Equilibrium reactions and the factors affecting them | 16-18 years

In this activity, students decide whether statements about equilibrium reactions are true or false and review their decisions in a group discussion.

The resource is designed to check and make explicit the following misconceptions that:

• All of a reactant added to an equilibrium mixture is used up before equilibrium is restored.
• When additional reactant is added to a system in equilibrium the value of the equilibrium constant increases.
• Equilibrium is reached when the concentration of reactants equals the concentration of products.
• A high value of the equilibrium constant is associated with a fast reaction.
• The rate of the forward reaction increases from mixing reactants until equilibrium is established.
• Catalysts affect forward and reverse reactions in an equilibrium system in different ways.
• Heating an equilibrium mixture decreases the rate of an exothermic reaction.

Learning objectives

Students will understand:

• The effect of concentration, catalyst and temperature on equilibrium reactions.

Sequence of activities

Introduction and demonstration

1. Demonstrate the consequence of adding a reactant to a system in equilibrium.
   • Mix together in a large conical flask a little iron(III) chloride solution and potassium thiocyanate solution and dilute with distilled water.
   • Describe the equilibrium system: Fe³⁺(aq) + SCN^-(aq) → [FeSCN]²⁺(aq)
     • In terms of appearance: yellow Fe³⁺(aq) + colourless SCN^-(aq) → blood red [FeSCN]²⁺(aq)
   • Add some potassium thiocyanate to the flask.
   • Ask students about their observations.
2. If possible, show an animation of the formation of an equilibrium mixture before sharing the learning objective with students.

Activity: stage 1

Hand out a worksheet to each student. Circulate and support the students as they:

• Work individually.
• Complete the table on the worksheet to decide whether the provided statements are true or false.
• Explain why the statements ticked as false are wrong.

Activity: stage 2

Arrange the students into groups of three. Ask them to:

• Compare the judgements they have made with each other.
• Agree a group response to each statement.
• Select a spokesperson.

Plenary

Draw the groups into a plenary and invite each spokesperson to comment on one of the statements. Encourage other groups to add to the comments.

Before finishing

Ask each student to:

• Write down changes to their thinking as a result of talking and listening to others.
• Hand in their worksheets.

Feedback

Give written feedback. Acknowledge achievement and give suggestions that help students to move forward.

Commentary

The initial demonstration (adding a reactant to a system in equilibrium) focuses student attention to the learning objective.

Working in a small group, the students articulate and assess their ideas. The plenary provokes a further evaluation of their initial responses to the statements.

The teacher provides written feedback on the students’ ideas to acknowledge achievement and to provide comment that leads students to recognise their next steps and how to take those steps.

Demonstration notes

Apparatus

• Conical flask, 500 cm³

Chemicals

• Potassium (or ammonium) thiocyanate solution, 0.5 mol dm^-3 (HARMFUL)
• Iron(III) chloride solution, 0.5 mol dm^-3 (IRRITANT)
• Distilled water

Other equipment

• Animation showing the formation of an equilibrium mixture (if available)

Health, safety and technical notes

• Read our standard health and safety guidance.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.

Answers

1. False: some, but not all, of the added reactant will be used up as the system reaches equilibrium.
2. True: the value of the equilibrium constant is not altered by adding a reactant to a system in equilibrium.
3. False: the concentrations of reactants and products do not have to be equal for an equilibrium to be reached.
4. False: there is no link between the equilibrium constant and the rate of the reactions in the equilibrium system.
5. False: the rate of the forward reaction will be at a maximum when reactants are first mixed and will fall as they are used up reaching a constant value equal to the rate of the reverse reaction when equilibrium is reached.
6. True: a catalyst increases both forward and reverse reactions to the same extent and therefore does not alter the equilibrium position or the value of the equilibrium constant.
7. True: if the forward reaction in an equilibrium system is endothermic then the reverse reaction must be exothermic.
8. False: if a system in equilibrium, where the forward reaction is endothermic, is heated, then the rate of reaction of both forward and reverse reactions will increase. However, the rate of the forward reaction will increase more than that of the reverse reaction so that the position of equilibrium moves towards the products.