Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 9.9 Dynamic equilibrium-Study Notes - New Syllabus

9.9 Dynamic Equilibrium

(i): Equality of Forward and Backward Reaction Rates

Many chemical reactions are reversible, meaning they can proceed in both the forward and backward directions. In a closed system, such reactions can reach a state known as dynamic equilibrium, where both reactions continue to occur but with no overall change in the system.

Dynamic Equilibrium (Rate Perspective)

At dynamic equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.

Initially, when reactants are present and products are absent, only the forward reaction occurs at a significant rate. As products begin to form, the backward reaction starts to occur. Over time:

• The rate of the forward reaction decreases as reactant concentration decreases.
• The rate of the backward reaction increases as product concentration increases.
• Eventually, both rates become equal, establishing dynamic equilibrium.

At this point, reactions are still occurring in both directions, but there is no net change in the amounts of reactants and products because they are being converted at equal rates. This is why the equilibrium is described as dynamic rather than static.

Illustration with a Reversible Reaction

\( \mathrm{N_2(g) + 3H_2(g) \ \rightleftharpoons \ 2NH_3(g)} \)

In this system:

• The forward reaction forms \( \mathrm{NH_3} \) from \( \mathrm{N_2} \) and \( \mathrm{H_2} \).
• The backward reaction decomposes \( \mathrm{NH_3} \) back into \( \mathrm{N_2} \) and \( \mathrm{H_2} \).
• At equilibrium, both processes occur at equal rates, so there is no overall change in composition.

Key Features of Equal Reaction Rates

• The equality of rates means there is no net change in the system at equilibrium.
• Both forward and backward reactions continue simultaneously.
• The system must be closed to prevent loss of reactants or products.
• The dynamic nature arises because particles are continuously reacting, even though macroscopic properties remain unchanged.

 (ii): Constant Concentrations of Reactants and Products

In a reversible reaction at dynamic equilibrium, the concentrations of reactants and products remain constant over time. This does not mean the reactions have stopped, but rather that the system has reached a balance due to equal rates of opposing reactions.

Constant Concentrations at Equilibrium

At dynamic equilibrium, the concentrations of reactants and products remain constant because they are being formed and consumed at equal rates.

As the reaction proceeds towards equilibrium:

• The concentration of reactants decreases as they are converted into products.
• The concentration of products increases as they are formed.
• Eventually, a point is reached where both concentrations stop changing.

This occurs because:

• The rate of the forward reaction equals the rate of the backward reaction.
• The amount of reactant used per unit time equals the amount regenerated.
• The amount of product formed per unit time equals the amount decomposed.

Therefore, although particles continue to react, there is no overall change in concentration, making the equilibrium appear constant on a macroscopic scale.

Illustration with a Reversible Reaction

\( \mathrm{N_2(g) + 3H_2(g) \ \rightleftharpoons \ 2NH_3(g)} \)

In this system:

• The concentrations of \( \mathrm{N_2} \), \( \mathrm{H_2} \), and \( \mathrm{NH_3} \) become constant at equilibrium.
• However, the forward and backward reactions continue to occur simultaneously.
• The constant concentrations are a direct result of equal reaction rates.

Key Features of Constant Concentrations

• Concentrations remain constant, but not necessarily equal to each other.
• The system must be closed to maintain constant concentrations.
• The equilibrium is dynamic because reactions continue at the particle level.
• Constant concentration is a consequence of equal forward and backward reaction rates.