Chemical Equilibria I

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1)Reversible reaction
A reversible reaction is a chemical reaction which can take place in both directions, i.e. reactant react to form products and products can also react to form the original reactants.

2)Dynamic equilibrium
A system is said to be in dynamic equilibrium occurs when the rate of forward reaction is equal to the rate of reverse reaction and the concentration of products and reactants remain constant. It is dynamic because reactants are being continuously converted to products and products are being continuously converted back to reactants.

3)Le Chatelier's Principle
Le Chatelier's Principle states that in a system in equilibrium, when a change is made to some external factor, the position of equilibrium shifts to oppose the change.

I will use the production of ammonia as an example.

4)Effects of changes in concentration
What happens when you increase the concentration of nitrogen gas. According to Le Chatelier's Principle the equilibrium reacts to remove the extra nitrogen gas that is added. Thus the equilibrium shifts to the right and produces more ammonia.

5)Effects of changes in pressure
Changes in pressure only affects reactions involving gases. From the chemical equation, we can see that 1 mol of nitrogen gas reacts with 3 mol of hydrogen gas to form 2 mol of ammonia gas, i.e. 4 mol of gaseous reactants react to form 2 mol of gaseous products. What happens when the pressure of the system is increased? According to Le Chatelier's Principle, the equilibrium shifts to the right to reduce the pressure.

6)Effects of changes in temperature
The formation of ammonia from nitrogen and hydrogen is an exothermic reaction, i.e. the forward reaction is exothermic . What happens when the temperature is increased? Based on Le Chatelier's Principle, the equilibrium will react to oppose the increase in temperature and the equilibrium shifts to the left.

7)Effects of a catalyst
A catalyst does not change the position of the equilibrium. It only increases the rate at which the system reaches equilibrium.

8)Equilibrium Constant Kc

If a reversible reaction is allowed to reach equilibrium, the product of the concentration of products divided by the product of the concentrations of reactants has a constant value at a particular temperature.

Changes in concentration and pressure and the presence of a catalyst does not change the equilibrium constant. Only temperature change will cause a change in the equilibrium constant. For an exothermic reaction, the equilibrium constant decreases as temperature is increased . For an endothermic reaction, the equilibrium constant increase with increasing temperature.

To be continued
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