Industrial Chemistry > 2. Many industrial processes involve manipulation of equilibrium reactions > Explain The Effect of Changing The Different Factors on Identified Equilibrium Reactions >
Explain the effect of changing the following factors on identified equilibrium reactions:
a. Pressure
b. Volume
c. Concentration
d. Temperature
LE CHATELIER’S PRINCIPLE
- general rule that helps in predicting the direction in which an equilibrium reaction will move when a change in pressure, volume, concentration or temperature occurs
- it states that “if an external stress is applied to a system in equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium
position”
Suppose that the equilibrium system given below is in a cylinder fitted with a movable piston.
CH4(g) + H2O(g) ⇌ CO2(g) + 3H2(g) (endothermic)
PRESSURE
- changes in pressure do not affect the concentration of the reacting species in condensed phases (e.g. solids and liquids) because solids and liquids are incompressible
- concentrations of gases are greatly affected by changes in pressure
- INCREASE in PRESSURE denotes an INCREASE in CONCENTRATION of the gaseous species present. This favors the reaction that decreases the total number of moles of gases. In the example given, the increase in pressure favors the reverse reaction.
- DECREASE in PRESSURE favors the net reaction that increases the total number of moles of gases. In the example given, the decrease in pressure favors the forward reaction.
VOLUME
- Changing the volume without changing the pressure does not alter the equilibrium established by the system
- However, it is possible to change the pressure of the system without changing its volume. The total pressure of the system can be increased by adding an inert gas.
- Addition of inert gas at constant volume increases the total gas pressure and decreases the mole fractions of the gaseous species. However, the partial pressure of each gas does not change. As such, the presence of an inert gas does not affect the equilibrium condition.
CONCENTRATION
- Changing the concentration of either reactants or products can disturb the equilibrium condition.
- An INCREASE in the concentration of the reactants shifts the equilibrium to favor formation of products. On the other hand, INCREASING the concentration of the products shifts the equilibrium to the left.
- In the example given, increasing the amount of hydrogen gas favors equilibrium to form CH4 and H2O. Decreasing the concentration of CH4 favors equilibrium to the left, thereby producing more CH4 and H2O.
TEMPERATURE
- Changes in concentration, pressure and volume may alter the equilibrium position but it does not change the value of the equilibrium constant. However, change in temperature
can alter the equilibrium constant. - In the example given, the forward reaction is ENDOTHERMIC (the system ABSORBS HEAT):
heat + CH4(g) + H2O(g) ⇌ CO2(g) + 3H2(g)
On the other hand, the reverse reaction is EXOTHERMIC (the system RELEASES HEAT):
CO2(g) + 3H2(g) ⇌ CH4(g) + H2O(g) + heat
- At equilibrium at a certain temperature, the heat effect is ZERO because there is no net reaction.
- An INCREASE in temperature favors ENDOTHERMIC reaction whereas a DECREASE in temperature favors the EXOTHERMIC reaction.