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2018 O Level Chem P1 Q34 — Energy Changes / Exothermic and Endothermic Solution

Energy Changes / Exothermic and Endothermic 2018 O Level Pure Chemistry · Paper 1 · Q34

Question

An equation for respiration is shown. C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂ ΔH = −2830 kJ/mol What can be deduced from the equation?

AEnergy from sunlight is needed for this reaction.
BGlucose is reduced.
CLarger molecules are made from smaller molecules.
DΔH for photosynthesis is +2830 kJ/mol.

Correct answer: D

Video Solution

Explanation

Answer: D. Photosynthesis is the reverse of respiration: \(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\).

Reversing a reaction keeps the same magnitude of \(\Delta H\) but flips its sign. Since respiration has \(\Delta H = -2830\) kJ/mol, photosynthesis has \(\Delta H = +2830\) kJ/mol, which is statement D.

A is wrong: sunlight is needed for photosynthesis, not respiration. B is wrong: glucose is oxidised in respiration. C is wrong: respiration breaks glucose into smaller molecules. Answer D.

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