Reversible Reaction


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 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.

The table below shows the effect of changing concentration (conc.) for a reversible reaction

Conc. of reactantsConc. of productsShift in position of equilibriumReaction yield
Increase$—$RightIncrease
Decrease$—$LeftDecrease
$—$IncreaseLeftDecrease
$—$DecreaseRightIncrease

The table below shows the effect of changing temperature (temp.) for a reversible reaction

Exothermic reactionEndothermic reactionShift in position of equilibriumReaction yield
Increase temp$—$LeftDecrease
Decrease temp$—$RightIncrease
$—$Increase tempRightIncrease
$—$Decrease tempLeftDecrease

The table below shows the effect of changing pressure for a reversible reaction

No. of moles of gaseous reactions $>$ no. of moles of gaseous productsNo. of moles of gaseous reactions $<$ no. of moles of gaseous productsShift in position of equilibriumReaction yield
Increase pressure$—$RightIncrease
Decrease pressure$—$LeftDecrease
$—$Increase pressureLeftDecrease
$—$Decrease pressureRightIncrease

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