Equilibrium and LeChatelier's Principle - Background

LeChatelier's Principle - Changes in Concentration



A system is at equilibrium when the rates of the forward and reverse reactions are equal. If additional reactant is added the rate of the forward reaction increases. As the rate of the reverse reaction is initially unchanged, the equilibrium appears to shift toward the product, or right, side of the equation. As the additional reactant is consumed the forward rate slows. When the rates of the forward and reverse reactions are again equal, the system has returned to equilibrium. It is convenient to think of this as the system shifting to the right to remove the added reactant.

While a disturbed system will return to equilibrium, it is not exactly the same equilibrium that existed before the stress was applied. One difference will be in the concentrations of the reactants and products. Consider the system below:

H2 + I2 2 HI


If we add some hydrogen to the system at equilibrium, the system will shift to the right in an attempt to remove the extra hydrogen. It can be shown mathematically that it will be unable to remove all of it. Therefore, the new equilibrium concentration of hydrogen will be higher than it was in the original equilibrium. The only way the system has to remove the hydrogen is by reaction with iodine. Therefore, the new equilibrium concentration of iodine will be lower than it was in the original equilibrium. When hydrogen and iodine react they form hydrogen iodide, so the concentration of hydrogen iodide will be higher than it was in the original equilibrium conditions. We could summarize the effects in a table like the one below in which brackets are used to represent concentrations.

applied
stress 		direction
of shift 		effect on
[H2] 		effect on
[I2] 		effect on
[HI]
H2 added right increased decreased increased


If hydrogen had been removed from the system, the equilibrium would shift to the left in an attempt to make more of it. We could prove mathematically that it will be unable to replenish all of it and that the new equilibrium concentration of hydrogen would be lower than it was in the original equilibrium. In order to make more hydrogen, the system must use up hydrogen iodide causing its new equilibrium concentration to be lower than it was in the original equilibrium. When hydrogen iodide decomposes it forms both hydrogen and iodine. The new equilibrium concentration of iodine will be higher than it was in the original equilibrium.

applied
stress 		direction
of shift 		effect on
[H2] 		effect on
[I2] 		effect on
[HI]
H2 removed left decreased increased decreased


Practice using LeChatelier's Principle by predicting what should happen if (1) iodine is added to the system at equilibrium, (2) iodine is removed from the system at equilibrium, (3) hydrogen iodide is added to the system at equilibrium and (4) hydrogen iodide is removed from the system at equilibrium. When you are done, check your answers.


Continue with changes in temperature as an equilibrium stress.


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