# Reversible reactions and dynamic equilibrium

6.2.1 Reversible reactions

Reversible reaction

• Products of reaction can react to produce original reactants
• Under certain conditions
• Occur in apparatus which prevent escape of reactants & products
• A + B ⇌ C + D

Direction of reversible reactions can be changed by changing the conditions

For example

Why is the experiment carried out in fume cupboard? (1)

Gases are toxic

6.2.2 Energy changes and reversible reactions

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction

Same amount of energy is transferred in each case

For example

6.2.3 Equilibrium

Equilibrium – forward & reverse reactions occur at the same rate

• Reversible reaction occurs in apparatus – prevents reactants & products to escape

6.2.4 The effect of changing conditions on equilibrium (HT only)

• If a system is at equilibrium & a change is made to any of the conditions, then the system responds to counteract the change
• Effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle

6.2.5 The effect of changing concentration (HT only)

6.2.6 The effect of temperature changes on equilibrium (HT only)

6.2.7 The effect of pressure changes on equilibrium (HT only)

In industry ethanol is produced by the reaction of ethene and steam at 300°C and 60 atmospheres pressure using a catalyst. The equation for the reaction is:

The forward reaction is exothermic. Use Le Chatelier’s Principle to predict the effect of increasing temperature on the amount of ethanol produced at equilibrium. Give a reason for your prediction. (2)

• ↑ temp
• Equilibrium position move to the left
• ↓ yield

Explain how increasing the pressure of the reactants will affect the amount of ethanol produced at equilibrium. (2)

• ↑ pressure
• Equilibrium position move to the right coz least no of moles
• ↑ yield