IB Chemistry SL R2.2 How fast? The rate of chemical change questions

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Question 1

Multiple choiceEasy1 mark

For the reaction shown, $E_\mathrm{a}=45\ \mathrm{kJ\,mol^{-1}}$ Which proposed collision involves the reactants and has at least this energy?

Show answer and explanation

Answer: D. The forward reaction uses H₂ and I₂ as reactants. The activation energy is the minimum energy colliding particles need for a successful collision, so 45 kJ mol⁻¹ meets the stated threshold.

A: uses the correct reactants, but 30 kJ mol⁻¹ is below the activation energy.
B: has sufficient energy but uses HI, which is a product in the forward reaction, instead of I₂.
C: uses the correct reactants, but 44 kJ mol⁻¹ is below the activation energy.
Remember: the collision energy must meet or exceed the activation energy.

Question 2

Paper 2Easy6 marks

A chemist compares the initial rate of a gas-phase reaction at two temperatures, with the same initial reactant concentrations. Figure 1 shows Maxwell–Boltzmann energy distributions for the reactant particles. The activation energy for the reaction is $8.0\ \mathrm{kJ\,mol^{-1}}$

(a)

State what is meant by the activation energy, $E_a$ of a reaction.

[1]

(b)

Use Figure 1.

(i)

Identify which curve represents the higher temperature.

[1]

(ii)

Shade the area under this curve that represents particles with enough energy to react.

[1]

(iii)

Explain why the initial rate of reaction is greater at the higher temperature.

[2]

(c)

A catalyst is added to the reaction. On Figure 1, draw and label a vertical line to show the activation energy for the catalysed reaction.

[1]

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