Factors affecting Equilibrium
Predict how the equilibrium concentrations of reactants and products are shifted by changes in concentrations of reactants and products, by pressure changes, and by temperature changes.
When a reaction reaches a state of chemical equilibrium under a particular set of conditions, no further changes in the concentrations of reactants and products occur. If a change is made in the conditions under which the system is at equilibrium, chemical change will occur in such a way as to establish a new equilibrium. The factors that can influence equilibrium are change in concentration, change in pressure (or volume), and change in temperature.
What effect does a change in one of these factors have on the extent of reaction? This question can be answered qualitatively by using Le Chatelier's principle: When a stress is applied to a system in a state of dynamic equilibrium, the system will, if possible, shift to a new position of equilibrium in which the stress is partially offset. To interpret this statement take, for example, the reaction
2NO2 ->N2O4(g)
Changes in Volume and Pressure.
The pressure of a system of gases in chemical equilibrium can be increased by decreasing the available volume. This change causes the concentration of all components to increase. The stress will be partially offset by a net reaction that will lower the total concentration of gas molecules. Consider our previous reaction:
2NO2(g) --> N2O4(g)
Changes in Temperature
If the temperature of a system is changed, a change in the value of Kc occurs. An increase in temperature always shifts the equilibrium in the direction of the endothermic reaction, while a temperature decrease shifts the equilibrium in the direction of the exothermic reaction. Therefore, for endothermic reactions the value of Kc increases with increasing temperature, and for exothermic reactions the value of Kc decreases with increasing temperature. In the case of our example reaction, Kc will decrease as the temperature is increased because the equilibrium will shift in the direction of the endothermic reaction, that is, in the reverse direction.
2NO2(g --> N2O4(g))
Example
For the reaction at equilibrium
2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)
state the effects (increase, decrease, no change) of the following stresses on the number of moles of sodium carbonate, Na2CO3, at equilibrium in a closed container. Note that Na2CO3 is a solid (this is a heterogeneous equation); its concentration will remain constant, but its amount can change.
Change of Concentration
When the concentration of any reactants or products in the reaction changes the equilibrium. The change of concentration further leads to minimising its effect.
Catalyst
A catalyst increases the rate of the chemical reaction and does not affect the equilibrium composition of a reaction mixture. It increases the rate of forward/reverse reactions that pass through the same transition state without affecting equilibrium. Catalyst lowers the activation energy for the forward reactions and backward reactions by exactly the same amount.