IGCSE Chemistry Paper-1: Specimen Questions with Answers 7 - 8 of 99

Question 7

Question

MCQ▾

Three electrolysis cells are set up. Each cell has inert electrodes. The electrolytes are listed below.

  • cell 1 aqueous sodium chloride
  • cell 2 dilute sulfuric acid
  • cell 3 molten lead (II) bromideIn which of these cells is a gas formed at both electrodes?

Choices

Choice (4)

a.

3 only

b.

1 and 2

c.

1 and 3

d.

2 only

Answer

b.

Explanation

Electrolysis of aqueous sodium chloride yields hydrogen and chlorine, with aqueous sodium hydroxide remaining in solution. ‎‎The reaction at the cathode is:

H2O (l) + 2e − ⇾ H2 (g) + 2OH

The reaction at the anode is:

Cl − ⇾ 12Cl2 (g) + 1e

The overall reaction is as follows:

Electrolysis of dilute sulphuric acid yields oxygen:

NaCl (aq) + H2O (l) ⇾ Na + (aq) + OH (aq) + H2 (g) + 12Cl2 (g)

The positive anode electrode reaction:

2H2O(l) + 4e ⇾ 4H +(aq) + O2 (g) (oxygen gas)

The negative cathode electrode reaction:

2H +(aq) + 2e ⇾ H2 (g) (hydrogen gas)

Question 8

Question

MCQ▾

Which process is not exothermic?

Choices

Choice (4)

a.

obtaining lime from limestone

b.

radioactive decay of 235U

c.

burning a fossil fuel

d.

reacting hydrogen with oxygen

Answer

a.

Explanation

burning a fossil fuel and a radioactive decay are both exothermic. Reaction of hydrogen and oxygen, or otherwise combustion of hydrogen is also exothermic. Lime (Cao) is obtained from limestone (CaCO3) by thermal decomposition of limestone, i.e.. we heat (PROVIDE energy) for this decomposition. hence this process is endothermic, not exothermic.

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