Page 422 - SUBSEC October 2017_Neat

02112032/CAPE/SPEC/MS/2017
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CHEMISTRY
UNIT 1 — PAPER 032
MARK SCHEME
KC UK XS
Question 3

S.O.: Module: 2 ----6.26

(i) Hypothesis
Any correct answer, for example:

A is a strong monobasic acid
B is ...
C is ... 1
D is ...
[1 mark]

(ii) Apparatus and materials [2 marks]
2
(iii) Method

3
Any reasonable workable set of procedures, for example:

 use of pH meter for weak and strong acids;
 react acids with metal or carbonate and record time
taken for reaction to stop;
 for each reducing agent, set up a cell with H 2O 2 as one
half-cell and the reducing agent in the other half-
cell. Measure the voltage on a high resistant
voltmeter.
[3 marks]

(iv) Variables
Correct and relevant [2 marks]

2

(v) Expected results
Any reasonable answer [2 marks]
2

(vi) Chemical principles

 Weak acid has low [H ] and strong acid has high [H ].
+
+
+
pH = - log 10 [H ]. Therefore, weak acid has high pH, and
strong acid has low pH.
3
 Low [H ] in solution produces slow rate of reaction and
+
vice versa.

 H 2O 2 half-cell reaction:

+
H 2O 2 + 2H + 2e  2H 2O(l) E = + 1.77V

 Reducing agent half-cell reaction:

-e Product E = + 1.39V

E cell = + 3.16V

 E cell is produced for the weak reducing agent.

[3 marks]

(vii) Sources of error/assumptions/limitations