3.4.2 Equilibria - Qualitative effects of changes of temperature and concentration
Students should:
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The equilibrium constant
Kc is a constant which represents how far the reaction will proceed at a given temperature.
When Kc is greater than 1, products exceed reactants (at equilibrium). When much greater than 1, the reaction goes almost to completion. When Kc is less than 1, reactants exceed products. When much less than 1 (Kc can never be negative...so when it is close to zero) the reaction hardly occurs at all.
The only thing which can change the value of Kc for a given reaction is a change in temperature. The position of equilibrium, however, can change without a change in the value of Kc.
Effect of Temperature
The effect of a change of temperature on a reaction will depend on whether the forward reaction is exothermic or endothermic. When the temperature increases, Le Chatelier's principle says the system will respond in such a way as to counteract this change, i.e. lower the temperature. Therefore, reactions will move towards the side of endothermic change.
Increase temperature = system moves in the endothermic direction and establishes a new edquilibrium
The reverse is true for a lowering of temperature.
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Example: The Haber process This is an equilibrium which is exothermic in the forward direction, i.e. towards the production of ammonia. N2 + 3H2 Increasing the temperature decreases the proportion of ammonia at equilibrium. So the reaction must be carried out at a temperature that is high enough to achieve equilibrium rapidly, but not too high as to restrict the proportion of ammonia. The temperature chosen in about 450ºC, at which the equilibrium forms 15% ammonia. |
Changing the temperature is the only factor that changes the value of Kc for a given equilibrium.
Effect of Concentration
When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products. When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants.
When the system is at equilibrium the rate of the forward reaction equals the rate of the reverse reaction. If more of the reactants are added the rate forward increases and the system is no longer at equilibrim. The forward reaction proceeds faster then the reverse reaction until the equilibrium is re-established.
- Add to the left and the system moves to the right
- Add to the right and the system moves to the left.
The same logic applies for removal of a component from the system. If one of the products is removed from the equilibrium then that decreases the rate fof the reverse reaction. The forward rate is now faster then the reverse rate and the system moves in the forward direction to re-establish equilirbrium.
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Example: The Haber process Under the conditions used only about 15% of the reactant gases produce ammonia. N2 + 3H2 The mixture of gases are then raidly cooled when the ammonia liquefies and is removed from the mixture. The gases are then recycled back through the reaction vessel where the forward reaction proceeds once again to 15%. This is continuously carried out. |
- Remove from the right and move to the right
- Remove from the left and move to the left
At no time does the value of kc change. The original equilibrium is disturbed and the system responds to re-establish the equilibrium proportions in all cases.
Effect of Pressure
Changes in pressure will only affect equilibria if the number of moles of gas on either side of the reactin are unequal.
H2 + I2 
2HI
This equilibrium remains unaffected by changes in pressure.
An equilibrium in which there are more moles of gas on the right hand side will be driven in the reverse reaction by an increase in pressure. The system is responding to a change in conditions in such a way as to counteract the change. Moving towards the side of fewer moles of gas releases the increased pressure.
Changes in pressure moves the system to one direction or the other (depending on the moles of reactant and product gases), but does not affect the value of kc or kp.
The equilibrium is being desturbed by the pressure change and responds to re-establish the value of the equilibrium constant. The may seem counter-intuitive at first sight and many students have a hard time accepting this.
Effect of catalysts on equilibrium
A catalyst does not effect either Kc or the position of equilibrium, it only effects the rate of reaction. As the rate of forward reaction and reverse reaction is affected equally then the equilibrium cannot be affected.