Pupil sheet · Reversible reactions and equilibrium

Pink or blue? Both ways at once — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. state

Supplement. Which two statements are true of a reversible reaction at equilibrium in a closed system?

Tick every correct statement.

There are equal amounts of reactants and products
The forward and reverse reactions have stopped
The rate of the forward reaction equals the rate of the reverse reaction
The concentrations of the reactants and products no longer change

Nudge (only if you are stuck): Equilibrium is dynamic: think about what is still happening, and what isn't changing.

2. predict / explain

Supplement. A violet cobalt solution is at equilibrium: pink form + chloride ions ⇌ blue form + water, and the forward reaction, to the blue form, is endothermic. A tube of it is put into hot water. What happens?

Circle one answer.

It turns bluer, because heating moves every equilibrium to the right
No change: temperature does not affect an equilibrium
It turns bluer: heating favours the endothermic forward reaction
It turns pinker: heating favours the reverse reaction

Nudge (only if you are stuck): Heating favours the direction that takes in heat.

3. predict / explain

Supplement. For the same cobalt solution, pink form + chloride ions ⇌ blue form + water, and the forward reaction, to the blue form, is endothermic. Water is added to a blue tube of it. Which way does the position of equilibrium move?

Circle one answer.

To the right: adding a substance always pushes the reaction forward
To the right: water dilutes the products
It doesn't move: water is not part of the reaction
To the left: water is a product, so adding it favours the reverse reaction

Nudge (only if you are stuck): Is water a reactant or a product here?

4. predict / explain

Supplement. Methanol is made from carbon monoxide and hydrogen: CO(g) + 2H₂(g) ⇌ CH₃OH(g). The forward reaction is exothermic. Predict, with a reason, the effect on the yield of methanol of (a) a higher pressure, (b) a higher temperature, (c) adding a catalyst.

(a) Higher pressure

(b) Higher temperature

Nudge (only if you are stuck): Count the gas molecules on each side for (a).

5. explain

Supplement. Nitrogen and oxygen react: N₂(g) + O₂(g) ⇌ 2NO(g). Explain why changing the pressure does not change the position of this equilibrium.

Gas molecules on each side

Why the position doesn't move

Nudge (only if you are stuck): Count the gas molecules on each side.

6. explain

Supplement. A pupil says: "Adding a catalyst to an equilibrium mixture makes more product." (a) Explain why the pupil is wrong. (b) What does the catalyst change?

(a) Why the pupil is wrong

(b) What the catalyst changes

Nudge (only if you are stuck): A catalyst speeds up a reaction. Which one?

7. interpret / explain

Supplement. Reactants are sealed in a flask and the concentration of the product is measured every minute (table). (a) After how many minutes does the mixture reach equilibrium? (b) A pupil says: "After that time the reaction has stopped." Explain what is really happening.

Time / min	0	1	2	3	4	5	6
Concentration of product / mol/dm³	0	0.20	0.32	0.38	0.40	0.40	0.40

(a) Time to reach equilibrium: ______________ unit ________

(b) What is really happening.

Nudge (only if you are stuck): Look for the first time the concentration stops changing.

8. predict / explain

Supplement. Potassium chromate solution is yellow and potassium dichromate solution is orange. In solution: 2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O. Predict and explain the colour change when (a) acid, which adds hydrogen ions, is added to the yellow solution, (b) then alkali, which removes hydrogen ions, is added.

(a) Acid added

(b) Then alkali added

Nudge (only if you are stuck): Hydrogen ions are a reactant.