Dynamic Equilibrium II

Learning Objective 7.2.A Explain the relationship between the direction in which a reversible reaction proceeds and the relative rates of the forward and reverse reactions.

Quick Notes

A reaction proceeds in the direction where the reaction rate is greater.
If the forward rate > reverse rate, then more products are forming.
If the reverse rate > forward rate, then more reactants are reforming.
Dynamic Equilibrium is reached when both rates are equal.

Full Notes

How Reactions Proceed: Forward vs. Reverse Rates

In a reversible reaction, both the forward and reverse processes happen at the same time:

Forward reaction: Reactants → Products
Reverse reaction: Products → Reactants

These two reactions compete – and the overall direction of change depends on which one is happening faster.

Comparing Rates: What Happens Before Equilibrium

If the forward reaction is faster, the system moves towards forming more products — there is a net conversion of reactants to products.
If the reverse reaction is faster, the system shifts back — there is a net conversion of products to reactants.

Over time, the system naturally shifts toward a balance, where the rates of both processes become equal.

Example: Establishing Equilibrium in the Formation of Hydrogen Iodide

When hydrogen gas (H₂) and iodine gas (I₂) are first mixed, they begin to react to form hydrogen iodide (HI) in a forward reaction:

H₂(g) + I₂(g) ⇌ 2HI(g)

AP Chemistry diagram showing the establishment of equilibrium in the H2 + I2 ⇌ 2HI reaction, with forward and reverse rates balancing over time.

At the start, the concentrations of H₂ and I₂ are high, so the forward reaction rate is much faster than the reverse. HI starts to form rapidly and there is a net formation of products from the forward reaction.

As more HI builds up, it begins to decompose back into H₂ and I₂ through the reverse reaction. This reverse rate gradually increases.

Eventually, the rates of the forward and reverse reactions become equal. At this point, the system reaches dynamic equilibrium. The concentrations of all species remain constant — not because the reaction has stopped, but because the forward and reverse processes are happening at the same rate.

Why It Matters

Understanding how the rates of the forward and reverse reactions affect the system helps us:

Predict which direction the reaction will move (see Topic 7.3).
Determine how close the system is to equilibrium.
Make informed decisions in industrial and laboratory settings to control yields and reaction efficiency (see Le Chatelier's Principle).

Summary

The relative rates of the forward and reverse reactions determine the direction of change in a reversible system.
If the forward rate is greater, the system produces more products.
If the reverse rate is greater, the system shifts back toward reactants.
At equilibrium, the two rates are equal, and concentrations remain steady.
This balance between rates is central to understanding how reversible reactions behave.