O-LevelChemistryRates of Reaction5070/21 · May/June 2017 · Q67 Marks

Photosynthesis is a reaction that occurs in the leaves of green plants. Carbon dioxide and water are converted into glucose and oxygen. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ The reaction is endothermic and is catalysed by enzymes.

(a)Draw an energy profile diagram for photosynthesis using the axes shown. Label • the axes, • the enthalpy change, • the reactants and products.[3]
(b)(i)What is an enzyme?[1]
(b)(ii)State the effect of enzymes on reactions.[1]
(c)Explain why the rate of photosynthesis increases as the temperature increases.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

A6(a) Axes - energy / enthalpy on vertical axis AND progress of reaction / course of reaction on horizontal axis (1)
Reactant and product including the relative position of lines – reactant level below products AND reactant and product labelled (1)
Enthalpy change – shown by upward arrow AND labelled enthalpy change or ∆H (1)
A6(b)(i) Biological catalyst (1)
A6(b)(ii) Speeds up a reaction / lowers activation energy (1)
A6(c) Particles have more kinetic energy / particles moving faster (1)
More successful collisions / more fruitful collisions / more energetic collisions / more particles with energy equal to or above activation energy(1)

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