3.2.2 Kinetics - Effect of concentration
Students should:
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The effect of Concentration changes
For a system at equilibrium the concentrations of both reactants and products are constant (but not the same). The value of Kc depends on these concentrations and Kc is constant unless the temperature is changed.
If a further quantity of reactant is added to the mixture already at equilibrium then the value of [products]/[reactants] no longer equals the value of Kc and the equilibrium must make adjustments to reestablish the equilibrium concentrations.
In this example the value of the ratio [products]/[reactants] is too small as more reactants have been added. The only way to adjust this is by making more product (and at the same time using up some of the reactants)
The equilibrium responds by making more product until the equilibrium ratio is reestablished at the Kc value.
In summary the effect of adding reactants has been to make more product.
This gives some general rules:
- If we add to the left hand side of an equilibrium we make more of the right hand side
- If we add to the right hand side of an equilibrium we make more of the left hand side
- If we remove from the left hand side the equilibrium makes more of the left hand side component
- If we remove from the right hand side the equilibrium makes more of the left hand side component
Explanation
All of this can be easily explained by considering the rates of the forwards and back reactions.
At equilibrium the rates of the forward and back reactions are equal
rate forwards = rate backwards
When we study rates of reaction one of the first conclusions drawn is that the rate of a chemical reaction depends on the concentrations of the reactants. The greater the concentration the more collisions occur and the faster the rate of the reaction.
When we add more reactant the forward rate will now be greater than the back rate. The reaction is now not at equilibrium and the forward reaction proceeds faster than the back reaction until the equilibrium conditions are reestablished.
Similiarly removal of a component from one side will reduce the rate of its reaction and case the equilibrium to make more of it.
In all of these changes the value of Kc remains unchanged. The equilibrium is temporarily disturbed and then the equilibrium concentrations are re-established.
