Chemistry SCORE A+

2011

GUIDELINES ON ANSWERING

QUESTIONS IN

CHEMISTRY PAPER 2 AND PAPER 3

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 should give.

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 chemical energy. 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 observation Chlorine 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 galvanometer needle.

Example 2Write the formula for the product formed between sodium and oxygen. Answer:

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

Example 3Write the ionic equation for the reaction between dilute hydrochloric acid and aqueous sodium hydroxide. Answer:HCl + NaOH NaCl + H2O H+ + OH– H2O 2H+ + 2OH– 2H2O

Example 4Given equation of reaction: 4Na + O2 2Na2O Name the product formed. Na2O sodium oxide.

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.

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  Wrong Correct 1. Purple solution disappears. Purple solution becomes colourless2. No reaction. No change 3. Colour of water is clear/white Colourless/no colour4. Rate of reaction is faster/slower. Rate of reaction is higher/lower5. Zinc donates two electrons. Zinc atom donates two electrons6. Copper molecule Copper atom7. Bromine ion. Bromide ion

8. H+ ion increase. Concentration / Number of moles of hydrogen ion, H+ increases.

9.Electrons flow from zinc metal to copper 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 : J

13. 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 to

verify 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 heard verifying 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 Correct

1 Moist litmus paper turns red. Moist litmus paper turns from blue to red

2Acidified potassium manganate(VII) turns colourless

Acidified potassium manganate(VII) turns from purple to colourless.

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

7. Writing clearly the working of the calculation.

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

= 0.25

0.25 x 32 = 8 g

Number of mole sodium= = 0.25

Mass of oxygen gas = 0.25 x 32 = 8 g

8. Plotting the graph Both 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 the page. 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 is obtained.

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?

GUIDELINES ON ANSWERING PAPER 2 ESSAY QUESTIONA. 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.

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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 /Aim

It 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 and material

List all the apparatus and material separately. If you are unable to do so, you can group all under one heading : “Apparatus and material”

3. Set–up of apparatus 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 must be stated; especially for experiment which include calculation. Steps taken while carrying out the experiment must be written in the correct order. [The correct order of the steps will enable you to carry out the experiment successfully.] Write: “Readings or/ and measurements are recorded”. Do not write “being read, observed, etc” You must ensure that the chemical substances you choose are correct and will react accordingly to give the desired result–giving the correct products. Chemical substances can be written in words or chemical formulae. If chemical 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 or pieces or ribbon, acidified or neutral, etc.

4. Observation / theory / explanation

1. Answer according to the needs of the experiment. If certain facts are not asked, you can write briefly. 2. Include chemical equation / half equation or graph in your answer. These will make your answer more easily understood.

5. Tabulation of data / 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. Symbols can be used for the responding variable. Time (second) 30 60 90 120Temperature (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.

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Item Description

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 dm–3, dilute hydrochloric acid 2 mol dm–3, water, paper

Procedure:1. 50.0 cm3 of sodium thiosulphate 0.1 mol dm–3 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 mark ‘X’ 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.(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

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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 time for 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 is represented 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 and

the ions moves faster.

3. Hence the frequency of collision between S2O32- ions and H+ ions will increase.

4. The increase in temperature also increases the percentage of ions with energy more than or equal to the activation energy.

5. Therefore the frequency of effective collision between the particles will also increase.

6. Subsequently the rate of reaction will increase,

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) [Drawing of diagrams will help you and increases your chances to score higher marks]1. Latex is a colloidal solution. It consists of rubber particles dispersed in water.

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2. Each rubber particle 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 to occur when they are near each other.

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

5. Hence latex could not coagulate.

(b) Sample answer1. Each rubber particle 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 to occur when they near each other.

3. Hence the repulsion between the negatively-charged particles prevents the rubber particles from 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 acid that 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.

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

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

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 Format

Time Type of instrumentNumber of question

Construct Marks

1 ½ jam

Structure item Open response

item Report item

2 or 3Compulsory

Question 1 & 2 Evaluate Aspect of Scientific

Process Skills 33

Question 3 Evaluate Aspect of Experimenting

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2. Allocation of marksScore Description

3 Excellent: Candidate gave the best response.2 Satisfactory : Candidate gave an average response1 Weak: Candidate gave an inaccurate response.0 Candidate gave no response or wrong response

3. Scientific skills (Assesed in structured question. Total marks: 33)1. Observing KK5012. Classifying KK5023. Measuring and using numbers KK5034. Inferring KK5045. Predicting KK505 6. 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.

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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 unitRuler One decimal place 9.6 cmElectronic balance One decimal place 10.5g

Stop watch

One decimal place. The decimal place must end with 0 or 5.

35.0 s26.5 s

Measuring cylinder 50 cm3 25.0 cm3

Pipette 25 cm3 25.0 cm3

Thermometer 0 – 110 0C 35.0 0CVoltmeter 0 – 3V 2.5 V

Burette 50 cm3Two decimal places. The second decimal place must end with a 0 or 5.

12. 20 cm3

12. 00 cm3

12. 35 cm3

Question 3 [17 Marks] Involves “Planning 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! 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 for both the variable. (the direction may be right or

wrong)

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(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 reactionMarble (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 experiment

To investigate the effect of total surface area of marble on the rate of reaction.

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

(c) Hypothesis The 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.

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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 dm–3, 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 dm–3 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 started

immediately. [The conical flask is stopped immediately ]5. The volume of carbon dioxide gas collected at intervals of 30 seconds is recorded until the

reaction stopped. [or until 50 cm3 of gas]6. Step 1 to step 5 is repeated by using large marble to replace the small marble chips.

Tabulation of data

Time (s) 0 30 60 90 120 150 180 210Burette reading (cm3)Volume of CO2 evolved (cm3)

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