MINISTRY OF EDUCATION
FIJI SCHOOL LEAVING CERTIFICATE
EXAMINATION
2008
CHEMISTRY
EXAMINER’S REPORT
CHEMISTRY
General Comments
The 2008 paper followed a similar format as other Chemistry papers of the past years.
Students continue to lose marks for not following instructions. Questions should be read slowly and carefully and answered accordingly. For e.g where name of a chemical is required the formula should not be written as indicated by the allocation of marks. Some students were not neat in presenting answers. Application of white ink(twink) was very common in many scripts.
Practicals – students must be encouraged to do practicals after which a thorough discussion of the results should follow. During the course of discussion safety rules/precautionary
measures/sources of errors and corrections of these errors should also be discussed.
Teaching of definitions/terms should be carefully discussed eg. dehydrating agent, hydrated, anhydrous, water of crystallization and so on. It was also noticed that many students still showed weakness in application and analysis type questions.
SECTION A
The responses given in a random sample of 100 scripts are tabulated below. The correct response in each case is marked by an asterisk. (*)
Question No. A B C D
1 *85 - 10 5
2 5 20 *40 35
3 25 20 *45 10
4 20 *55 - 25
5 10 15 50 *25
6 75 10 *15 -
7 10 *55 10 25
8 10 5 5 *80
9 30 30 *10 30
10 50 *30 15 5
11 *50 15 15 20
12 20 15 15 *50
13 - *60 25 15
14 *65 5 10 20
15 30 30 15 *25
16 *60 20 10 10
17 *25 10 30 35
18 40 25 *20 15
19 15 *50 15 20
20 10 5 60 *25
Overall, Section A was very well attempted by majority of the students. Quite a number of them scored very highly in this Section. Some students showed difficulty in arriving at correct answers in Q5 and 6 which was on Quantitative Chemistry.
In Question 9 some students showed difficulty in deciding Choice C – Cl2 and Ca (OH)2 as
reactants for the preparation of bleaching powder. In Question 10 some students failed to distinguish between ionic/covalent bonding and hence selecting PCl3 as the correct answer.
SECTION B
QUESTION 1
(a) (i) Well done.
Correct response was diffusions.
(ii) Well done.
Students were able to tell the formation of “white ring” or “white cloud”.
(iii) Equation was well written as
NH
(g)
3 + HCl(g) NH4Cl(s) by a majority of students.
(iv) Poorly answered. It raises the issue of the experiment done at various schools. Correct responses include – do not inhale ammonia; don’t spill HCl/NH3 on the skin; wear gas mask; use in an airy place or fume cupboard.
(b) (i) Many students could not state the Law of Conservation of mass. They wrote that matter is neither created nor destroyed instead of saying that mass is conserved during a chemical reaction.
(ii) Students could not reason out why the mass of the flask and its contents remained the same before and after the reaction. Incorrect responses such as “the flask was closed so no gases escaped” were given instead of saying that “mass is conserved during a chemical reaction”. Teachers should focus properly on the teaching of this concept during the practical session.
(iii) Well done.
Good answers such as formation of a white precipitate ; formation of a milky solution. Cloudy appearance in the flask were given.
(iv) Poorly done by some students who gave formulas such as Ba2SO4/Ba SO 2 4 /
Ba2
+ SO2
QUESTION 2
(a) Well done.
However some students gave the name of the gases instead of the formula while some wrote both. The correct response should have been (i) CO2 (ii) O2 (iii) SO2.
(b) (i) Step 1 : Well done by most students.
Correct response was “formation of a blue precipitate.”
Step 2 : Many students gave wrong observations. They could not tell the difference between a precipitate and a solution. The correct response should have been formation of a light blue
precipitate/curdy blue precipitate or formation of a precipitate.
(ii) Poorly done. Most of the students could not identify the type of chemical reaction occurring. While conducting this practical teachers must explain thoroughly the observations and the type of reaction. Correct responses are as follows :
Step 1 neutralisation Step 2 precipitation Step 3 decomposition
(c) (i) Well done.
Students were able to name the apparatus as “separating funnel”. However it should be noted that students must be made aware of the type of apparatus they use and for what purpose.
(ii) Well answered.
QUESTION 3
(a) (i) Satisfactorily done.
Mixed answers were given i.e. some students gave incorrect formula for lead sulphide such as Pb2S/PbS2 instead of PbS more emphasis should be made on writing of correct chemical formulas.
(ii) Not well done.
Incorrect responses include
Wrong Mr calculation for PbS. This probably was a result of giving the wrong chemical formula in a(i).
Students calculated the % as 43 x 100% = 75%
Correct calculation should have been :
239
207x 100% = 86.6%
(iii) Very poorly done.
Students were unable to explain that all the lead reacts with sulphur. Instead explanations such as sulphur melts easily and SO2 gas escapes into the air were given.
(iv) Satisfactorily done.
Correct response is “it will be low or decrease”.
(v) Well done
Students knew the precautions to be taken while burning sulphur in the laboratory.
(b) (i) and (ii) well answered.
Students were able to correctly draw the structure of the ester propyl ethanoate.
(iii) Generally well done. However some students continue to write that
QUESTION 4
(a) (i) Poorly done by some students. These students were not able to complete the equation correctly. The correct products are :
C2H2(g) and Ca(OH)2(g) - to be written in any order.
(ii) Very poorly attempted.
Common error was “heat absorbed by sand”. Instead of “to distribute the heat since the reaction is exothermic”.
(iii) Well done.
Correct response was “rapid decolourisation of bromine from orange to clear”.
(iv) Very poorly done. Students failed to compare the reactivity of brome with ethene to that of reactivity of bromine with ethyne and hence lost marks. A common mistake was “bromine water will decolourise” instead of making a comparison.
(b) Generally well done. However some students did not follow the instructions in the question and wrote names in parts (i) and (iii). Correct responses.
(i) Cl
(ii) Deep red clear solution (iii) CO 2
3
(iv) Dilute hydrochloric acid.
Students need to be very specific while giving answers.
QUESTION 5
(a) (i) Poorly done.
Students were able to indicate the anode and cathode correctly but they failed to put the appropriate signs. As a result marks were lost. Correct response was P : Anode (+) Q : Cathode (-)
(ii) Well attempted.
Correct response was Q/Cathode/(-)
(iii) Well done. Few students wrote Cu instead of Copper – again they lost marks for not reading the question properly.
(iii) Fair attempt.
Correct response : Cu2
(b) (i) Fairly attempted.
Some students did not seem to understand that water is a poor conductor of electricity and that by acidifying it, its conductivity increases.
(ii) Fair attempt.
Again many students seemed to have misread the questions and gave 1 : 2 as their answer instead of 2 : 1.
(iii) Poorly attempted.
Students failed to explain that sea water is cheap and readily available.
SECTION C
QUESTION 1
(a) (i) Well attempted.
Students who get this wrong had failed to show 8 electrons around each atom while some also used dashes to indicate the share of electrons.
Correct Structure
(ii) Poorly attempted.
Some students gave properties associated with ionic compounds. Correct response :
low melting point low boiling point liquid/solid/gas state non-electrolyte
(b) (i) Well attempted.
Lot of students were able to give the mass number as 24.
(ii) Well done. But some students wrote Mg 2
instead of M 2
Students were not asked to identify M.
(iii) Well done. Correct response was M 2N 2 or MN. A few students wrote MgO
and therefore lost marks. Well done. However few students wrote ionic bond
(c) (i) Well done. Common errors.
6 C atoms were shown in the structure of PVC molecule instead of 4 the H & Cl atoms where not shown in alternating sequence
the PVC molecule was not drawn with open ends.
Correct response H H H H
C C C C
H Cl H Cl
(ii) Well done. Correct response
rigid
hard/does not decompose.
In some responses the property & use of PVC did not match.
(d) (i) to (iii) Poorly done.
Some students were not able to identify the main product and draw the structural formula. Correct responses
Cl Cl (i) 1, 1, 2, 2 – tetra chloro ethane
H C C H
Cl Cl
H H
OR 1, 2 – dichloroethene C C
Cl Cl
H H H
(ii) propane H C C C H
H H
(iii) ethanol H C C OH
H H
QUESTION 2
(a) (i) Equation was poorly written
Correct response: NH3 + H2O NH4 + OH many students
were not able to indicate correctly the size of the arrows.
(ii) Poorly done.
Many students were not able to explain that water donate 1 proton (H ) to NH3.
(b) (i) Calculation for the molecular formula was well done. molecular formula : C2H6O.
(c) (i) Well attempted.
A few students did not mention the colour change as “orange to green” but instead wrote green. The other observation was sharp /pungent/vinegar like smell.
(ii) Well done.
Some students got the structure wrong but circled the functional group correctly.
H Correct answer H C C
H
(d) (i) to (iv) Poorly attempted
(i) Many students gave only part of the answer instead of comparing graphite with diamond while some students could not relate the property to correct structure and bonding.
O
(ii) Students showed a lack of understanding about polar and non-polar bonding.
(iii) There was poor explanation about iodine molecules having weak Vander Waals forces in between which breaks easily upon the slightest heating.
(iv) Many students continue to talk about mobile ions carrying current in metals instead of mobile electrons
QUESTION 3
(a) (i) Well done.
Many students were able to calculate the concentration as 0.14 molL 1
.
(ii) Very poorly done.
Students failed to explain that there is no water of crystallisation that will interfere with the concentration. Some students showed confusion between anhydrous and hydrated.
(iii) Well done.
Correct response 0.0035 moles or 3.5 x 10 3
moles.
(iv) Fair attempt. Few students did not use the correct mole ratio or correct volume of HCl. Correct response 0.466 molL 1 or 0.47 molL 1.
(v) Well done.
Suitable indicator was methyl orange.
(vi) Poorly done. Several students failed to give the correct colour of the indicator.
Correct response 1. Red 2. Yellow
(vii) Very poorly attempted. A very common explanation given was that water will not affect the end point because it is neutral this answer was not accepted instead of saying that near the end point [H ] = [OH ] or amount of H = amount of OH therefore the end point will not be affected.
(b) (i) to (ii) Poorly attempted.
In (i) some students used 80 (2 x 40) as the Mr of calcium therefore got incorrect number of moles of Ca.
n = Mr m = 40 16
(ii) Some students used incorrect mole ratio :
Ca : CaO is 1:1 n CaO = 0.4 moles
(iii) Students used 112 (2 x 56) as the Mr of CaO.
Mass of CaO = n x Mr = 0.4 x 56 = 22.4 g
QUESTION 4
(a) (i) and (ii) Very well attempted correct responses.
(i) Experiment 1 – HCl is very dilute.
(ii) Experiment 3 – HCl is the most concentrated.
(iii) Poorly done.
Some incorrect response included increase the concentration of HCl or use a catalyst instead of use powdered or crushed coral.
(iv) Poorly done.
Most students knew what the rate of a reaction was but could not put it into proper words. Rate of reaction refers to the speed of a reaction or how fast/slow a reaction occurs.
(b) Poorly done.
Students showed poor understanding of reactivity of metals. Some explained that oxide coating on the surface of magnesium causes it to react slowly. Correct explanation : sodium is very reactive therefore reacts fast while magnesium is less reactive therefore reacts slowly.
(c) (i) Well done.
Students were able to explain that the reaction is endothermic.
(ii) and (iii) poorly done.
Incorrect partial explanations were given.
(ii) Correct explanation was less CO will form and more of CH4 and H2O will form
(iii) Correct explanation was more CO will form. Reaction is endothermic, increase in temperature will favour forward reaction.
(iv) Well done.
Many students showed good understanding about the role of a catalyst and its effect on the equilibrium. Correct explanation – the catalyst speeds up the rate but, does not affect equilibrium constant KC.
(d) The definitions of Le Chatelier’s Principle was very poorly done. Correct definition – if a system in equilibrium is subjected to a stress then the system readjust itself to oppose or relieve this stress.
QUESTION 5
(a) (i) 1 and 2 – Well done.
1. HCl(aq) + H2O(l) H + Cl
or H3O + Cl
2. CH3COOH(aq) + H2O(l) H + CH3COO
Or H3O + CH3COO
(ii) Satisfactorily done.
Concept of strong acid and weak acid was not well understood by students. Correct explanation – HCl is a strong acid undergoes complete dissociation while CH3COOH is a weak acid undergoes incomplete dissociation.
(b) (i) Satisfactorily done.
Some students wrote CUSO4 + H2O as products instead of CUSO4 + x H2O.
(ii) Fairly well done.
Some students failed to mention “to drive out all the water of crystallization” which was the correct answer. While some said to evaporate the water which was not accepted as the answer.
(iii) to (iv) Well done.
(iii) n =
160 60 1.
= 0.01 moles
(v) x =
01 0.
= 5, hence CU SO4. 5H2O
(v) hydrated salt is one that contains water of crystallization while an anhydrous salt does not contain water of crystallisation.
SECTION D OPTIONAL TOPICS
1. Chemistry of Copper and its Compounds
most popular choice
Most students who attempted this topic had good knowledge about the copper Chemistry.
Some areas of weakness included :
use of copper in everyday life. name and colour of complex ion.
Importance of copper in the human body.
2. Chemistry of Food
Second most popular choice. Students showed a good understanding of this topic. Some areas of weakness included
not able to differentiate between starch soluble and starch insoluble polymers i.e. amylate & amylopectin
poor explanation of test for glucose denaturation process.
3. Chemistry in the Environment
Third popular choice many students who attempted this topic showed a poor understanding of eutrophication, thermal pollution, weathering and soil Chemistry.
4. Consumer Chemistry
Least popular choice and poorly attempted. Many students were careless in giving correct responses to manufacture of soaps and detergents, uses of bleaches, polymer Chemistry and types of water.
