Question 5
- ((i) Explain each of the following processes in terms of electron transfer.
I. oxidation;
II. reduction.
(ii) Give one example each in 5(a)(i). [4 marks] - Consider the following oxidation – reduction reaction equation:
Cr2O72-(aq) + NO(g) H+ Cr3+(aq) + NO3-(aq)
(i) Write the:
I. oxidation half equation;
II reduction half equation.
Determine the overall balanced equation for the reaction. [7 marks] - (i)Describe briefly the extraction of tin from its ore.
(ii) Write a balanced chemical equation for the reaction.
(iii) Write an equation for the reaction of tin with:
I. oxygen;
II. chlorine.
(iv) State two uses of tin.
(v) State one condition necessary for the reaction in 5(c)(iii) I to occur. [14 marks]
Observation
Majority of the candidates avoided this question.
In part (a), the few candidates that responded to the question were able to give the definitions of oxidation and reduction reactions correctly.
In part (b), majority of the candidates could not write the oxidation and reduction half equations for the given reaction equation.
In part (c), few candidates described the extraction of tin from its ore. Also, few of them could write a balanced chemical equation for the reaction.
The expected answers include:
- (i) I – oxidation is electron(s) loss
II – reduction is electron(s) gain
(ii) Oxidation e.g. – Fe 2+→ Fe3+ + e-
e.g. Reduction
Cl2 + 2e-→ 2Cl- -
- (i) Its ore is concentrated/ crushed and washed with water, it is then roasted in airto remove impurities . The product is mixed with charcoal / coke and heated to reduce the oxide. Limestone is added to obtain pure tin
(ii) SnO2(s) + 2C(s) →Sn(l) + 2CO(g) / SnO2 + C Sn + CO2
(iii) I. Sn(s) + O2(g) → SnO2(s)
II. Sn(s) + 2Cl2(g)→ SnCl4(s)
(iv) - for coating iron or steel
- for making alloys
- for making cans for food / drinks
- as tin foil
(v) High temperature (above 1,000 0C)