7.8.4 Effect of Temperature Change

Whenever an equilibrium is disturbed by a change in the concentration, pressure or volume, the composition of the equilibrium mixture changes because the reaction quotient, Qc no longer equals the equilibrium constant, Kc.

However, when a change in temperature occurs, the value of equilibrium constant, Kc is changed.

In general, the temperature dependence of the equilibrium constant depends on the sign of ∆H for the reaction

The equilibrium constant for an exothermic reaction (negative ∆H) decreases as the temperature increases.

The equilibrium constant for an endothermic reaction (positive ∆H) increases as the temperature increases.

Temperature changes affect the equilibrium constant and rates of reactions.

Production of ammonia according to the reaction,

N2(g) + 3H2(g) ⇔ 2NH3(g)

H = – 92.38 kJ mol-1

is an exothermic process. According to Le Chatelier’s principle, raising the temperature shifts the equilibrium to left and decreases the equilibrium concentration of ammonia. In other words, low temperature is favourable for high yield of ammonia, but practically very low temperatures slow down the reaction and thus a catalyst is used.

 

Effect of Temperature – An experiment

Effect of temperature on equilibrium can be demonstrated by taking NO2 gas (brown in colour) which dimerises into N2O4 gas (colourless).

2NO2(g) ⇔ N2O4(g); ∆H = – 57.2 kJ mol-1

NO2 gas prepared by addition of Cu turnings to conc. HNO3 is collected in two 5 mL test tubes (ensuring same intensity of colour of gas in each tube) and stopper sealed with araldite.

Three 250 mL beakers 1, 2 and 3 containing freezing mixture, water at room temperature and hot water (363K), respectively, are taken (Figure) Both the test tubes are placed in beaker 2 for 8-10 minutes.

After this one is placed in beaker 1 and the other in beaker 3.

The effect of temperature on direction of reaction is depicted very well in this experiment.

At low temperatures in beaker 1, the forward reaction of formation of N2O4 is preferred, as reaction is exothermic, and thus, intensity of brown colour due to NO2 decreases.

While in beaker 3, high temperature favours the reverse reaction of formation of NO2 and thus, the brown color intensifies.

Effect of temperature can also be seen in an endothermic reaction,

[Co(H2O)6]3+(aq)  +   4Cl-(aq)    ⇔   [CoCl4 ]2-(aq) + 6 H2O(l)

Pink                    colorless               blue

 

At room temperature, the equilibrium mixture is blue due to [CoCl4]2–. When cooled in a freezing mixture, the color of the mixture turns pink due to [Co(H2O)6]3+

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