7.2 The Position of Equilibrium

Deduce KcLe Chatelier’s Principle

Key Points thus far…

• A reaction is at equilibrium if :-The rate of the forward reaction = the rate of the

backward reaction, therefore quantities remain constant

Macroscopically there is no change however microscopically there is constant change

Equilibria only exist in a CLOSED SYSTEM – from which no energy exchange is possible

An equilibrium can favour products ( rhs ) or reactants (lhs)

Kc and the Magnitude of Kc

• Kc is the equilibrium constant, where….

• When Kc >> 1, the reaction goes almost to completion.

• When Kc << 1, the reaction hardly proceeds.

Some Examples

• For the synthesis of an ester…….

OK give me another example

• Ammonia synthesis

And just one more

• Contact Process

Position of Equilibirum

• Consider Ammonia synthesis …..

• What will be the effect of removing Ammonia ?• What will be the effect of adding Hydrogen ?

Le Chatelier’s Principle

• If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

• If you remove something ( same as decreasing concentration ) the equilibrium will seek to replace it.

Good old Ammonia

• Impact of removing– 1. Ammonia– 2. Hydrogen– 3. Nitrogen

• Impact of increasing pressure ( GASES ONLY )– As pressure is a consequence of the impact of gas

molecules on its container, the equilibrium will shift to the side with LESS GAS MOLECULES to counteract the change.

So What About Temperature ?

• This is a little more complicated.

• If exothermic

• If endothermic

Is that everything ?

• Well……..NO !• Catalysts • A catalyst has absolutely no effect on the

position of equilibrium, it merely increases the rate at which equilibrium is reached.

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