Effects of Changing Conditions

Effect of Changing the Concentrations

When a system at equilibrium is disturbed by adding or removing one of the substances (thus changing its concentration), all the concentrations will change until a new equilibrium point is reached.

If the concentration of a reactant in the forward action is increased, the equilibrium is displaced to the right, favoring the forward reaction

For example, in the reaction:

H2 + I2 <---> 2HI

If you add more hydrogen gas, the equilibrium will be stressed to the right therefore favoring the forward reaction and producing more hydrogen iodide.

Effect of Temperature on Equilibrium

If the temperature of a given equilibrium reaction is changed, the reaction will shift to a new equilibrium point. If the temperature of a system in equilibrium is raised, the equilibrium is shifted in the direction that absorbs heat.

For example, in the reaction:

N2 + 3H2 <---> 2NH3 + heat (at equilibrium)

If the heat of the system is increased, the system will get rid of this excess heat by forming more N2 and H2 from NH3.

Note: This reaction is called the Haber process, named after Fritz Haber, a German Chemist who developed it in 1913. This process is attributed by some to be "the cause" of World War I. The Haber process increased the industrial output of Ammonia, a key ingredient in NH4NO3 , a military explosive. Without a sufficient amount of Ammonia, Germany would have not been able to fight a war thus "avoiding" World War I...

Effects of Pressure on Equilibrium

A change in pressure affects only equilibria in which a gas or gases are reactants or products. Le Chatelier's Law can be used to predict the direction of displacement. If it is assumed that the total space in which the reaction occurs is constant, the pressure will depend on the total number of molecules in that space. An increase in the number of molecules will increase pressure; a decrease in the number of molecules will decrease pressure. If the pressure is increased, the reaction that will be favored is the one that will lower the pressure, that is, decrease the number of molecules.

For example, in the reaction:

N2 + 3H2 <---> 2NH3 + heat (at equilibrium)

If the concentrations of the nitrogen and hydrogen are increased, the forward reaction is increased. At the same time, if the ammonia (NH3) produced is removed by dissolving it into water, the forward reaction is again favored.