2. Chemistry of gases 3. Industrial chemistry 4. Organic Chemistry 5. Physical chemistry 6. Electrochemistry 7. Chemistry of metals The topics covered in these areas includes: 1. General Chemistry • Periodic table • Chemical families and trends across the periods. 1

Tactics of Chemistry • Structure and bonding • Separation techniques 2. Chemistry of gases (18 gases) which are distributed as follows. • Form 1: O2 and H2 • Form 2 : CO and CO2 • • NSFuoOrlp2m,hNu3Hr:3aNnidtrsoogmeneaonfditistscocmompopuonudnsd:sS: ON22,, N2O, NO, • Chlorine and its compounds: Cl2, HCl SO3, H2S • CO3rHga6)n,iaclckheenmesis(tertyh:eAnlek,aCn2eH(4m),eatlhkaynnee,sC(Het4haynnde,eCth2 Han2)e, 3. Industrial chemistry • Carbon and some of its compounds: Solvay process • Sulphur & some of its compounds: Frasch and contact process • fNoirtrNoigterinca(nVd) ascoimd)eaonfditHs acboemrpPoruoncdes: sOftosrwaamldmporonciae.ss • Chlorine and its compounds: Manufacture of hydrochloric acid 4. Organic Chemistry I & II • Hydrocarbons • Alkanols and Alkanoic acids 5. Physical chemistry • Energy changes in physical & chemical processes • Reaction rates and reversible reactions • Solubility and solubility curve 6. El••e ctEErolleeccchtterrmoocliyhstteircmycieclalsl c(eElllesctrolysis) 7. Ch• emSiosdtriyumof, metals 2

Learning chemistry made easier • Aluminium •• Iron Zinc • Lead • Copper Paper 3: This is a practical Paper, with a total marks of 40. It has three questions. Frequently covered areas are 1. Titration/Volumetric analysis/quantitative analysis 2. Physical chemistry • Enthalpy change • Solubility and solubility curves • Reaction rates 3. Qualitative analysis Calculating your mean To get your KCSE mean, KNEC adopts several methods. The method chosen depends on the performance in the national exam for that year. The method adopted changes from year to year. In every conversion, the practical paper takes a bigger percentage. All the same you have to work hard in the three papers. If you pass in all the three papers, conversion does not matter. Students who don`t want to work hard for a particular paper have a big problem with conversion. If you work to score all the marks in the three papers then you have no issue against the conversion because either way you will end up scoring an better grade. For your average mean in any standard exam you sit for in your school we recommend you consult your chemistry teacher or the exam officer who will inform you accordingly of the method adopted to arrive to your average mean. To pass chemistry, you must know your weak areas which makes you fail. This book will make you up to date with the current marking tips. This will help you to have the most current question answering tactics so that you can have smooth, easy and develop confident when handling chemistry exams. 3

The Periodic Table I II Group III IV VI VII VIII 0 Period s - block Alkali Metals Key Non Metals 4.0 1 Alkaline Earth Metals Halogens 1.0 Transitional Metals Relative Atomic Mass Noble Gases He Basic Metals Lanthanides p - block 2 H Semimetals Element Symbol Actinides Helium Atomic number 14.0 20.2 1 Element name Hydrogen N Ne 10 Tactics of Chemistry6.9 9.0d - block10.812.0 7 16.0 19.0 Neon Nitrogen 40.0 42 Li Be B C O F 34 31.0 Ar Lithium Beryllium 5 6 8 9 23.0 24.3 Boron Carbon P Oxygen Fluorine 18 27.0 Argon 3 Na Mg 28.1 15 32.1 35.5 11 12 Al Phosphorous Sodium Magnesiu Si S Cl m 13 39.1 40.1 Aluminium 14 16 17 Silicon Sulphur Chlorine 4 K Ca 19 20 45.0 47.9 50.9 52.0 54.9 55.9 58.9 58.7 63.5 64.5 69.7 72.6 74.9 79.0 79.9 83.8 Potassium Calcium 85.5 85.5 Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 25 5 Rb Sr 23 24 23 24 Manganese 26 27 28 29 30 31 32 33 34 35 36 37 38 Scandium Titanium Vanadium Chromium 99.0 Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium 101.1 85.5 106.4 107.9 112.4 114.8 121.8 132.9 132.9 88.69 91.2 92.9 95.9 Tc 118.7 127.6 126.9 134.3 Ru Rh Pd Ag Cd In Sb 6 Cs Ba Y Zr Nb Mo 43 Sn Te I Xe 55 56 41 42 Technetium 44 45 46 47 48 49 51 Caesium Barium 41 42 Niobium Molybdenum Ruthenium Rhodium Palladium Silver Cadmium Indium 50 Antimony 52 53 54 223.0 223.0 Yttrium Zirconium 180.9 183.9 186.2 197.0 204.4 Tin Tellurium Iodine Xenon 190.2 132.9 195.1 200.6 207.2 209.0 210.0 222.0 7 Fr Ra 178.5 Ta W Re Au Tl 210.0 87 88 Os Ir Pt Hg Pb Bi At Rn Francium Radium Hf 73 74 75 79 80 81 Po Tantalum Tungsten Rhenium 76 77 78 Gold Mercury Thallium 82 83 85 86 72 Osmium Iridium Platinum 272.0 277.0 Unknown Lead Bismuth 84 Aststine Radon Hafnium 262.1 263.1 262.1 Unknown Unknown 289.0 Unknown Polonium Unknown Unknown 269.0 Rg Cn Uut Unknown 261.1 Db Sg Bh Hs Mt 113 Fl Uup Uus Uuo Ds 111 112 Ununtrium 115 Lv 117 118 Rf 105 106 107 108 109 Roentgeniu Copemicium 114 Ununpentium Ununseptium Ununocium Dubrium Seaborgium Bohrium Hassium Meitnerium 110 Flerovium 116 104 Damstadtium m Livermorium Rutherfodium Lanthanides 138.9 140.1 140.9 144.2 146.9 140.1 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0 Actinides La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 60 61 62 67 69 57 58 59 Neodymium Promethium Samarium 63 64 65 66 Holmium 68 Thulium 70 71 Lanthanium Cerium Praseodymium 232.0 Europium Gadolinium Terbium Dysprosium Erbium 258.1 Ytterbium Lutetium 232.0 257.1 227.0 231.0 238.0 237.1 Pu 243.1 247.1 247.1 251.1 252.1 Md 259.1 260.1 Th Fm Ac Pa U Np 94 Am Cm Bk Cf Es 101 No Lr 90 93 Plutonium 95 96 100 Mendelevium 89 Thorium 91 92 Neptunium Americium Curium 97 98 99 Fermium 102 103 Actinium Protactinium Uranium Berkelium Califomium Einsteinium Nobelium Lawrencium

Learning chemistry made easier SECTION A PAPER 233/1 & 2 THEORY PAPERS

Tactics of Chemistry CHEMICAL FORMULAE & EQUATION. Chemistry uses a particular scientific communication style to deliver it`s information. Just like in English or any other subject, communication is delivered in a unique way as per the demands of that subject, chemistry too has its unique way of communication. Chemistry is a science which deals with how substances react. It relays it`s information by chemistry sentences. These sentences are represented by chemical equations, and chemical equations are represented by chemical symbols. Chemical equations represents chemical reactions. It is very important to note that when a chemical equation is written wrongly then it delivers a wrong information. The Tactic In Marking KCSE chemistry the following areas are keenly checked and penalized i f not well done. I. Chemical Representation & Casing Casing refers to writing of Capital or small letters where applicable when using symbols to represent elements , compounds and ions. Chemical symbols and formulae must be written correctly and in the right order for all elements, compounds and ions. The following rules will help you to understand how casing is done in the right way. 6

Learning chemistry made easier Elements Compounds Right Wrong Right Wrong CO2 hH Co2 H2O H2O PbI2 PB Pb pbi2 KI HG Hg ki, Ik or Ki CL Cl mg Mg kmno4/KMNO4 KMnO4 NH4SO4 (NH4)2SO4 AL Al Note 1. Where an element is represented by a single alphabet, then the letter MUST be capital for an element and in compounds. For 2. iWnshtearnecaeninelheymderongteisnrseuplrpehsiednetegdasbHy2tSwnooatlHph2sa.bets the first letter must be capitalized. e.g Na not na or NA or nA. The Tactic Wrong casing and wrong chemical representation is penalized fully. 3. Questions in the periodic table may have an element represented by a letter, ensure to use the letter for your answer or the actual symbols of the element provided they are correctly identified. Sample question 1. Element A and B has atomic number 17 and 12 respectively. Write the formula of the compound between A and B. The correct Answer BA2 or MgCl2 If the symbols representing elements are not correctly identified in a chemical equation, the answer is not acceptable. A student confused magnesium with calcium and ended up failing. The Wrong Answer CaCl2 7

Tactics of Chemistry 4. Ions must also be cased correctly. The charge is always written as a superscript and after the number for ions with a charge greater then one. For instance Mg2+, Fe2+, Fe3+, Na+, Al3+, O2-, OH-, SO42- 5. When ions are represented in a statement, it is always important to specify the oxidation state of the element if it has more than one oxidation state. For instance • Iron (III) or Iron (II) • Copper (I) or Copper (II) • Manganate (VII) or Manganese (II) 6. When writing the ionic compounds, metals must start followed by the non metal, for instance • The correct way to write sodium chloride is NaCl and NOT • UPClbsNeI2ar,NuClOoeTpnpIu2emPrbb(.eIIr)3sualspehlaatbeorisatCeudSiOn4sNamOTpleSOq4uCeus,tiLoena1d. Iodide 7. Where a compound comprises of two non metals, the oxidised species (reducing species) comes first. eg HCl, HF, NO, NO2, NH3 , H2O 8. In organic chemistry ensure you have given complete structure of compounds and avoid skeleton structures. Sample Question 2: Draw the structure of butane. The correct Answer H HHH H C C C CH HHHH 8

Learning chemistry made easier The Wrong Answer C C CC When required to draw the structure of other organic compounds such as alkanols and alkanoic acids, all bonds must be shown. Sample Question 3 Draw the structure of ethanol and ethanoic acid The correct Answer The Wrong Answer HH HH H C C OH H C C OH HH HH Note the presentation of the bond between the functional group of the ethanol -O-H is the correct way, which shows the bond between oxygen and hydrogen. See also the correct structure of ethanoic acid. The Wrong Answer The correct Answer HO HO H C C O H H C C OH HH HH When representing the four bonds in a carbon chain note that there is a difference when structure is represented as =C=C= which is wrong instead of C=C or C=C which are correct. The four bonds must be put in the right way not just filling bonds anyhow to be four. 9

Tactics of Chemistry Wrong casing gives a wrong impression of your chemical symbols. This leads to improper interpretation or meaning. See how wrong casing can alter the intended meaning and give a very wrong impression. Formula Intended Meaning Actual Meaning Co CO (Carbon (II) oxide) Cobalt mg Mg (Magnesium) milligram HG Hg (Mercury) Hectogram H20 H2O (Water) A molecule of hydrogen has 20 atoms Na+ Na+ (Sodium ion) Sodium plus SO42- SO42- (Sulphate ions) so 42 minus/hyphen Co2 Carbon (IV) oxide Cobalt molecule (Does not exist) You also need to know the oxidation state of elements and their valencies. They are very important when writing compounds. Put in practice the valencies of the first 20 elements and other used elements and radicals in secondary school chemistry syllabus. Valence does not only mean the outermost electron, but the combining power of an element of radical. It is important to note that an element can have more than one oxidation state (combining power) as indicated below. Element Oxidation Number Valency Common Ion Hydrogen, H +1 or -1 1 H+ , H- Helium, He 0 0 - Lithium, Li +1 1 Li+ Beryllium, Be +2 2 - Boron, B +3, -3 3 - Carbon, C +2, +4 2, 4 C4-, (C22-) -3, -2, -1, (+1), +2, +3, +4,+5 N3- Nitrogen, N 1, 2, 3, 4,5 10

Learning chemistry made easier Oxygen, O -2 2 O2- Fluorine, F -1, +1 1 F- Neon, Ne 0 - Sodium, Na 0 1 Na+ Magnesium, Mg +1 2 Mg2+ Aluminium, Al +2 3 Al3+ Silicon, Si +3 4 Phosphorous, P -4, (+2), +4 1, 3, 5 P3- Sulphur, S -3, +1, +3, +5 2, 4, 6 S2- Chlorine, Cl -2, +2, +4 or +6 1, 2, 3, 5, 7 Cl- Argon, Ar -1, +1, (+2), (+4), +3, +5, +7 0 - Potassium, K 0 1 K+ Calcium, Ca +1 2 Ca2+ +2 The valencies of other common metals which you need to practice are: Element Oxidation Number Valency Common Ion Barium, Ba +2 2 Ba2+ Vanadium, V +2, +3, +4, +5 2, 3, 4, 5 V+5 Chromium, Cr 2, 3, 6 Cr3+ or Cr6+ Manganese, Mn +2, +3 or +6 2, 4,6,7 Mn2+ or Mn7+ Iron, Fe +2, (+3), +4, (+6), +7 2, 3, 6 Fe2+ or Fe3+ Cobalt, Co 2, 3 Co2+, Co3+ Nikel, Ni +2, +3, (+4), (+6) 1, 2, Copper, Cu +2, +3, (+4) 1, 2 Ni2+ Silver, Ag +1, +2, (+4) Cu+ or Cu2+ Zinc, Zn 1 Gold, Au +1 or +2 2 Ag+ Mercury, Hg +1, (+2), (+3) 1, 3 Zn2+ Bromine 1, 2 Au+ Iodine +2 1, 5, 7 Hg+ +1, (+2), +3 1, 5, 7 Br- +1, +2 I- -1, +1, (+3), (+4), +5 -1, +1, (+3), (+4), +5, +7 11

Tactics of Chemistry Lead +2, +4 2, 4 Pb2+, Pb4+ Platinum (+1), +2, (+3), +4, +6 2, 4, 6 Pt2+, Pt4+, Pt6+ Those in brackets are not very common. It is very important to know the valencies of the common radicals. They include Radical Ion Oxidation Number Valency Ammonium ion NH4+ +1 1 Hydroxonium H3O+ +1 1 Hydroxide -1 1 Cyanide OH- -1 1 Nitrate CN- -1 1 Nitrite -1 1 Hypochlorite NO3- -1 1 Chlorite NO- -1 1 Chlorate OCl- -1 1 Perchlorate -1 1 Hydrogen carbonate ClO2- -1 1 Hydrogen sulphate ClO3- -1 1 Hydrogen sulphite ClO4- -1 1 Dihydrogen phosphate HCO3- -1 1 Acetate -1 1 HSO4- Manganate VII -1 1 Formate HSO3- -1 1 Chromate -2 2 Dichromate H2PO4- -2 2 CCH2H3C3OOO2- - MnO4- CHO2- CrO42- Cr2O72- 12

Learning chemistry made easier Peroxide O22- -2 2 Sulphate SO42- -2 2 Sulphite SO32- -2 2 Thiosulphate S2O32- -2 2 Carbonate CO32- -2 2 Oxalate C2O42- -2 2 Hydrogen phosphate HPO42- -2 2 Silicate SiO32- -2 2 Phosphite PO33- -3 3 Phosphate PO43- -3 3 Arsenate AsO43- -3 3 Borate BO3- -3 3 II. Contacts Contact is writing of chemical symbols touching or joined to each other. This may result from careless writing of letters, due to either ignorance, writing in a hurry or poor handwriting. You need to note all the letters with tails and ensure they are independent. Contacts may make your intended meaning altered leading to loss of marks. You must note that letters are written independently from the others but ensuring the right casing as discussed earlier. Make a tendency to write elements as many times as possible to ensure that compounds and other two-letter elements are written correctly. Look at the examples below. 13

Tactics of Chemistry Compounds Elements Independent Independent Contact Contact Na Na NH4NO3 NH4NO3 NaOH NaOH Al Al Cl Cl CuSO4 CuSO4 Mg CH3CH2OH CH3CH2OH Mg Cu NaHCO3 NaHCO3 Cu Co CH3COOH CH3COOH Co The letters must also be written in its complete format. For instance letter O must be different from Letter U etc. The Tactic Contacts in chemical symbols is wrong. It is penalized. Beware III Spacing Letters must be independent but ensuring that the spaces between them are not too big to be misinterpreted. Unnecessary gapping gives element or compounds a distinct impression. Lets look at some of the unnecessary gapping in the following. Actual Spacing Impression Presentation Co These spaces changes the intended Co meaning of the symbols completely. NaOH N aO H They appear as separate or CuSO4 Cu SO4 incomplete elements and compounds. These spaces should be avoided . CH3CH2OH CH3 CH2 OH NaHCO3 Na H C O3 CH3COOH CH3 C O O H Spacing creates wrong impression resulting to failure. 14

Learning chemistry made easier IV. Balancing Balancing involves working out the number of the particles (the particles referred to are atoms and ions) involved in a reaction. The number of particles reacting must be equal to the number of particles formed. Balancing also involves proper writing of the compounds to ensure they don’t change their chemical composition. Be careful not to create or destroy mass. Sample Question 4: Write an equation for the reaction which takes place when copper is heated in air. i. Cu (s) + O2(g) The Wrong Answers CuO2(s) (Wrong CuO formula) ii. Cu(s) + O2(g) CuO(s) (Equation not balanced) These answer are wrong due to unbalanced chemical equation. In the two cases above • Check the number of atoms in (i) above. Copper and oxygen have one mole each of atom and molecules respectively on both reactant and product sides. It`s true that the matter has not been destroyed, but the chemical composition of the existence of the compounds has been changed. This has oxidised copper from +4 which is not true for this compound . • In answer (ii) above, the oxygen atoms have been destroyed. The mass of the products and the reactants must be equal. (Cu = 63.5, O = 16.0) Reactants Products = 63.5 + (16.0 x 2) = 63.5 + 16.0 = 95.5g = 79.5g 15

Tactics of Chemistry This shows that the mass have been destroyed. The mass of the reactants; 95.5g differs from that of the products 79.5g. To ensure that we do not destroy matter, the equation has to balance. 2Cu(s) + O2(g) 2CuO(s) Reactants Products = (63.5 + 16.0) x 2 = (63.5 + 16.0) x 2 = 159g = 159g Proper balancing does not destroy mass. The Tactic Note that giving unbalanced equations gives wrong information which will make you fail. When dealing with chemical equations 1. Don`t destroy matter 2. Don`t change the chemical composition of compounds (chemical formulae) Avoid inserting numbers anywhere anyhow during balancing of chemical equations. The composition of magnesium is written as MgO and not MgO2. Mg(s) + O2(g) MgO2(s) Tchheemniucaml bfoerrm2uilna/McogmOp2 ohsaistiobneeonf inserted without considering the magnesium oxide. The combining power (valency) of magnesium is 2 and that of oxygen is 2, therefore one magnesium atom will bond completely with one oxygen atom. When balancing chemical equations, whole numbers are used for moles in all topics apart from energy changes in physical and chemical processes, where the fractions may be used to refer to the formation of one mole of a substance. 16

Learning chemistry made easier Example. CO(g) ΔH=-393kJmol-1 C(s ) + 1/2O2(g) H2O(l) ΔH=-286kJmol-1 H2(g) + 1/2O2(g) The Tactic Unbalanced chemical equations earn ZERO mark. A well balanced chemical equation has the following properties. 1. The number of moles of a particular particles in reactant side MUST be equal to the number of the same particles on the product side. 2. The chemical composition of the compound must not be changed or altered. 3. Should have proper state symbols. V. State symbols Chemical reactions require a specification of what physical state does reactants react and what physical states are the products formed. State symbols are represented as an initial of the state of matter, and as an abbreviation in case of ionic compounds. The states include Solids (s) Gases (g) Liquids (l) Aqueous Solution (aq) A state symbol is always put in brackets; ( ), to distinguish it from a chemical symbol or the number of atoms in a molecule or a compound. For instance Pb(s), O2(g), Mg(NO3)2(aq), H2O(l) A well balanced chemical equation must have correct state symbols as a subscript after the compound (s) formula or element (s) symbol. The appropriate level of the subscript is that of a subscript number. Example of a well balanced equation with state symbols. 2Pb(NO3)2(s) Heat 2PbO(s) + 4NO2(g) + O2(g) CH3CH2OH(l) + 3O2(g) 2CO2(g) + 3H2O(l) 17

Tactics of Chemistry The Tactic If your chemical equations are well balanced and have the correct state symbols, you earn yourself full marks. You are penalized in case you miss to write the state symbols or write wrong state symbols. VI. Naming Naming of the chemical compounds is done using the accepted International Union of Pure and Applied Chemistry (IUPAC) rules. Compounds are named using their oxidation state if they have more than one. Avoid using common names, they can lead to penalization. When writing the oxidation state of an element it is indicated using Roman numbers and in brackets. The Roman number must be capital e.g Carbon (II) oxide but not carbon ii oxide or carbon (ii) oxide, nitrogen (I) oxide but not nitrogen i oxide or nitogen (i) oxide. Other examples N/B. Common names are not used nowadays Compound IUPAC Name Common name Carbon monoxide CO Carbon (II) oxide Carbon dioxide CO2 Carbon (IV) oxide Potassium permanganate KMnO4 Potassium manganate (VII) Ferric chloride Iron (II) chloride FeCl2 Iron ( III) chloride Ferrous chloride FeCl3 Potassium permanganate K2Cr2O7 Potassium dichromate (VI) Sulphuric acid H2SO4 Sulphuric (VI) acid Sulphurous acid H2SO3 Sulphuric (IV) acid Potassium chromate K2CrO7 Potassium chromate (VI) Fe2O3 Ferrous oxide Iron (III) oxide 18

Learning chemistry made easier Compounds of the elements with only one oxidation state are not indicated. They include Zinc oxide; ZnO Magnesium oxide; MgO When naming the organic compounds ensure • A number is separated from a word using a hyphen (-) e.g 2-chloropropane, 2-methylbut-2-ene. • A number is separated from another number using a comma (,) e.g 2,2-dimethylpropane, Propan-1,2,3-triol. • Only the first letter starts with a capital letter if the name does not start with a number or if it is not at the beginning of a sentence. No capital letter for the parent name if it is not the first word in a sentence e.g Bromomethane... The name of the compound however large must be one word. Avoid gaps between the names Let us now look at the following examples where student fail to write the names correctly. Cl Cl This compound should be written as H C C H 1-bromo-2-methylpropane And not 1-bromo-2-methyl propane Cl Cl H HH This compound should be Br C C C H written as HH 1,1,2,2-tetrachloroethane HC H and not 1,1,2,2-Tetra chloro ethane H There has been a lot of mistakes with students when required to draw and name isomers. 19

Tactics of Chemistry Sample question 5 Draw and name all the isomers of butane Students have been committing the following mistake. i. ii. H H H HH HH H C C C H H C C C C H HH H H H H Butane HC H Butane H iii H H H In the three answers H C C C H Structure i and ii are the same. Note that an isomer is not made by branching a methyl at the end. HH It still remain the longest HC H carbon chain as in i. H 2-methylpropane If you were to count the number of carbons in isomer i and ii, both have 4 carbons in the longest chain. The Tactic Branching one or two ends of a long carbon chain hydrocarbon does not become a new structural formula. When counting the longest carbon chain it can follow any direction provided it is in a continuous chain. The number of carbons in the longest continuous chain in isomer iii is 3, and a branch attached to the second carbon. Therefore the structure of hydrocarbon (iii) is different from that in (i) and (ii). 20

Learning chemistry made easier VII Calculation using balanced chemical equation Balanced chemical equations are useful in quantitative chemistry. Calculations are based from the mole ratio of the reactants and the products. Therefore if the equation is wrong the mole ratio will be wrong hence wrong calculation. Common questions requiring a well balanced chemical equation include; moles, molarity, volume, energy changes, quantity of electricity, Sample question 6 What volume of oxygen will completely react with 10cm3 of propane. This question can be answered effectively using a balanced equation. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) 10cm3 10cm3 x 5 =50cm3 Sample question 7 What quantity of electricity is required to liberate 75cm3 of oxygen gas when electrolyzing dilute sulphuric acid. (Molar gas volume =24dm3, 1F = 96,500 Coulombs) This question can be answered effectively using a balanced equation. Water is simply electrolysed OH- ions loses the 4 electrons which are gained by hydrogen ions. 4OH-(aq) 2H2O(g) + O2(g) + 4e- Moles of oxygen gas = /75 24,000 = 0.003125moles of O2== 96,500 x 4 moles of electron For 1 mole 386,000 Coulombs Electricity required = 386,000 x 0.003125 = 1,206.25Coulombs Other example are found in titration and thermal chemistry parts. 21

Tactics of Chemistry Tactical Practise A 1. Draw and name the isomers of butyne (2 marks) 2. A student was asked to prepare Lead (II) chloride salt using the following ingredients; Nitric (V) acid, lead (II) oxide and hydrochloric acid. Using ionic equations only explain how the salt can be prepared. (2 marks) 3. The table below shows the pH values of solution A, B, C and D. Solution AB CD PH 27 11 14 Select solutions in which a sample of lead (II) hydroxide is likely to dissolve. Give reasons for each solution selected (3 marks) 4. Calculate the oxidation number of the Mn in KMnO4. (1 mark) 5. The electronic configuration of ion K3+ and P- are 2.8 and 2.8.8 respectively. i. Write the electronic configuration of element of K and P. ( 1 mark) ii. Write the compound formed between K and P. ( 1 mark) iii. What is the nature of the aqueous solution of the compound formed between K and P. 6. A compound Y reacts with bromine to form another compound, whose formula is: H Br CH3 CH2 C C CH3 Br H What is the formula and name of the compound Y? (2 marks) 7. Which species undergo oxidation when iron (III) reacts with chlorine. Show using the oxidation number. ( 2 marks) 8. Calculate the oxidation number of nitrogen iTniNClO3.2WanhdaNt His4+t.h(e2 marks) of its 9. The formula for a chlorine of titanium is formula sulphate? (1 mark) 10. Aluminium oxide reacts with both acid and bases. a. Write an equation for the reaction between aluminium oxide and hydrochloric acid. (1 mark) b. Using the equation in (a) above, calculate the number of moles of hydrochloric acid that would react completely with 153.0g of aluminium oxide. (Al=27.0, O=16.0) (2 marks) 11. Complete the table below by writing the products formed at the electrodes during the electrolysis of the electrolytes given in the table. (3 marks) 22

Learning chemistry made easier Electrolyte Product at Product at cathode anode Aqueous sodium sulphate using inert electrodes Aqeous copper (II) sulphate using copper electrodes. 12. Giving a reason in each case, identity an acid and a base in the equation. H3O+(aq) + NH3(g) NH4+(aq) + H2O(l) Acid:........................................... (1 mark) Reason:...................................... (1 mark) Base:........................................... (1 mark) Reason:...................................... (1 mark) 13. Given the following information Cu(s) Cu2+(aq) + 2e- Eθ = +0.34V Ni(s) Ni2+ (aq) + 2e- Eθ = -0.23V Give the cell representation of the above cell. (1 mark) 14. Write equations to show the effects of heat on each of the following: a. Sodium hydrogen carbonate (1 mark) b. Silver nitrate ( 1 mark ) c. Anhydrous iron (II) sulphate ( 1 mark) 15. a. A student electroplated a spoon with copper metal. Write an equation for the process that took place at cathode. ( 1 mark) b. Calculate the time in minutes required to deposit 1.18g of copper if a current of 2 amperes was used. (1 Faraday = 96,500 coulombs, Cu = 63.5) ( 2 marks) H H 16. Hydrazine gas burns in oxygen to form N N H H nitrogen gas and steam. a. Write an equation for the reaction. ( 1 mark) b. Using the bond energies given below, calculate the enthalpy change for the reaction in (a) above. (2marks) 23

Tactics of Chemistry Bond Bond energy (kJ per mole N=N 944 N-N 163 N-H 388 O=O 496 H-O 463 17. The scheme below shows some reaction sequence starting with solid N. Study it and answer the question that follow. Solid HCl(aq) Solution A few drops of NH3(aq) White N Step I P Step II solid Mg(s) Exces Step IV NH3(aq) Colourless Solution Q a. Write the formula of the complex in the solution Q.( 1 mark) b. Write an equation for the reaction in step IV. (1 mark) 18. When fuel burn in the internal combustion engine at high temperature, one of the products formed is nitrogen (II) oxide. a. Write the equation for the formation of nitrogen (II) oxide. ( 1 mark) b. Give a reason why nitrogen (II) oxide is not formed at room temperature. ( 1 mark) 19. A student investigated a property of acids M and N by reacting equal volumes of acids M and N of the same concentration with equal volumes of 2M potassium hydroxide. The results were recorded in the table below. Acid Rise in temperature (ΔT) K M 4 N 2 a. Which of the acids is likely to be a weak acid? Explain. ( 2 marks) b. Write the equation for the reaction between ethanoic acid and potassium hydroxide. ( 1 mark) 20. Draw the structure and give the names of three alkanes having molecular formula C5H10. (3 marks) 21. A scrap iron metal was added to copper (II) sulphate solution. Write an ionic equation for the reaction that took place. (1 mark) 24

Learning chemistry made easier 2. KEY WORDS USED IN QUESTIONS I. State and Explain Observations. Chemistry as a science uses observations as its basics. Note that your ability to recognize or having skills to recognize changes in a chemical reaction is very important. Once an observation has been made you need to know the scientific principles behind the observations you have made. Observations are made from our four senses • Seeing • Hearing • Smelling • Feeling The Tactic Tasting is not included. Remember the rule; Never taste anything in the laboratory. Whenever you are looking for observations note the following changes. SEE • The colour of reactants and products, colour of residue when hot and cold e.g, from colorless to purple. Green to orange, pink to colourless, black to reddish brown... • fSd otirsamstoeelvdse, sso,ufablgimmaasatitsitofeno,rrcmroyefsdt/raelllaiibcztaeatrniaottnsedae,ntacd.sothlied products, e.g solid deposited, a liquid • When heating solids, note the changes on gentle and strong heating. • You can also see the behaviour of reaction like violent, vigorous, and formation of effervescence. • Temperature change using a thermometer • The nature as crystalline or powdered solid 25

Tactics of Chemistry HEAR • Hissing sounds • Pop sound • Cracking sound SMELL • Fruity smell • Chocking smell • Odourless • Pungent FEEL • Temperature change • Slippery, soft or course Once you identify the observation, technical terms are then used to explain principles behind what you have observed. Among the technical terms used are; • Oxidized/Reduced/Oxidising agent/reducing agent • Deliquescent, hygroscopic, efflorescent • Dehydrated • Hydrolyzed Mixing of words and formulae in the explanation is accepted, provided the rules on the oxidation state is obeyed. Many questions under state and explain requires proper colour observation for you to be able to answer the questions properly. A colour change in an experiment or in a chemical reaction communicates that a certain reaction has taken place. Lets look at the following reactions. • When potassium manganate (VII) changes from purple to colourless. This means that manganate (VII) ions has been REDUCED to manganese (II) ions, which is colourless. See the diagrams below. 26

Learning chemistry made easier Manganese (II) ions are colourless. Manganate ions are purple in colour • When copper changes from brown to black after being heated in air, shows that copper has been OXIDISED. + Oxygen Brown copper Black Copper (II) oxide • When sugar changes to a black mass after adding concentrated sulphuric (VI) acid, shows that the water elements; hydrogen and oxygen has been removed from the sugar. Carbon is left and therefore sugar is DEHYDRATED. This makes it charred /change to a spongy black mass. Thus concentrated sulphuric (VI) acid is a dehydrating agent. Samples of sugar Charred/Blackened sugar • White colour of anhydrous copper (II) sulphate changes to blue on addition of water. This indicates that copper (II) sulphate has been HYDRATED. It also shows that CUSO4 is hygroscopic. White unhydrous Blue hydrated Copper (II) sulphate Copper (II) sulphate 27

Tactics of Chemistry Sample question 8 In the reaction below, hydrogen gas is passed over hot Copper (II) oxide. Anhydrous Copper Copper (II) oxide (II) sulphate Dry hydrogen gas Heat State and explain the observation taking place in the combustion tube. Observation: Black copper (II) oxide changes to brown and white anhydrous copper (II) sulphate turns blue. Explanation; Hydrogen reduces copper (II) oxide to copper, and itself oxidised to water which hydrates the anhydrous copper (II) sulphate hence turn blue. NOTE: When words are used, ensure that the oxidation states is well stated to avoid confusion. For instance when metals are mentioned, oxidation state should distinguish the metals from one another. For example which Copper oxide (Cu2O) - Copper oxide… copper? (I) or copper ( II) oxide (CuO) - Lead oxide., Which lead oxide? lead (II) oxide (PbO) or lead (IV) oxide (PbO2). There are other tips you must be aware of to pass your exam which includes colours of chemicals. Colours of the Gas Mixed colors should not be used to avoid ambiguity. Where you are not sure/ very clear of the colour use the dominating colour. Common colours you should know to includes: 28

Learning chemistry made easier Chlorine gas Bromine water Nitogen (IV) Oxide Yellow green in Yellow, orange, red, brown Fluorine gas Brown in colour colour Pale yellow Bromine gas Common Yellow Brown or red in colourless colour gasses: O2, H2, N2O, NO, NH3, SO2, CO, CO2, Dinitrogen tetraoxide pale yellow The list of colourless gas is large. They include H2, O2, NO, N2O, H2S, SO2, SO3, NH3, HCl, Common oxides includes Zinc Oxide Cold Lead (II) oxide Lead (IV) Oxide Manganese Oxide White Yellow Red Black Copper Oxide Magnesium Oxide Iron (III) Oxide Calcium Oxide Black White Browm White 29

Tactics of Chemistry Colours of some salts Hydrated Copper Hydrated Iron (III) Hydrated Iron (II) (II) salts - Blue salts - Brown salts - Green Potassium Potassium Copper (II) chloride dichromate (VI) chromate (VI) salt Orange Yellow Blue - green White coloured salts are quite a number, for most metals except those of copper, iron (II) and Iron (III) The summary below gives the colour of salts excluding those of Cu2+, Fe3+, Fe2+ 1. Nitrates - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. 2. Chlorides - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. 3. Carbonates - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. 4. Hydrogen carbonates - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. 5. Sulphates - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. 6. Sulphites - of Zn, Mg, Ca, Na, K, Li, NH4+, Pb are white. NOTE: the color of the salts may be the same but the texture may differ due to the size of the crystals. 30

Learning chemistry made easier Colours of common solutions Iron (II) salt Iron (III) salt dichrCorm2Oa7t2e- (VI) Cr3+ solution solution Orange chromium (III) Green Brown/Yellow Green mangMannOat4e- (VI) Mn2+ chroCmraOt4e2- (VI) dicChrr2oOm72a- te manganese (II) Yellow (VI) Orange Purple Colourless Colours of common metals Copper powder Zinc powder Lead powder Brown Grey Grey 31 Magnesium metal Iron powder Grey Greyish black

Tactics of Chemistry The safest way is to use the dominating colour only, or the most noticeable colour. For instance Copper is brown, CuO is black, MnO4– is purple, Cr2O72– is orange, Cr3+ is green... Colours like dirty green, brick red should not be used. A sample of a few bricks will defer in colour. Sample the bricks below and notice the colour difference. Sample question 9 In a closed system aqueous iron (III) chloride reacts with hydrogen sulphide gas as shown in the equation below. 2FeCl3(aq) + H2S(g) 2FeCl2(aq) + 2HCl(aq) + S(s) (Brown/Yellow) (Green) State and explain the observations that would be made if dilute hydrochloric acid is added to the system. (2 Marks) The Wrong Answers The equilibrium shifts to the left to consume/ reduce the concentration of hydrogen ions in the system. How do you know that the equilibrium shifts to the left? The correct Answer Observation - the yellow/brown colour of the mixture increases/ intensifies and the green colour of the mixture decreases/fades or the yellow deposits of the sulphur also dissolves. Explanation - Iron (II) ions Fe2+ are oxidized to iron (III), Fe3+, sulphur is reduced to H2S. Hence the equilibrium shifts to the left. 32

Learning chemistry made easier Sample question 10 State two observations made when magnesium ribbon is placed in a test tube containing hydrochloric acid. The Wrong Answers • A gas is produced • Bubbles/ effervescence of a gas • Magnesium ribbon disappears • Magnesium ribbon dissolves Looking keenly at this answer it does not say much, the following questions comes up. i. A gas is produced - How do you know a gas is or has been produced? ii. Bubbles/ effervescence of a gas. No colour of the gas is stated iii. Magnesium ribbon disappears. This is a mysterious statement iv. Magnesium ribbon dissolves. An error of omission of important details. To where does it dissolve? What colour does if form? The correct Answer • Bubbles or effervescence of a colourless gas is produced • Magnesium gradually diminishes as it reacts with the acid. Sample question 11 eAqtu2a0t0iCo,nNOb2ealonwd.N2O4 gases exist in equilibrium as shown in the 2NO2(g) N2O4(g) ΔH= -ve (Red brown) (Pale yellow) State and explain the observation that would be made when. 1. A syringe containing the mixture at 200C is immersed in ice cold water. [11/2 marks] 2. The volume of the gaseous mixture in syringe is reduced. [11/2 marks] 33

Tactics of Chemistry On such questions the common answers given by many students are The Wrong Answers The equilibrium shifts to the right. Or the forward reaction is favoured. Its true that the reaction shifts to the right, but how do you come to conclusion/ justify that the equilibrium shifts to the right? Failure to state the observation makes the answer incorrect. Stating the observation first makes the answer correct but not conclusion first, then observation. The correct Answer • Pale– yellow colour intensifies, forward reaction is exothermic. Lowering the temperature shifts the equilibrium to the right. • Pale-yellow colour intensifies, reducing the volume of the syringe increases the pressure and the side with less number of moles/ particles is favoured, this makes the equilibrium shifts to the right. How the reaction shift is noted is stated as observation and what makes the equilibrium shift to the right is then explained. This answer is appropriate and satisfactory and it raises no question. The Tactic Dealing with questions which requires stating and explaining an observation, ensure you state • The observation first • Explain the cause of your observation, either using words or balanced chemical equations. If the observation is about a colour, explain what brings the colour or what happens that results to the observations you have made. 34

Learning chemistry made easier Tactical Practise B 1. Explain how the hotness of a Bunsen burner flame can be increased (1 mark) 2. Dry ammonia gas was passed over heated lead (II) oxide and the products passed over anhydrous copper (II) sulphate as shown in the diagram below. Lead (II) oxide Anhydrous copper (II) sulphate Dry ammonia gas State a. the observation made in the combustion tube (2 marks) b. the property of ammonia gas shown in this property (1 mark) 3. Study the set up below and answer the question that follow. Gas ammonia Heat Water with few drops of universal indicator a. Write an equation for the reaction between ammonia and copper (II) oxide. (1 mark) b. During the experiment, the colour of the contents in the water trough changed. State the colour change observed and give an explanation. (2 marks) 4. Aluminium is extracted through electrolysis process while copper is extracted through reduction process. Explain. (2marks) 5. Explain the observation made when a burning magnesium is lowered into a glass jar of sulphur (IV) oxide. (1 mark) 6. A measuring cylinder fitted with moist steel wool was inverted in a trough of water as shown in the diagram below. 35

Tactics of Chemistry Moist steel wool Pure water a. state and explain the observations made on the: i. Moist steel wool after four days ( 1 mark) ii. Water in the measuring cylinder after four days. (1 mark) b. What would be the effects of using steel wool moistened with salty water. (1 mark) 7. A gas jar of chlorine water was inverted over water and allowed to stand for sometime. Gas jar Water trough Water a. State and explain two observation made in the gas jar after some time. (2 marks) b. Write the equation for the reaction between chlorine and hot concentrated potassium hydroxide. (1 marks) 8. Aqueous hydrogen chloride reacts with potassium manganate (VII) to produce chlorine gas, while a solution of hydrogen chloride in methybenzene has no effects on potassium manganate (VII). Explain this observation. (2 marks) 9. a. What would be observed if sulphur (IV) oxide is bubbled through acidified potassium manganate (VII)? ( 1 mark) b. In an experiment, sulphur (IV) oxide was dissolved in water to form a solution L i. What would be observed if a few drops of barium nitrate solution were immediately added to solution L? ( 1 mark) ii. Write an ionic equation for the reaction that occurred between solution L and aqueous barium nitrate in (b) (i) above. ( 1 mark) 36

Learning chemistry made easier 10. State and explain what would happen if a dry red litmus paper was dropped in a gas jar of dry chlorine. ( 2 marks) 11. Carbon (II) oxide is described as a “silent killer”. a. state one physical property of carbon (II) oxide that makes it a “silent killer” (1 mark) b. state and explain one chemical property that makes carbon) oxide poisonous to human beings. (2 marks) 12. Explain why the pH of 1.0M hydrochloric acid is 1.0M while that of 1.0M ethanoic acid is 5.0. (2mks) 13. In an experiment hydrogen sulphide was passed through 1m CuSO4(aq) boiling tube as shown in the diagram in a Hydrogen sulphide 1M CuSO4(aq) a. State the observation made in the boiling tube. (1mark) b. Write the ionic equation for the above reaction. (1mark) c. What precaution should be taken in carrying out this experiment?. Give a reason? (1mark) 14. Consider the following reaction at equilibrium. PCl5(g) PCl3(g) + Cl2(g) Complete the table below to show the effect of different factors on the position of equilibrium. (2 marks) Factors Effects on equilibrium position Decrease in pressure Removing chlorine Adding helium to the mixture 15. State one observation made when a gas jar of moist hydrogen sulphide is inverted over a gas jar of dry chlorine gas. (1mark) 37

Tactics of Chemistry II. Law statements When stating the law, the condition(s) under which the law operates must always be stated. A condition is the control which must be maintained for the law to be applicable. Failure to state a condition the law is in applicable and incorrect thus earning no marks. Sample question 12 State Boyle`s law. The Wrong Answers It states that the volume of a fixed mass of a gas is inversely proportional to pressure. The above law statement is incomplete because it does not have condition under which it operates. The complete statement with the condition should read The correct Answer The complete statement is that. It states that the volume of a fixed mass of a gas is inversely proportional to pressure exerted on it provided temperature is kept constant.. The underlined statement is the condition of the law and makes the law complete and applicable. • Another mistake easily committed is when you are required to state the law but you express the law instead. Expression is where mathematical formula or symbols or equations are used to represent a particular law. A statement is where words are used to represent a law. The Tactic Mathematical formula is an expression not a statement. Distinguish the two. 38