Physical Chemistry
7 marks · EasyA student investigates the thermal decomposition of ammonium chloride in an open dish and a sealed container.
Explain why changing the pressure has little effect on the rate of the decomposition when ammonium chloride is heated as a solid in an open dish.
Write a balanced chemical equation, using the correct symbol, to show that the thermal decomposition of ammonium chloride is a reversible reaction.
If the experiment is repeated in a sealed container, what substances will be present when equilibrium is reached? Give your answer as a list.
Explain why all these substances are present at equilibrium, even though the reaction appears to have stopped.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) In solids, particles are already packed closely together, so increasing pressure does not change how often they collide. Therefore, changing the pressure has little effect on the rate.
(b) NH4Cl(s) ⇌ NH3(g) + HCl(g)
(c) NH4Cl(s), NH3(g), HCl(g)
(d) At equilibrium, the forward and reverse reactions happen at the same rate, so all substances are being formed and used up equally. Their amounts do not change, even though the reaction is still happening.
Common mistakes: Saying pressure affects rate of all reactions; Forgetting to use reversible arrow; Listing only one substance at equilibrium; Saying reaction stops at equilibrium
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States or explains that changing pressure does not affect solids or liquids because particles are already close together.
A1 (a) · 1 mark · Links to collision frequency or rate specifically.
B1 (b) · 1 mark · Writes correct reversible equation using the ⇌ symbol.
A2 (b) · 1 mark · Balanced equation with correct states.
B2 (c) · 1 mark · Lists all 3 substances present at equilibrium (NH4Cl(s), NH3(g), HCl(g)), any order.
M2 (d) · 1 mark · States that at equilibrium, the reaction continues forwards and backwards at the same rate.
A3 (d) · 1 mark · Explains that the amounts/concentration of substances stay constant even though reactions continue (dynamic equilibrium).
Worked solution
a. In solids (and liquids), particles are already packed very closely together, so increasing pressure makes no significant difference in how closely they can be packed. As a result, the frequency of particle collisions does not really increase, so the rate of decomposition is almost unaffected by changes in pressure.
b. The correct reversible equation is
c. At equilibrium in a sealed container, all three substances are present: NH4Cl(s), NH3(g), and HCl(g).
d. Because equilibrium is dynamic, the forward and reverse reactions continue to happen at the same rate. This means that the amounts of all substances remain constant even though reactions are still taking place, so it appears that the reaction has stopped.