Pink or blue? Both ways at once — answer key

1. XQ8 · Practice

Model answer. At equilibrium both reactions keep going at the same rate, so the concentrations stay the same. The reactions haven't stopped, and the amounts don't have to be equal.

A full answer includes

• The rate of the forward reaction equals the rate of the reverse reaction.
• The concentrations of the reactants and products no longer change.

Also accept

• Equal rates; concentrations no longer change

Watch for

• “At equilibrium the reactions have stopped.” — Both reactions are still going, at the same rate, so the amounts stay the same.
• “At equilibrium there are equal amounts of reactants and products.” — The concentrations are steady, not equal: either side can be larger.

2. XQ9 · Practice

Model answer. The forward reaction is endothermic, so heating moves the position of equilibrium to the right, using up some of the extra heat. More of the blue form is made and the solution turns blue.

A full answer includes

• It turns bluer: heating favours the endothermic forward reaction.

Also accept

• Bluer; the endothermic forward reaction is favoured

Watch for

• “Heating always moves an equilibrium to the right.” — Heating favours the endothermic direction. If the forward reaction is exothermic, heating moves the position to the left.

3. XQ10 · Practice

Model answer. Water is on the right of the equation, a product. Adding more of a product moves the position to the left, using up some of the added water: more of the pink form is made.

A full answer includes

• To the left: water is a product, so adding it favours the reverse reaction (pinker).

Also accept

• To the left; water is a product

Watch for

• “Adding anything pushes a reaction forward.” — Adding a reactant moves the position to the right; adding a product moves it to the left.

4. XQ11 · Practice

Model answer. (a) 1 + 2 = 3 gas molecules on the left, 1 on the right. A higher pressure moves the position to the side with fewer gas molecules, the right: more methanol. (b) The forward reaction is exothermic, so the reverse is endothermic; heating moves the position to the left: less methanol. (c) A catalyst speeds up both reactions equally: the same yield, reached sooner.

A full answer includes

• (a) More methanol: 3 gas molecules on the left, 1 on the right; higher pressure favours fewer.
• (b) Less methanol: heating favours the endothermic reverse reaction.
• (c) No change in yield: equilibrium is reached sooner.

Also accept

• More; less; no change (sooner)

Watch for

• “Heating always moves an equilibrium to the right.” — Heating favours the endothermic direction. If the forward reaction is exothermic, heating moves the position to the left.
• “A catalyst gives more product at equilibrium.” — A catalyst speeds up the forward and the reverse reactions equally. Equilibrium comes sooner, but the position does not move.

5. XQ12 · Practice

Model answer. A change of pressure moves the position towards the side with fewer gas molecules. Here the left has 1 + 1 = 2 gas molecules and the right has 2. Neither side has fewer, so pressure makes no difference to the position.

A full answer includes

• There are 2 gas molecules on each side (1 + 1 on the left, 2 on the right), so neither side has fewer.

Also accept

• The same number of gas molecules on each side

Watch for

• “A higher pressure always moves the position to the right.” — A higher pressure moves it to the side with fewer gas molecules, which may be either side; with the same number on each side it makes no difference.

6. XQ13 · Practice

Model answer. (a) The catalyst speeds up both reactions equally, so the rates stay equal and the position of equilibrium is the same: no more product. (b) The mixture reaches equilibrium in less time.

A full answer includes

• (a) A catalyst speeds up the forward and reverse reactions by the same amount, so the position doesn't move.
• (b) Equilibrium is reached sooner.

Also accept

• Both directions equally, same position; equilibrium sooner

Watch for

• “A catalyst gives more product at equilibrium.” — A catalyst speeds up the forward and the reverse reactions equally. Equilibrium comes sooner, but the position does not move.

7. XQ14 · Practice

Model answer. (a) The concentration rises until 4 minutes and then stays at 0.40 mol/dm³: equilibrium. (b) The reactions have not stopped. Product is made as fast as it turns back into reactants, so its concentration no longer changes.

Checked automatically

• (a) Time to reach equilibrium: 4 min (exact value)
  • Common wrong answer 6 : The concentration is already steady at 4 minutes: that is when equilibrium is reached.

A full answer includes

• (a) 4 minutes.
• (b) The forward and reverse reactions are still going, at equal rates, so the concentration stays the same.

Also accept

• 4 minutes; both reactions continue at equal rates

Watch for

• “At equilibrium the reactions have stopped.” — Both reactions are still going, at the same rate, so the amounts stay the same.

8. XT2 · Apply

Model answer. (a) Acid adds H⁺, a reactant, so the position moves to the right to use some of it up: more orange dichromate. (b) Alkali removes H⁺, so the position moves to the left to replace it: more yellow chromate.

A full answer includes

• (a) It turns orange: adding a reactant moves the position to the right.
• (b) It turns yellow again: removing a reactant moves the position to the left.

Also accept

• Orange (right); yellow again (left)

Watch for

• “Alkali adds something, so the position moves to the right.” — The alkali takes hydrogen ions away. Removing a reactant moves the position to the left.