O-LevelChemistryStoichiometry5070/22 · May/June 2020 · Q27 Marks

Hydrogen reacts with oxygen as shown in the equation. 2H2(g) + O2(g) → 2H2O(l) A sample containing 1.00 mol of hydrogen, H2, is completely combusted. This sample releases 286 kJ of heat energy.

(a)Calculate the heat energy released when 25.0g of hydrogen is completely combusted. heat energy released ..................................................................... kJ [2][2]
(b)Use ideas about bond breaking and bond forming to explain why this reaction is exothermic.[2]
(c)(i)The reaction shown also represents the overall process that occurs within a hydrogen-oxygen fuel cell. Describe one advantage of using a hydrogen-oxygen fuel cell to power a motor vehicle rather than burning gasoline.[1]
(c)(ii)Complete the equations for the two electrode reactions that happen in a hydrogen-oxygen fuel cell. H2 → ............................................ + 2e- O2 + 4H+ + 4e¯ → ....................................[2]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] 2(a) (moles of H2) = 25 / 2 OR 12.5 (1)
[1] 2(a) (energy = 12.5 × 286) = 3575 / 3580 (kJ) (1)
[1] 2(b) bond breaking is endothermic and bond making is exothermic / bond breaking absorbs energy and bond making releases energy (1)
[1] 2(b) more energy released (during bond making) than absorbed (during bond breaking) (1)
[1] 2(c)(i) no carbon dioxide made (during use) / only makes water as a product / more energy efficient (during use) / saves fossil fuels (1)
[1] 2(c)(ii) H2 → 2H+ + 2e¯ (1)
[1] 2(c)(ii) O2 + 4H+ + 4e¯ → 2H2O (1)

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