Reversible reactions and dynamic equilibrium: WASSCE Chemistry

Most reactions you have met so far go one way: once wood has burned, it cannot "unburn". But many reactions can run both forwards and backwards. In a closed container they settle into a balance where both directions continue at the same rate. Understanding this dynamic equilibrium is the key to making ammonia, sulphuric acid and many other industrial chemicals efficiently.

Learning objectives

By the end of this lesson you should be able to:

distinguish reversible from irreversible reactions, with examples
explain what is meant by dynamic equilibrium
state the characteristics of a system at equilibrium
interpret graphs of concentration and rate against time as equilibrium is reached
describe simple reversible reactions that show colour changes

Reversible and irreversible reactions

Definition

A reversible reaction is one that can go in both directions: the products can react to re-form the reactants. It is shown by the sign ⇌.

Irreversible (one way)	Reversible
Burning charcoal, cooking an egg, rusting	Heating hydrated copper(II) sulphate
Neutralisation of a strong acid by a strong alkali	Thermal decomposition of ammonium chloride
Most precipitation reactions	Haber process (ammonia), Contact process (SO₃)

Examples of reversible reactions:

Hydrated copper(II) sulphate:

\text{CuSO}_4\cdot5\text{H}_2\text{O(s)} \rightleftharpoons \text{CuSO}_4\text{(s)} + 5\text{H}_2\text{O(l)}

\begin{gathered}\text{CuSO}_4\cdot5\text{H}_2\text{O(s)} \\[6pt] \rightleftharpoons \text{CuSO}_4\text{(s)} + 5\text{H}_2\text{O(l)}\end{gathered}

Heating drives off water: blue → white. Adding water reverses it: white → blue, with heat given out.

Ammonium chloride:

\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}

\begin{gathered}\text{NH}_4\text{Cl(s)} \\[6pt] \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}\end{gathered}

On heating it decomposes; the gases recombine as white solid on the cool upper part of the test tube.

Making ammonia:

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

\begin{gathered}\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \\[6pt] \rightleftharpoons 2\text{NH}_3\text{(g)}\end{gathered}

Dynamic equilibrium

When a reversible reaction takes place in a closed system (nothing can enter or leave):

At first, only the forward reaction occurs, quickly.
As products build up, the backward reaction starts and speeds up, while the forward reaction slows (reactants are used up).
Eventually the forward and backward rates become equal. From then on, the concentrations stay constant.

Concentration against time: reactant concentration falls and product concentration rises until both become constant at equilibrium

Figure 1. Concentrations level off when equilibrium is reached. WASSCE Tutor diagram.

Definition

Dynamic equilibrium is the state of a reversible reaction in a closed system in which the rate of the forward reaction equals the rate of the backward reaction, so the concentrations of reactants and products remain constant.

It is called dynamic because both reactions are still happening; they simply cancel each other out. Think of someone walking up a downward-moving escalator at the same speed: they stay in one place, but are still moving.

Characteristics of a system at equilibrium

Characteristic	Meaning
Closed system	Equilibrium can be set up only if no substance escapes
Rates equal	Forward rate = backward rate
Concentrations constant	But not necessarily equal; there may be much more reactant than product, or the reverse
Dynamic	Particles keep reacting in both directions
Reached from either side	The same equilibrium mixture forms whether you start with reactants or with products (same conditions)
Macroscopic properties constant	Colour, pressure and density stop changing
Affected by conditions	Changing concentration, pressure or temperature shifts the position (next lesson)

Seeing equilibrium: colour changes

Worked example

Chromate and dichromate. In solution:

2\text{CrO}_4^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \rightleftharpoons \text{Cr}_2\text{O}_7^{2-}\text{(aq)} + \text{H}_2\text{O(l)}

\begin{gathered}2\text{CrO}_4^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \\[6pt] \rightleftharpoons \text{Cr}_2\text{O}_7^{2-}\text{(aq)} + \text{H}_2\text{O(l)}\end{gathered}

The left side (CrO₄²⁻) is yellow and the right side (Cr₂O₇²⁻) is orange.

Add acid (more H⁺): the solution turns orange, as the equilibrium shifts to the right.
Add alkali (which removes H⁺): it turns yellow, as the equilibrium shifts to the left.

The change can be reversed again and again, which shows the reaction is reversible.

Another example: brown nitrogen(IV) oxide, NO₂, is in equilibrium with colourless dinitrogen tetroxide, N₂O₄:

2\text{NO}_2\text{(g, brown)} \rightleftharpoons \text{N}_2\text{O}_4\text{(g, colourless)}

\begin{gathered}2\text{NO}_2\text{(g, brown)} \\[6pt] \rightleftharpoons \text{N}_2\text{O}_4\text{(g, colourless)}\end{gathered}

A sealed tube of the mixture is darker brown when hot and paler when cold.

In Sierra Leone

A bottle of soft drink is a closed system at equilibrium. Carbon(IV) oxide gas above the drink is in equilibrium with CO₂ dissolved in it. While the cap is on, the dissolving and escaping happen at equal rates and the drink stays fizzy. Opening the bottle breaks the closed system: gas escapes, the equilibrium can no longer hold, and the drink slowly goes flat.

Common mistake

At equilibrium the concentrations are constant, not equal. An equilibrium mixture may contain mostly products, mostly reactants, or anything between.

Key points

Reversible reactions (⇌) can go in both directions; examples: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O, NH₄Cl ⇌ NH₃ + HCl, N₂ + 3H₂ ⇌ 2NH₃.
In a closed system, a reversible reaction reaches dynamic equilibrium: forward rate = backward rate.
At equilibrium, concentrations are constant but not necessarily equal; reactions continue in both directions.
Equilibrium can be reached from either side.
Colour changes (chromate/dichromate, NO₂/N₂O₄) show equilibria shifting.

WASSCE examination focus

WAEC tests this

Expect to define dynamic equilibrium, to state its characteristics, to give examples of reversible reactions, and to sketch or interpret concentration–time or rate–time graphs. Explain why equilibrium is called "dynamic".

Quick check

What does the sign ⇌ mean in an equation?
Why is chemical equilibrium described as dynamic?
State two characteristics of a system at equilibrium.
What colour change is seen when acid is added to potassium chromate solution?

Answers:

The reaction is reversible: it can go forwards and backwards.
Both the forward and backward reactions continue to happen, at equal rates.
Any two: closed system; forward and backward rates equal; concentrations constant; reachable from either side; macroscopic properties constant.
Yellow to orange.

Reversible reactions and dynamic equilibrium