A reversible reaction ($\rightleftharpoons$) is a reaction that can take place in both directions – i.e. forward reaction and backward reaction.
When the rate of forward reaction is equal to the rate of backward reaction, the reaction is said to have reached a state of dynamic equilibrium.
When a chemical reaction reaches a state of dynamic equilibrium:
- the rate of forward reaction is equal to the rate of backward reaction
- the concentration of reactants and products remain unchanged (constant)
The composition of an equilibrium mixture in a reversible reaction can be altered by changes in:
- concentration
- temperature
- pressure
Le Chatelier’s Principle
Le Chatelier’s Principle is used to predict the change in composition of an equilibrium mixture. Le Chatelier’s Principle states that:
If the conditions of a system in dynamic equilibrium are altered, the system will move so as to oppose the change.
Changing Concentration
The table below shows the effect of changing concentration (conc.) for a reversible reaction
Conc. of reactants | Conc. of products | Shift in position of equilibrium | Reaction yield |
---|---|---|---|
Increase | $—$ | Right | Increase |
Decrease | $—$ | Left | Decrease |
$—$ | Increase | Left | Decrease |
$—$ | Decrease | Right | Increase |
Changing Temperature
The table below shows the effect of changing temperature (temp.) for a reversible reaction
Exothermic reaction | Endothermic reaction | Shift in position of equilibrium | Reaction yield |
---|---|---|---|
Increase temp | $—$ | Left | Decrease |
Decrease temp | $—$ | Right | Increase |
$—$ | Increase temp | Right | Increase |
$—$ | Decrease temp | Left | Decrease |
Changing Pressure
The table below shows the effect of changing pressure for a reversible reaction
No. of moles of gaseous reactions $>$ no. of moles of gaseous products | No. of moles of gaseous reactions $<$ no. of moles of gaseous products | Shift in position of equilibrium | Reaction yield |
---|---|---|---|
Increase pressure | $—$ | Right | Increase |
Decrease pressure | $—$ | Left | Decrease |
$—$ | Increase pressure | Left | Decrease |
$—$ | Decrease pressure | Right | Increase |