P27MS

CHEMISTRY
PAPER 3
MARKING SCHEME

1. Table I
Experiment 1 2 3 4 5
Volume of solution F(cm3) 10 9 8 7 6
Volume of water(cm3) 0 1 2 3 4
Time taken (seconds) 36 45 52 62 86
Rate of reaction = 1 0.02778 0.02222 0.01923 0.01613 0.01163
Time

- Complete table filled with realistic values (1mark)

- Accept reciprocals given in decimal or standard form.
- Penalize ½ mk for any unfilled space
- Reject fractions given for 1/t.
- Consistent decimals (tied to the raw time only). (1mark)
- Accept whole numbers or decimals used consistently for time otherwise penalize fully.
- Accuracy + 1 sec to the school value. (1mark)
Trend tied to the 1st time reading (time increases as water is added. (1mark)
(i) Graph
- Labelling of axes (with units indicated) (½ mark)
Penalize full for wrong units used or inverted axes.
- Scale (¾ of squares used on both axes) (½ mark)
- Plotting (4 points or more correctly plotted. (1mark)
Award ½ mk for 3 points plotted correctly otherwise penalize fully
- Straight line (line best fit with +ve gradient. (1mark)
Award 4 there is evidence the use of a ruler.
(ii) Correct showing on the graph (½ mark)
Work out time (1mark)
Correct answer as per graph (½ mark)
(iii) Rate increases as the concentration of solution F increases // Rate is directly proportional to concentration of solution F
(1mk) when concentration is high there are more particles in a given space. .: the particles collide more often increasing
the rate of reaction. (1mk)
Table II
1 2 3
Final burette reading (cm3) 17.4 33.7 16.4
Initial burette reading (cm3) 0.0 17.4 0.0.
3
Volume of solution H used (cm ) 17.4 16.3 16.4

- Complete table with all 3 titrations (1mark)

- Complete with only 2 titration. (½ mark)
Penalize ½ mk once for wrong arithmetic inverted table unrealistic valves.
- Consistent decimals (only 1 or 2 decimal place) (1mark)
If 2 decimal place used then 2nd must either be 0 or 5 otherwise penalized fully.
- Accuracy (compared candidate value with school value. (1mark)
Titre values within +0.1 of school value. (1mark)
Titre values within +0.2 of school value (½ mark)
- Principles of averaging (b) (i) (1mark)
Values averaged should be within +0.2 of themselves otherwise penalize fully.
If all the three values are consistent and average. (1mark)
If only 2 values are consistent and average (1mark)
Penalize fully if inconsistent values averaged.
If 3 values are consist and only 2 are averaged penalize fully.
b) (i) Final answer (correct average value obtained compared to school value. (1mk)
If the value is within +0.1 of school value (1mk)
If the value is within +0.2 of school value (½ mark)
(ii) Av. titre x 3.75 = Ans moles
1000
 Mole ratio NaOH : HCl 1 :1
250 x ans above = Ans in moles
25
Penalize (½ mark) for wrong units and answer given to less than 4 significant figure unless it work out exactly.

(iii) 40 x 2 = 0.08moles
1000
0.08 – ans in b(ii) = Ans in moles
(iv) Mg + 2HCl Mg Cl2 + H2
Mole ratio 1:2
Ans in b(iii) x 24 = Answer g
2

Observations Inferences
2. (i) Colourless gas that relight a glowing NO3- ions present
splint, wet litmus turns red
Brown gas
Residue yellow hot white cold Zn2+ ions

(ii) White precipitation formed SO42- ions

(iii) White precipitation formed SO42- ions
(iv) White precipitation formed Zn2+, Al3+, Pb2+
two mentioned
All
(v) White precipitate soluble Zn2+ ions confirmed
NB: Zn2+ should have been mentioned above
(b) Observations Inferences
(i) Light blue flame without smoke Absence of C = C or C = C
Presence of - C - C - or
Organic compound of L, M M
(ii) miscible / uniformly dissolve Presence of polar organic
or dissolves forming colourless solution Substance reject : polar compound
(iii) pH = 4 / pH = 5 Weakly acidic reject: acidic substance
Reject: pH colour and pH range R-COOH present
(iv) Oily sweet smelling substance Penalize fully for any contracting group

(v) Orange colour of K2Cr2O7 persist R - OH absent penalize full for any contracting functional group.
or Orange colour of K Cr O do not change to green Presence of H+ / H3O+
2 2 7
Reject: Orange colour forms / colour remains
unchanged / no effect K2Cr2O7
(vi) Rapid effervescence / bubbles of colourless gas/ fizzing / R - COOH . penalize fully for any contractory ion / group.