Reversible Reactions

Definition of Reversible Reactions

Reversible reactions are chemical reactions where the reactants can form products, and those products can react to reform the original reactants. This means the reaction can proceed in both directions: forwards and backwards.

In chemical equations, reversible reactions are shown using the symbol 1CC1C, which indicates that both the forward and reverse reactions occur.

For example, when hydrogen gas reacts with iodine gas, hydrogen iodide is formed, but hydrogen iodide can also break down back into hydrogen and iodine:

$\text{H}_2(g) + \text{I}_2(g) \; \rightleftharpoons \; 2\text{HI}(g)$

Dynamic Equilibrium

In a reversible reaction, when the forward and reverse reactions occur at the same rate, the system is said to be at dynamic equilibrium. This means:

• The concentrations of reactants and products remain constant over time.
• Both forward and reverse reactions continue to happen, but there is no overall change in the amounts of substances.
• The equilibrium is dynamic, not static, because the reactions are still occurring continuously.

For example, in the reaction between nitrogen and hydrogen to form ammonia:

$\text{N}_2(g) + 3\text{H}_2(g) \; \rightleftharpoons \; 2\text{NH}_3(g)$

At equilibrium, ammonia is being made at the same rate as it breaks down into nitrogen and hydrogen.

For instance, if the forward reaction produces 5 moles of ammonia per minute and the reverse reaction breaks down 5 moles per minute, the total amount of ammonia remains constant, even though both reactions are ongoing.

Factors Affecting Reversible Reactions

According to Le Chatelier's Principle, if a system at equilibrium is disturbed by changing concentration, temperature, or pressure, the system shifts its equilibrium position to counteract the disturbance and restore balance.

Effect of Concentration

Changing the concentration of reactants or products shifts the equilibrium position:

• If the concentration of reactants increases, the equilibrium shifts to produce more products.
• If the concentration of products increases, the equilibrium shifts to produce more reactants.

This happens because the system tries to counteract the change and restore balance.

For example, if more hydrogen gas is added to the reaction:

$\text{N}_2(g) + 3\text{H}_2(g) \; \rightleftharpoons \; 2\text{NH}_3(g)$

The equilibrium will shift to the right to use up the extra hydrogen and produce more ammonia.

Effect of Temperature

Temperature changes affect the equilibrium position depending on whether the forward or reverse reaction is exothermic or endothermic:

• If temperature increases, the equilibrium shifts in the direction that absorbs heat (endothermic direction).
• If temperature decreases, the equilibrium shifts in the direction that releases heat (exothermic direction).

This is because the system tries to oppose the temperature change by favouring the reaction that counteracts it.

For example, in the reaction:

$\text{N}_2(g) + 3\text{H}_2(g) \; \rightleftharpoons \; 2\text{NH}_3(g) + \text{heat}$

The forward reaction is exothermic (releases heat). So, increasing temperature shifts equilibrium to the left (towards reactants), while decreasing temperature shifts it to the right (towards products).

Effect of Pressure

Pressure changes only affect reactions involving gases. The equilibrium shifts to the side with fewer or more gas molecules to reduce the pressure change:

• Increasing pressure shifts equilibrium towards the side with fewer gas molecules.
• Decreasing pressure shifts equilibrium towards the side with more gas molecules.

For example, in the ammonia synthesis reaction:

$\text{N}_2(g) + 3\text{H}_2(g) \; \rightleftharpoons \; 2\text{NH}_3(g)$

There are 4 moles of gas on the left (1 N92 + 3 H92) and 2 moles on the right (2 NH3). Increasing pressure shifts equilibrium to the right, producing more ammonia.

This is why industrial ammonia production uses high pressure to increase yield.

Example: For instance, if the forward reaction produces 5 moles of ammonia per minute and the reverse reaction breaks down 5 moles per minute, the total amount of ammonia remains constant, even though both reactions are ongoing.