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ChemistrySCORE A+

2011

GUIDELINES ON ANSWERING

QUESTIONS IN

CHEMISTRY PAPER 2 AND PAPER3

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GUIDELINES ON ANSWERING QUESTIONS IN CHEMISTRY PAPER 2

GUIDELINES ON ANSWERING STRUCTURED QUESTION

1. General(a)As with other subjects, you should read all questions carefully and circle all important key words.(b)Answers can be short but they must be precise and complete.(c)The space provided for your answer gives an indication as to how long the answer should be.(d)Always look at the marks allotted for each question as a guide to how many points you shouldgive.

2. Answers must be PRECISE.

Need not write in complete sentence but your answer must be PRECISE.Example:

State the conversion of energy that occurs during the electrolysis.

Answer:

The energy conversion that occurs during electrolysis is from electrical energy to chemicalenergy.

To chemical energy. [Answer incomplete. 0 mark!] From electrical energy to chemical energy.

3. Answers must relate to the needs of the question.Do not give answers that are not related to the need of the question.Read the question carefully and underline/circle important information or keywords.Example 1:

State the observation(s). State what you experienced with your five senses only. [see, hear, smell,touch, taste].

Incorrect observation Correct observationChlorine gas released. Effervescence/Gas bubbles released/ Light green gas given out.

Copper formed. Brown solid deposited. [Brown deposits]

Deflection of the galvanometer Deflection of the galvanometerneedle.

Example 2

Write the formula for the product formed between sodium and oxygen.

Answer:

Na + O2 Na2O Na2O or? 4Na + O2 2Na2O

Example 3

Write the ionic equation for the reaction between dilute hydrochloric acid and aqueous sodiumhydroxide.

Answer:

HCl + NaOH NaCl + H2O H+ + OH H2O 2H+ + 2OH 2H2O

Example 4

Given equation of reaction: 4Na + O2 2Na2O

Name the product formed.

Na2O sodium oxide.

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3. Use the correct terminology.Must use the correct terminology.If name of a person is used as unit or a chemical process, it must be in capital letter.Do not use abbreviation such as E.C.S , RAM or RMM.The followings are some of the common inaccurate/incomplete answers given by students.

Wrong Correct

1.

Purple solution disappears. Purple solution becomes colourless2. No reaction. No change3. Colour of water is clear/white Colourless/no colour4. Rate of reaction isfaster/slower. Rate of reaction is higher/lower5. Zinc donates two electrons. Zinc atom donates two electrons6. Coppermolecule Copperatom7. Bromine ion. Bromide ion8. H+ ion increase. Concentration / Number of moles of hydrogen ion, H+

increases.

9. Electrons flow from zinc metal tocopper metal.

Electrons flow from zinc metal to copper metal through

the wire.

10. Forces of attraction are weak. Intermolecular forces of attraction are weak.11. Soap consists of two parts. Soap anion consists of two parts.12. Energy unit: j (joule) Unit joule : J13. Position of zinc is above copper. Position of zinc is above copper in the electrochemical

series. or zinc is more electropositive than copper

14. Reagent to verify iodine.starch Starch solution.4. Describing chemical test: Description must have (1) correct reagent, (2) procedure, and (3)

expected observation.

Do not give the reagent only.

You must state how you would carry out the test using the reagent and what is the observation toverify the identity of the substance.

Example:

Name gas Y and describe a method to verify its identity.

Answer:

Hydrogen. Test with wooden splinter

Hydrogen. Place a lighted splinter into the test tube.

Hydrogen. Place/ Put/Near a lighted wooden splinter into the test tube. A pop sound is heardverifying the gas is hydrogen.

5. Writing balanced chemical equation.

Step 1: Write the correct formula and symbol for the reactants and products.Step 2: Balance the equation.Mole ratio in the equation must be in simple ratio.Zn + 2HCl ZnCl2 + H2 2Zn + 4HCl 2ZnCl2 + 2H2

6. Stating correctly the CHANGES in observation or state. .

You must state the change from the INITIAL colour/state to the FINAL colour/state.The following are some of the common mistakes made by students.

Wrong Correct1 Moist litmus paperturns red. Moist litmus paper turns from blue to red

2 Acidified potassium Acidified potassium manganate(VII) turns from purple to

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manganate(VII) turns colourless colourless.

3 Oxidation numberchanges to +2 Oxidation number changes from 0 to +2

7. Writing clearly the working of the calculation.

Spaces provided is to be used for writing the workings/steps clearly and not to be used for roughcalculation.

Marks are also given for workings and hence they must be written clearly.Unit must be correct.Example:

Wrong working and answer Correct working and answer16

64= 0.25

0.25 x 32 = 8 g

Number of mole sodium= 1664

= 0.25

Mass of oxygen gas = 0.25 x 32 = 8 g

8. Plotting the graphBoth axes must be labelled with the correct variable and unit (if any) must be written.

If no scale is given, use the appropriate scale so that the graph drawn covers at least half of thepage.

Points plotted must be correct.Draw the best straight line /curve.If value of the variable needs to be obtained form the graph, you must show how this value isobtained.

9. Drawing the set up of apparatus.Set up of apparatus drawn must be functional.Materials and main apparatus must be labelled.Areas representing solids, liquids and solutions must be shaded.A two dimensional diagram should be drawn.Below are some diagrams of the set up of apparatus for certain experiments. Can you identify

the mistakes in each diagrams?

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GUIDELINES ON ANSWERING PAPER 2 ESSAY QUESTION

A. General

Read all questions carefully. Always note the marks allotted for each part of the question. This will an indication as to how

many points/facts you need to answer.

Wherever possible, substantiate your answer with labelled diagrams or equations. Answer according to the needs/instructions of the question. Marks are allotted for each

instruction.

B. Why students lose marks1. Did not give enough facts/points or incomplete facts.2. Did not give the correct conditions of the experiment.3. Did not write chemical equation.4. Wrong words or terms used.

SUGGESTED FORMAT TO USE WHEN ANSWERING PAPER 2 ESSAY QUESTION

(ESSAY INVOLVING CARRYING OUT AN EXPERIMENT)

Item Description

1. Title /AimIt would be good if you can write a title or aim of the experiment. This would

enable you to stay focused and hopefully you will not write out of point!

Just read the question and write a statement based on the need instruction of

the question.

2. Apparatus andmaterial

List all the apparatus and material separately.If you are unable to do so, you

can group allunder one heading : Apparatus and material

3. Setup ofapparatus diagram

Normally two marks are allotted

1st mark: The apparatus set up is functional. 2nd mark: All chemical substance and main apparatus are labelled.(solutions, solids must be shaded)

4. Procedure

You need to briefly explain how to set up the apparatus for the experiment.You must describe how you measure and pour the chemical substances

into appropriate containers.

Values of concentration and volume of the solution and/or other data mustbe stated; especially for experiment which include calculation.

Steps taken while carrying out the experiment must be written in thecorrect order. [The correct order of the steps will enable you to carry out the

exper iment successful ly.]

Write: Readings or/ and measurements are recorded. Do not write beingread, observed, etc You must ensure that the chemical substances you choose are correct andwill react accordingly to give the desired resultgiving the correct products.

Chemical substances can be written in words or chemical formulae. Ifchemical formulae are used, they must be correct.

The physical state of the chemical substances must also be stated. Example:solution or solid or gas, powdered orpieces or ribbon, acidified or neutral, etc.

4. Observation /theory /

explanation

1. Answer according to the needs of the experiment. If certain facts are notasked, you can write briefly.

2. Include chemical equation / half equation or graph in your answer. Thesewill make your answer more easily understood.

5. Tabulation ofdata / result

The readings normally need to be presented in a table. Your table must have the

correct headings and units. Values must be given for the manipulated variable.

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Item Description

Symbols can be used for the responding variable.

Time (second) 30 60 90 120

Temperature (0C) t1 t2 t3 t4

6.

Calculation

If real values are not given, you can use unknown such as x, y or z in your

calculations.

All steps must be clearly shown.Units must be written.

7. Conclusion A conclusion must be written based on the aim of the experiment.

SAMPLE ANSWERS FOR PAPER 2 ESSAY QUESTION

Sample 1(a) Describe an experiment to investigate the effect temperature on rate of reaction. [12 marks]

(b) With reference to the collision theory, explain how this affects the rate of reaction. [4 marks]

Sample Answer

(a) Effect of temperature on rate of reaction.

[Note: (a) The set up of apparatus drawn is functional. (b) All the materials are labelled]

Apparatus and material: Measuring cylinder 50 cm3, measuring cylinder 5 cm3, stopwatch,

thermometer, 5 conical flask of the same size, Bunsen burner, wire gauze, match box, cloth, sodium

thiosulphate 0.1 mol dm3, dilute hydrochloric acid 2 mol dm3, water, paper

Procedure:1. 50.0 cm3 of sodium thiosulphate 0.1 mol dm3 is measured using a measuring cylinder.2. The solution is then poured into a clean conical flask.3. The temperature of the solution is measured using the thermometer and the reading recorded.4. 5.0 cm3 of dilute hydrochloric acid 2 mol dm-3 is measured using another measuring cylinder.5. The acid is then poured into the flask and the stopwatch is started immediately.6. The flask is swirled and placed on a piece of paper with markX drawn.7. The mark X is observed through the solution vertically and the time taken for the mark X to

disappear from sight is recorded.8. Step 1 to step 7 of the experiment is repeated using solutions of sodium thiosulphate heated to

different temperatures as shown in the table.

9. The solution is heated as shown in the diagram10. For each of the experiment, the other variables are fixed. They are:

(a)volume and concentration of sodium thiosulphate.

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(b)volume and concentration of dilute acid.(c)Size of the conical flask.

11. To investigate the effect of temperature on the rate of equation, two graphs are drawn, i.e.(a)Graph of temperature against time,(b)Graph of temperature against 1/time

Tabulation of data

Experiment 1 2 3 4

Temp (oC) 28 38 48 58

Time / (s) t1 t2 t3 t4

1/time (s-1

) a1 a2 a3 a4

Graph (i) Graph (ii)

From graph (i), it is shown that the higher the temperature of sodium thiosulphate, the shorter the timefor the mark X to disappear from sight.

From graph (ii), it is shown that as the temperature of sodium thiosulphate increases, the rate of

reaction given by 1/time increases.

Chemical equation

Na2S2O3 + 2HCl 2NaCl + H2O + SO2 + S(p)

ConclusionWhen the temperature of the reactant (sodium thiosulphate) increases, the rate of reaction increases.

(b) Collision theory1. The reaction between sodium thiosulphate and dilute hydrochloric acid to form solid sulphur isrepresented by the following ionic equation:

S2O32- + 2H+ SO2+ H2O + S

2. When the temperature increases, the kinetic energy of S2O32- ions and H+ ions increases andthe ions moves faster.

3. Hence the frequency of collisionbetween S2O32- ions and H+ ions will increase.4. The increase in temperature also increases the percentage of ions with energy more than or equalto the activation energy.

5. Therefore the frequency of effective collision between the particles will also increase.6. Subsequently the rate of reaction will increase,

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Sample 2

(a) Why latex does not coagulates when it is inside the rubber tree? [3 marks]

(b) Explain how coagulation of latex occurs when it is exposed to air. [7 marks]

Sample answer

(a) [D rawing of diagrams wil l help you and increases your chances to score higher marks]

1. Latex is a colloidal solution. It consists ofrubber particles dispersed in water.2. Each rubberparticle is made up of many long-chain rubber molecules enclosed by a protein-

like membrane which is negatively charged.

3. The negative charges around the rubber particles cause repulsion between these particles tooccur when they are near each other.

4. Hence the repulsion between the negatively-charged particles prevents the rubber particlesfrom coming close to each other.

5. Hence latex could not coagulate.

(b) Sample answer

1. Each rubberparticle is made up of many long-chain rubber molecules enclosed by a protein-like membrane which is negatively charged.

2. The negative charges around the rubber particles cause repulsion between these particles tooccur when they near each other.

3. Hence the repulsion between the negatively-charged particlesprevents the rubber particlesfrom coming close to each other.

4. Coagulation of latex can occur when it is exposed to air for a few days.5. Coagulation is caused by bacteria in the air which enter the latex.6. The growth and spread of the bacteria produce acidthat causes the coagulation of the latex.7. Acid contain hydrogen H+ ions which neutralize the negative charges on the protein membrane

of the rubber particles.

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8. With the absence of the repulsive negative charges, the rubber particles can now come closetogether.

9. This enables them to collide with one another resulting in the breakage of the proteinmembrane.

10.The rubber molecules combine with one another and entangle and thus causing the latex to coagulate.

GUIDELINE ON ANSWERING QUESTIONS IN CHEMISTRY PAPER 3

1. Paper 3 FormatTime Type of instrument

Number of

questionConstruct Marks

1

jam

Structure item Open response

item

Report item

2 or 3

Compulsory

Question 1 & 2

Evaluate Aspect of ScientificProcess Skills

33

Question 3 Evaluate Aspect of Experimenting 17

2. Allocation of marksScore Description

3 Excellent: Candidate gave the best response.

2 Satisfactory : Candidate gave an average response

1 Weak: Candidate gave an inaccurate response.

0 Candidate gave no response orwrong response

3. Scientific skills (Assesed in structured question. Total marks: 33)1. Observing KK5012. Classifying KK5023. Measuring and using numbers KK503

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4. Inferring KK5045. Predicting KK5056. Communicating KK5067. Using space-time relationship KK5078. Interpreting Data KK5089. Defining operationally KK50910.Controlling variables KK51011.Hypothesizing KK511

4. Techniques on answering question(a) Classifying

Draw a table with a ruler. Every column must have a correct heading and units MUST be written (if any)

(b) Observing Write what you will see, hear, feel, taste or smell.

(c)

Making inference State the appropriate inference that explains the observation. You must give an inference that corresponds with the observation.

(d) Predicting: You need to analyse all the information given (observations, previous experiences or

data that can be trusted) and hence make a forecast (prediction) according to the need of

the question.

(e) Measuring and using numbers You must write the value of the measurement according to the accuracy of the apparatus

used. All the values written must be uniform and have the correct unit.

Apparatus Accuracy of readingExample of reading

and its unit

Ruler One decimal place 9.6 cm

Electronic balance One decimal place 10.5g

Stop watch

One decimal place. The decimal

place must end with 0 or 5.

35.0 s

26.5 s

Measuring cylinder 50 cm 25.0 cm

Pipette 25 cm 25.0 cm

Thermometer 0110 C 35.0 C

Voltmeter 03V 2.5 V

Burette 50 cm3

Two decimal places. The second

decimal place must end with a 0or 5.

12. 20 cm

12. 00 cm312. 35 cm

3

Question 3 [17 Marks]

InvolvesPlanning on carrying out an experiment. Answer according to the need of the question. Write the report according to the headings

required in the question.

You must write the headings and not the question number or part question.Technique on answering question 3

(a)Aim of experiment Read the question carefully and the aim for the experiment is clearly stated in the question!

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The aim must be written accurately.(b)Problem statement The problem statement must be a question ending with a question mark ?. The problem statement should use certain key word such as: What ........; How .... Need to be accurate and complete.

(c)Hypothesis The statement correlates the manipulated variable with the responding variable. The manipulated variable must be written in front of the statement and followed by the

responding variable.

Need to state the direction of change forboth the variable. (the direction may be right orwrong)

(d)All variables Manipulated variable: Materials that we use and change in the experiment. Responding variable: Measurement (using apparatus, through observation or product of the

reaction) that is recorded for each material that we change in the experiment.

(e)List of apparatus and materials State the name of all materials (including its concentration) and all apparatus. A labelled diagram of the set-up of apparatus may complement the list.

(f) Procedure Volume and concentration must be written. State the quantity of the material used. Do not write about three gram. The steps in the procedure must be in order. Use passive sentences.

(g)Tabulation of data Must draw table using a ruler. Every column must have a heading and its unit (if any) Manipulated variable must be on the left side of the table and responding variable on the right

side.

SAMPLE ESSAY FOR PAPER 3

Topic: Rate of reaction

Marble (Calcium carbonate) reacts with dilute hydrochloric acid to form calcium chloride, water and

carbon dioxide gas. Different sizes of calcium carbonate will give different rate of reaction.Plan an experiment to investigate the effect of total surface area of calcium carbonate on the rate of

reaction. Your planning should include the following headings.

(a)Aim of experiment(b)Problem statement(c)Hypothesis(d)Variables(e)Apparatus and materials(f) Procedure(g)Table of data

Sample Answer

(a)Aim of experimentTo investigate the effect of total surface area of marble on the rate of reaction.

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(b)Problem statementHow does the total surface area of marble affect the rate of reaction?

(c)HypothesisThe bigger the total surface area of marble, the higher is the rate of reaction.

(d)Variables(i) Manipulated variable: Total surface area(ii) Responding variable: Volume of carbon dioxide given off(iii) Fixed variable: Mass of marble, concentration of hydrochloric acid, volume of

hydrochloric acid, temperature of reactants.

Apparatus: Burette 50 cm3, measuring cylinder, 50 cm3, electronic balance, conical flask, delivery

tube with stopper, retort stand with clamp, basin, stopwatch

Material: 5.0 g large marble chips, 5.0 g small marble chips, dilute hydrochloric acid 0.2 mol dm3,

water.

Procedure:1. Water is added into the burette and inverted as shown in the diagram. The water level is

adjusted to the 50 cm3

mark.

2. The water used is saturated with carbon dioxide gas before the experiment.3. Using a measuring cylinder, 50.0 cm3 of dilute hydrochloric acid 0.2 mol dm3 is measured

and poured into a conical flask.

4. 5.0 g of small marble is weighted and added into the conical flask and the stopwatch is startedimmediately. [The conical flask is stopped immediately ]

5. The volume of carbon dioxide gas collected at intervals of 30 seconds is recorded until thereaction stopped. [or until 50 cm3 of gas]

6. Step 1 to step 5 is repeatedby using large marble to replace the small marble chips.Tabulation of data

Time (s) 0 30 60 90 120 150 180 210

Burette reading (cm )

Volume of CO2 evolved (cm )