Reversible reactions and equilibrium — GCSE Chemistry practice questions

Question 1

A museum keeps old paper maps in a glass display case. A humidity card in the case has a spot of cobalt(II) chloride on it. The spot changes colour when the air in the case becomes damp. To reuse the card, a technician dries it in a warm oven.

Which row is correct?

Accepted Answer

A: blue · pink · endothermic. Anhydrous cobalt(II) chloride is blue and hydrated cobalt(II) chloride is pink, so the spot is blue in dry air and pink in damp air. Removing the water takes in thermal energy from the oven, so drying the spot is endothermic; adding water back is exothermic.

Question 2

Phosphorus(V) chloride is heated to 250 °C in a sealed flask. It decomposes in a reversible reaction.

PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

After some time the mixture reaches equilibrium. Which statement about the mixture at equilibrium is correct?

Accepted Answer

C: The concentrations no longer change, but PCl₅ molecules are still decomposing.. At dynamic equilibrium both reactions continue, at the same rate. PCl₅ is still decomposing, but it re-forms just as fast, so every concentration stays constant. The concentrations do not have to be equal to each other.

Question 3

Calcium carbonate is heated in a sealed steel container until equilibrium is reached.

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

The forward reaction is endothermic. The temperature is then raised and the mixture reaches equilibrium again.

Which row compares the new equilibrium with the first one?

Accepted Answer

C: moves to the right · increases · increases. The forward reaction is endothermic, so raising the temperature moves the position to the right. A higher temperature also makes both reactions faster, because more collisions have energy equal to or greater than the activation energy. At the new equilibrium the two rates are equal again, and both are higher than before.

	colour of the spot in dry air	colour of the spot in damp air	drying the spot in the oven is
A	blue	pink	endothermic
B	blue	pink	exothermic
C	pink	blue	endothermic
D	pink	blue	exothermic

	position of equilibrium	rate of the forward reaction	rate of the reverse reaction
A	moves to the left	decreases	increases
B	moves to the right	increases	decreases
C	moves to the right	increases	increases
D	moves to the right	increases	no change