What is chemical equilibrium?

Q: What is chemical equilibrium?

Chemical Equilibrium is a state in a reversible reaction where the rate of forward reaction equals the rate of backward reaction, resulting in no net change in the concentrations of reactants and products over time.

Chemical Equilibrium — Chemistry Keypoint

Chemical Equilibrium is a state in a reversible reaction where the rate of forward reaction equals the rate of backward reaction, resulting in no net change in the concentrations of reactants and products over time.

Quick Summary

Chemical equilibrium occurs when forward and backward reaction rates are equal
Concentrations of reactants and products remain constant (not necessarily equal)
Equilibrium constant (Kc) shows the position of equilibrium at a given temperature
Le Chatelier’s principle predicts how equilibrium shifts when conditions change
Common in industrial processes like Haber process for ammonia production

Understanding Chemical Equilibrium

Many chemical reactions do not go to completion. Instead, they reach a point where both reactants and products exist together. This is called chemical equilibrium. Think of it like a busy Lagos market where people enter and leave at the same rate – the number inside stays constant even though movement continues.

In a reversible reaction, two opposite processes happen at once. The forward reaction converts reactants to products, while the backward reaction converts products back to reactants. At equilibrium, both processes continue but at equal rates, so the amounts of substances present do not change.

For example, when nitrogen and hydrogen gases combine to form ammonia in the Haber process:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The double arrow (⇌) shows this is a reversible reaction. At equilibrium, nitrogen and hydrogen still combine to form ammonia, while ammonia simultaneously breaks down to form nitrogen and hydrogen. The concentrations remain steady.

Characteristics of Chemical Equilibrium

1. Dynamic Nature: Equilibrium is not static. Reactions continue in both directions but at equal rates. Molecules constantly react, but the overall amounts stay the same.

2. Constant Concentrations: The concentrations of all substances remain constant at equilibrium. However, these concentrations are not necessarily equal. You might have more reactants than products, or vice versa.

3. Achievable from Either Direction: You can reach the same equilibrium state whether you start with reactants only, products only, or a mixture of both. The final concentrations will be identical.

4. Requires Closed System: For equilibrium to establish and maintain, the system must be closed. No substances should escape or be added from outside.

5. Constant Temperature: Temperature must remain steady. Changing temperature shifts the equilibrium position and changes the equilibrium constant value.

The Equilibrium Constant (Kc)

The equilibrium constant (Kc) is a number that describes the position of equilibrium. It tells us whether products or reactants are favoured at equilibrium.

For a general reversible reaction:

aA + bB ⇌ cC + dD

The equilibrium constant expression is:

Kc = [C]^c[D]^d / [A]^a[B]^b

Where square brackets [ ] represent concentration in mol/dm³, and the small letters (a, b, c, d) are the coefficients from the balanced equation.

Interpreting Kc Values:

Kc >> 1 (very large): Products are strongly favoured. Equilibrium lies far to the right. Most reactants convert to products.
Kc = 1: Neither products nor reactants are favoured. Similar amounts of both exist at equilibrium.
Kc << 1 (very small): Reactants are strongly favoured. Equilibrium lies far to the left. Very little product forms.

Important Points About Kc:

Kc is constant at a given temperature for a particular reaction
Kc changes only when temperature changes
Kc has no units in WAEC exams (though it can have units in advanced study)
The value of Kc depends on how you write the equation (reversing the equation gives 1/Kc)

Le Chatelier’s Principle

Henri Le Chatelier discovered that when you disturb an equilibrium system, it shifts to oppose the change. This helps predict how equilibrium responds to changing conditions.

Effect of Concentration Changes:

Adding reactant: Equilibrium shifts right to make more product
Removing reactant: Equilibrium shifts left to make more reactant
Adding product: Equilibrium shifts left to make more reactant
Removing product: Equilibrium shifts right to make more product

Effect of Pressure Changes (gases only):

Increasing pressure: Equilibrium shifts to the side with fewer gas molecules
Decreasing pressure: Equilibrium shifts to the side with more gas molecules
If both sides have equal gas molecules, pressure change has no effect

Effect of Temperature Changes:

For exothermic reactions (release heat): Increasing temperature shifts equilibrium left (favours reactants). Decreasing temperature shifts right (favours products).
For endothermic reactions (absorb heat): Increasing temperature shifts equilibrium right (favours products). Decreasing temperature shifts left (favours reactants).

Effect of Catalysts:

Catalysts speed up both forward and backward reactions equally. They help reach equilibrium faster but do not change the position of equilibrium or the value of Kc.

Comparison of Different Factors Affecting Equilibrium

Factor	Effect on Equilibrium Position	Effect on Kc Value	Time to Reach Equilibrium
Concentration	Shifts position	No change	No change
Pressure (gases)	Shifts position	No change	No change
Temperature	Shifts position	Changes	No change
Catalyst	No change	No change	Decreases (faster)
Inert Gas (constant volume)	No change	No change	No change

Industrial Applications

1. Haber Process (Ammonia Production):

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ/mol

This exothermic reaction produces ammonia for fertilizers. Industrial conditions use high pressure (200 atm) to favour the product side (4 molecules become 2), moderate temperature (450°C) to balance speed and yield, and iron catalyst to speed up the reaction.

2. Contact Process (Sulphuric Acid Production):

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ/mol

Uses vanadium(V) oxide catalyst at moderate temperature and pressure to maximize SO₃ production.

Common Exam Mistakes

WAEC examiners frequently report these errors:

Confusing “equal rates” with “equal concentrations”: At equilibrium, reaction rates are equal, not concentrations. Students often write that concentrations are equal, which is wrong.
Saying reactions stop at equilibrium: Reactions continue at equilibrium, just at equal rates. Many students write that reactions stop.
Claiming catalysts shift equilibrium: Catalysts only speed up reaching equilibrium, they don’t change its position or Kc value.
Wrong Kc expressions: Students forget to raise concentrations to the power of their coefficients, or put products in the denominator instead of numerator.
Not specifying effect direction: When explaining Le Chatelier’s principle, students write “equilibrium shifts” without saying which direction (left or right).
Confusing ΔH and ΔG: At equilibrium, ΔG = 0 (not ΔH). ΔH tells if reaction is exothermic or endothermic.

Practice Questions

Multiple Choice Questions

1. In the reaction N₂O₄(g) ⇌ 2NO₂(g), what happens when pressure is increased?

Equilibrium shifts to produce more NO₂
Equilibrium shifts to produce more N₂O₄ ✓
No change in equilibrium position
The reaction stops completely

(Answer: b – Higher pressure favours the side with fewer gas molecules: 1 molecule of N₂O₄ vs 2 molecules of NO₂)

2. For the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), which statement is correct?

The concentrations of H₂, I₂, and HI are always equal
The rate of formation of HI equals the rate of decomposition of HI ✓
The reaction has stopped
Adding a catalyst increases the amount of HI produced

(Answer: b – At equilibrium, forward and backward rates are equal)

3. If Kc = 4.0 × 10⁻³ for a reaction, this indicates:

Products are strongly favoured
Reactants are strongly favoured ✓
Equal amounts of reactants and products exist
The reaction is very fast

(Answer: b – Kc << 1 means reactants are favoured)

4. For an exothermic reaction at equilibrium, increasing temperature will:

Increase the value of Kc
Decrease the value of Kc ✓
Not affect the value of Kc
Stop the reaction

(Answer: b – Heat is a product in exothermic reactions, so adding heat shifts equilibrium left, decreasing Kc)

Essay Questions

1. (a) Define chemical equilibrium. (2 marks)
(b) State three characteristics of a system in chemical equilibrium. (3 marks)
(c) For the reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ/mol
Explain how the equilibrium position would be affected by: (i) increasing pressure (ii) increasing temperature (iii) adding a catalyst. (6 marks)
(d) Write the expression for the equilibrium constant (Kc) for this reaction. (2 marks)

Tips: In part (c), clearly state the direction of shift (left or right) and explain why using Le Chatelier’s principle. Remember that catalysts don’t shift equilibrium position.

2. (a) Distinguish between a reversible reaction and an irreversible reaction. (4 marks)
(b) For the equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium constant Kc = 0.50 at a certain temperature.
(i) What does this value of Kc tell you about the position of equilibrium? (2 marks)
(ii) If the concentration of N₂ is 0.20 mol/dm³, H₂ is 0.30 mol/dm³, and NH₃ is 0.15 mol/dm³, show whether the system is at equilibrium. (5 marks)
(c) State Le Chatelier’s principle. (2 marks)

Tips: In part (b)(ii), calculate the reaction quotient Q using the same expression as Kc, then compare Q to Kc. If Q = Kc, system is at equilibrium.

3. (a) Explain what is meant by the term “dynamic equilibrium”. (3 marks)
(b) State four factors that affect the position of equilibrium in a chemical reaction. (4 marks)
(c) Give two reasons why a catalyst is used in industrial equilibrium processes even though it does not change the position of equilibrium. (4 marks)
(d) In the esterification reaction: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Suggest two ways to increase the yield of the ester. (2 marks)

Tips: For part (d), use Le Chatelier’s principle – consider removing products or adding reactants to shift equilibrium right.

Memory Aids

RICE Table: Remember how to solve equilibrium problems using:
Reaction (write the equation)
Initial (concentrations)
Change (in concentrations)
Equilibrium (final concentrations)

For Le Chatelier’s Principle: “If you stress it, it shifts to fix it” – The system shifts to oppose any change you make.

Kc Expression: “Products on Top” – Products in numerator, reactants in denominator.

Temperature Effects on Exothermic Reactions: “Heat is a Product” – For exothermic reactions, treat heat as a product. Adding it (increasing temperature) shifts left. Removing it shifts right.

Chemical Equilibrium