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Factors that Affect the Equilibrium of Reactions

The equilibrium of a reaction is affected by factors such as changes in concentration, pressure, and temperature. The likely resulting changes in equilibrium can be predicted on the basis of Le Chatelier's principle which can be stated as follows:


If we change the conditions (factors such as the ones mentioned above) of a reaction system in equilibrium, the system will shift in such a way as to reduce the imbalance caused by the stress. This is Le Chatelier's principle.


Based on this principle, we will look at some of the factors that affect the equilibrium of reactions.

Effect of Temperature

What is your first instinct when you think of a reaction scenario in which the temperature is changed? It is true that the reaction rate will change. We might even think that the reaction rates of all reactions invariably increase. This is not true. Even though many reactions speed up with the increase in temperature, that is not always the case. Consider the reaction given below:

image\25230 ch 11.png

The reaction between nitrogen and hydrogen to give ammonia is an exothermic reaction. The reaction produces heat energy during the forward reaction. For this reaction at equilibrium, if we increase the temperature, it will only facilitate the backward reaction. Now we will consider another reaction.

image\25242 ch 11.png

For this reaction, we are supplying heat. So what kind of reaction is it? It is an endothermic reaction. If we subject this reaction in equilibrium to an increase in temperature, the forward reaction will be favored. We can generalize our observations as follows:

An increase in temperature favors endothermic reactions. An increase in temperature decreases the reaction rate, if the reaction is exothermic.

Effect of Pressure

The effects of pressure are significant in reactions involving gases. Think about what happens when we increase the pressure of a reaction in equilibrium. When the pressure is increased, the equilibrium no longer exists. Consider the reaction given below.

image\25259 ch 11.png

The direction of reactions can be predicted based on Le Chatelier's principle. For the above reaction, if the pressure is increased the forward reaction is favored. Why is the forward reaction favored? The reason is that the forward reaction will result in less moles of gas, thereby decreasing the strain caused by the increased pressure. This influence of pressure will not affect the reaction if the total number of moles of reactants and the total number of moles of products are equal. The influence of pressure is not seen with reactants or products which are either in their solid or liquid state because they are almost non-compressible.

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