Reversible Reaction

• In some chemical reactions, the products can react to form the original reactants.
• This kind of reaction is called a reversible reaction and is represented by the ⇌ symbol in chemical equations.
• The reaction going to the right is called the forward reaction.
• The reaction going to the left is called the reverse reaction.

1. Dehydration of hydrated copper(II) sulfate

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

• Forward reaction: when hydrated copper(II) sulfate is heated, it loses its water of crystallization and becomes white.
• Reverse reaction: when water is added to anhydrous copper(II) sulfate, the solid becomes hydrated copper(II) sulfate and turns blue.

• Forward reaction: when ammonium chloride is heated, it decomposes into ammonia and hydrogen chloride gas.
• Reverse reaction: the gases react to form ammonium chloride solid.

Dynamic Equilibrium

A reversible reaction can reach dynamic equilibrium when it occurs in a closed system such as a sealed container.

Conditions for dynamic equilibrium:

1. Forward and reverse reactions are both occurring simultaneously.
2. The rates of forward and reverse reaction are the same.
3. The concentrations of the reactants and products remain constant.

Shifting of Equilibrium

Position of equilibrium: The position of equilibrium is a measure of the concentrations of the reactants and products.

• Equilibrium shifts to the right: It means the concentration of the forward reaction products increases.
• Equilibrium shifts to the left: It means the concentration of the reverse reaction products (or forward reaction reactants) increases.

Factors that Affect the Equilibrium Position

1. Catalyst

• Using a catalyst does not affect the equilibrium position.
• A catalyst increases the rate for both the forward and reverse reaction, so there is no overall change in the equilibrium position.
• A catalyst will shorten the time it takes for the reaction to reach equilibrium.

• When there is an increase in temperature, the equilibrium shifts in the endothermic direction. (If the forward reaction is endothermic, the equilibrium shifts to the right and produce more forward reaction products.)
• When there is a decrease in temperature, the equilibrium shifts in the exothermic direction. (If the forward reaction is endothermic, the equilibrium shifts to the left and produce more reverse reaction products.)

• An increase in pressure shifts the equilibrium so that fewer moles of gases are produced.
• A decrease in pressure shifts the equilibrium so that more moles of gases are produced.

Frequently asked in IGCSE Chemistry are questions on how equilibrium is shifted by changes in different factors 👇 

Question 1

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride. Iodine monochloride is dark brown in color while iodine trichloride is yellow.

ICl + Cl₂ ⇌ ICl₃ 

When the equilibrium mixture is heated, it becomes darker brown in color. Explain whether the backward reaction is exothermic or endothermic.

• An increase in temperature shifts the equilibrium in the endothermic direction.
• More iodine monochloride was produced, which is the product of the backward reaction.
• Therefore, the backward reaction is endothermic.

Question 2

Ammonia (NH₃) can be made by reacting nitrogen and hydrogen, in the presence of an iron catalyst, according to the equation

 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)  ∆H = –92 kJ/mol 

The reaction is reversible and the reaction mixture can, if left for long enough, reach a position of dynamic equilibrium. The graph shows how the percentage of ammonia at equilibrium depends on the temperature and pressure used. Use the graph to describe the effect of temperature and pressure on the percentage of ammonia at equilibrium.

• An increase in temperature decreases the percentage of ammonia at equilibrium. This is because...
  • The forward reaction is exothermic since ∆H is negative.
  • An increase in temperature shifts equilibrium in the endothermic direction.
  • The equilibrium shifts to the left to produce more of the reverse reaction products.
  • Therefore, percentage of ammonia at equilibrium decreases as temperature increases.
• An increases in pressure increases the percentage of ammonia at equilibrium. This is because...
  • The reverse reaction produces twice as many moles of gas than the forward reaction.
  • An increase in pressure shifts equilibrium so that fewer moles of gas is produced.
  • The equilibrium shifts to the right to produce more of the forward reaction products.
  • Therefore, percentage of ammonia at equilibrium increases as pressure increases.

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