Equilibrium and LeChatelier's Principle - Background

LeChatelier's Principle - Changes in Temperature



Changing the temperature of an equilibrium system also stresses the system. Just like with changes in concentration, the system will attempt to relieve the stress by shifting in one direction or the other. The direction of the shift depends on whether the forward reaction is exothermic or endothermic. While heat is not really a reactant or a product in a chemical reaction, it is convenient to treat it as such. For an exothermic reaction heat is released during the reaction and we can consider heat as a product.

A + B C + heat

For an endothermic reaction the reverse is true and we can consider heat as a reactant.

A + B + heat C

As the formation of hydrogen iodide from hydrogen and iodine (the forward reaction) is an endothermic process we could write our equilibrium reaction as:

H2 + I2 + heat 2 HI

Once heat is placed in the equation we can predict the direction of the equilibrium shift when a temperature change is made. If we were to raise the temperature of the above system it would shift to the right in an attempt to dissipate the heat. As a result the concentration of hydrogen iodide would increase while the concentrations of hydrogen and iodine would decrease.


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


Practice using LeChatelier's Principle by predicting what should happen when the system is cooled. When you are done, check your answers.


Continue with some practicalities.


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