This page contains the detailed and easy notes for Edexcel IGCSE Chemistry Reversible Reaction And Equilibria for revision and understanding.

Edexcel IGCSE Paper 1: Complete Revision Summary

REVERSIBLE REACTION

Reactions that proceed both in forward and reverse direction.

Eg – N_2(g) + 3H_2(g)  ⇌  2NH_3(g)

At the start the concentration of the reactants decreases. The reactants decreases and the concentration of products start to increase.

A ⇌ B

There comes a point where concentration of reactants and the products are same as the rate of appearance of products and rate of disappearance of reactants is the same. That point is the equilibrium point.

DYNAMIC EQUIBRIUM

• When rate of forward reaction is equal to the rate of reverse reaction.
• The reactions does not stop at equilibrium. The reactions takes place with the same rate in both the direction so overall we see no change.

CONDITIONS FOR DYNAMIC EQUILBRIUM

1. a) It has to be closed system. Nothing should leave or enter the system.
2. b) The rate of forward reaction should be equal to the rate of reverse reaction.

Le Chattelier’s Principle

When the system in equilibrium is subject to a change, the equilibrium is moved to a direction to counteract the chance.

Concentration ⇌

N_2(g) + 3H_2(g)  ⇌  2NH_3(g)

Forward direction ammonia is made in the reverse direction nitrogen and hydrogen are used up.

Add nitrogen                      Right

Add hydrogen                      Right

Add ammonia                      Left

Remove Ammonia                      Right

So in habers process nitrogen and hydrogen are continously added and unreacted are recycled and ammonia is removed as soon as it is formed.

Pressure

• More the gas molecules more the pressure.
• Less the pressure less the gas molecules.

N_2(g) + 3H_2(g)  ⇌  2NH_3(g)

• Increasing pressure = Less gas side = Right
• Decreasing pressure: = more gas side = Left
• So high pressure is required for manufacture of ammonia.

Temperature and Equilbrium

• If ΔH is negative the forward reaction is exothermic and produces heat.
• If the forward is exothermic the reverse is endothermic and vice versa.

EXOTHERMIC REACTION (Produces heat)

 ENDOTHERMIC REACTION (Takes in heat)

 Eg N_2(g) + 3H_2(g)  ⇌  2NH_3(g)

ΔH = -93KJ/mol

• Forward reaction is exothermic produces heat. Reverse reaction is endothermic and takes in heat.
• Increase in temperature will shift the equilibrium side which is left and decrease in temperature will shift to more heat side which is right.

KEY TERMS

1. a) Rate of a reaction – It is the decrease in the concentration of reactants per unit time, or the increase in the concentration of products per unit time,
2. b) Collision Theory – For the reactions to take place the particles should collide or bump into each other. Low concentration = few collisions. High Concentration = more collisions.
3. c) Activation Energy – the minimum energy required to start the reaction.
4. d) Catalysts – A catalyst is a substance that changes the speed of a chemical reaction without chemically altering it at the end of the reaction.
5. e) Reversible Reaction – Reactions that proceed both in forward and reverse direction.
6. f) Dynamic Equilibrium – When rate of forward reaction is equal to the rate of reverse reaction.
7. g) Le Chatellier’s Principle – When the system in equilibrium is subject to a change, the equilibrium is moved to a direction to counteract the chance.
8. h) Exothermic – Produces Heat
9. i) Endothermic – Takes in Heat

DISCLAIMER

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References:

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