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Chemistry · Class 10 · Chapter 9: Chemical Equilibrium

8 min

Key points

Chemical equilibrium is a state in which the rate of forward reaction equals the rate of backward reaction
At equilibrium, concentrations of reactants and products remain constant but not equal
Equilibrium can only be achieved in a closed system where no substance can escape
Le Chatelier's principle states that if stress is applied to a system at equilibrium, the system shifts to counteract the stress
Factors affecting equilibrium: concentration, pressure, temperature, and catalyst
Catalyst increases the rate of both forward and backward reactions equally, hence does not affect equilibrium position
Increasing pressure favors the side with fewer gas molecules
Increasing temperature favors endothermic reaction direction
Industrial applications include Haber process for ammonia production and Contact process for sulfuric acid

Definitions

Chemical Equilibrium: A state of balance in which the rate of forward reaction is equal to the rate of backward reaction and the concentrations of reactants and products remain constant.
Reversible Reaction: A chemical reaction in which reactants form products and products can also form back the reactants under the same conditions.
Le Chatelier's Principle: If a stress is applied to a system at equilibrium, the equilibrium shifts in such a direction as to undo the effect of stress.
Dynamic Equilibrium: An equilibrium in which the forward and backward reactions continue to occur at equal rates.

Worked example

Define chemical equilibrium and explain how increase in temperature affects the equilibrium position of an exothermic reaction. (5 marks)

Chemical equilibrium is the state of a reversible reaction where the rate of forward reaction becomes equal to the rate of backward reaction and the concentrations of reactants and products remain constant. When temperature is increased in an exothermic reaction, according to Le Chatelier's principle, the equilibrium shifts in the direction that opposes the change. Since exothermic reactions release heat, increasing temperature will shift the equilibrium towards the reactants (backward direction) to absorb the excess heat.

Common mistakes

Students confuse 'equal concentrations' with 'constant concentrations' at equilibrium
Forgetting that catalyst does not shift equilibrium position, only increases rate
Not understanding that equilibrium is dynamic - reactions continue but at equal rates
Mixing up the effect of temperature on exothermic vs endothermic reactions
Writing incomplete equilibrium expressions without proper arrows

Quick recap

Chemical equilibrium occurs when forward and backward reaction rates are equal with constant concentrations. Le Chatelier's principle states that equilibrium shifts to oppose any applied stress. Concentration, temperature, and pressure changes affect equilibrium position. Only dynamic equilibrium exists in reversible reactions.

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