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Change in Temperature


The effect of changing the temperature of a reaction at equilibrium depends on whether the reaction is exothermic or endothermic in nature. When the reactants are converted into products, heat is evolved in exothermic reactions, whereas heat is absorbed in endothermic reaction. To increase the temperature of the system we add heat to it.

According to the Le Chatelier principle, the equilibrium is shifted in a direction so as to consume (or reduce) the heat added to the system. Thus, for an endothermic reaction, the equilibrium is shifted to the right-hand side and for an exothermic reaction; the equilibrium is shifted to the left-hand side, as these are the directions of consumption of heat. These effects may be easily followed if we treat heat as one of the constituents of the reaction as described below.
  1. Exothermic reaction
    A + B C + D + q
  2. Endothermic reaction
    A + B + q C + D
where A, B, C and D are the reacting constituents and q is the heat involved in the reaction. Now the Le Chatelier principle as discussed earlier, in the case of the effect of adding one of the constituents, may be used directly.

If we add q (or raise temperature), for an exothermic reaction, equilibrium is shifted to the left-hand side, whereas in an endothermic reaction, equilibrium is shifted to the right-hand side, as these directions are the directions in which heat is consumed (or reduced).

We may also use the above way of writing chemical reactions to discuss the effects produced when the temperature of the system is lowered. Here, we are removing heat and thus reaction moves in a direction to produce more heat. Thus, for an exothermic reaction, the equilibrium is shifted to the right-hand side, whereas is an endothermic reaction, it is shifted to the left-hand side.

Note that the equilibrium in the present case is primarily caused by the variation of the equilibrium constant of the reaction. From the conclusions drawn above, the effect of temperature on equilibrium constant may be deduced.

Effect of temperature on equilibrium constant
Reaction Temperature Concentration of species on
Right-hand side Left-hand side Equilibrium constant

Exothermic

Increase Decrease

Decrease Increase

Increase Decrease

Decrease Increase

Endothermic

Increase Decrease

Increase Decrease

Decrease Increase

Increase Decrease


Problem
The ionic products of water at 273 K and 373 K are 1.1 10-15 (mol dm-3)2 and 5.1 10-13 (mol dm-3)2, respectively. Predict whether the ionization of water is exothermic or endothermic.

Solution
Since the ionic product increases with temperature, the ionization of water will be endothermic in nature.




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