Introduction to kinetics and equilibria — WCH12 IAL Chemistry practice questions · Study-IAL

Introduction to kinetics and equilibria

WCH12Topic 2.43 questions

Collision theory, Maxwell-Boltzmann distributions, catalysts and dynamic equilibrium.

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Question 14 marks
Use the Maxwell–Boltzmann distribution to explain why a small increase in temperature causes a large increase in the rate of a reaction.
Mark scheme
- At higher temperature the distribution curve flattens and its peak moves to higher energy (the area under the curve is unchanged) (1)
- The area under the curve beyond $E_a$ increases, so a much greater proportion of molecules have energy $\geq E_a$ (1)
- Molecules also move faster, so collide more frequently (1)
- So there are many more successful collisions per unit time (1)
Question 23 marks
Dinitrogen tetroxide (colourless) and nitrogen dioxide (brown) are in dynamic equilibrium in a sealed syringe:

$\text{N}_2\text{O}_4(\text{g}) \rightleftharpoons 2\text{NO}_2(\text{g}) \qquad \Delta H = +58\,\text{kJ mol}^{-1}$

⇌

2NO2(g)ΔH=

+58kJ mol−1

(a) State and explain the effect on the equilibrium yield of $\text{NO}_2$ of increasing the pressure at constant temperature.

(b) State the effect on the equilibrium position of increasing the temperature, giving a reason.

Mark scheme

(a) The yield of $\text{NO}_2$ decreases / equilibrium shifts to the left (1)
(a) Because the left-hand side has fewer moles of gas (1 mol compared with 2 mol) (1)
(b) Shifts to the right, because the forward reaction is endothermic (1)