Reversible Reactions and Equilibrium

Reversible Reactions

A reversible reaction is one that can go in both directions — the products can react to form the original reactants again.

This is shown with the equilibrium symbol (⇌):

A + B ⇌ C + D

The forward reaction (left to right) produces C and D.

The backward (reverse) reaction (right to left) reforms A and B.

Example: Ammonium Chloride

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

When heated, ammonium chloride decomposes into ammonia and hydrogen chloride (forward — endothermic). When cooled, the gases recombine to form ammonium chloride (backward — exothermic).

Example: Hydrated Copper Sulfate

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

(blue) (white)

Heating blue hydrated copper sulfate drives off water, leaving white anhydrous copper sulfate (endothermic). Adding water to the white powder turns it blue again and releases heat (exothermic). This is used as a test for water.

Energy in Reversible Reactions

If the forward reaction is exothermic, the reverse reaction is endothermic by the same amount, and vice versa. The energy changes in each direction are equal and opposite.

Dynamic Equilibrium

When a reversible reaction takes place in a closed system (where no substances can enter or leave), eventually the rate of the forward reaction equals the rate of the backward reaction. This is called dynamic equilibrium.

At equilibrium:

• The forward and reverse reactions are both still happening (it is "dynamic" — not stopped)
• The rates of the forward and reverse reactions are equal
• The concentrations of reactants and products remain constant (but not necessarily equal)

Equilibrium can only be reached in a closed system — if products can escape, the reaction will continue in the forward direction.

Le Chatelier's Principle (Higher Tier)

Le Chatelier's principle states that if a system at equilibrium is subjected to a change in conditions, the position of equilibrium will shift to oppose the change.

The "position of equilibrium" describes whether the mixture contains mostly reactants or mostly products. If it shifts to the right, more products are formed; if it shifts to the left, more reactants are reformed.

Effect of Temperature

Consider: A + B ⇌ C + D (forward reaction is exothermic)

• Increase temperature → equilibrium shifts to the left (endothermic direction) to absorb the extra heat → more reactants, less product, yield decreases
• Decrease temperature → equilibrium shifts to the right (exothermic direction) to release more heat → more product, yield increases

Changing temperature changes both the position of equilibrium and the rate.

Effect of Pressure (Gaseous Reactions Only)

The equilibrium shifts towards the side with fewer moles of gas when pressure is increased.

Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Left side: 4 moles of gas (1 + 3). Right side: 2 moles of gas.

• Increase pressure → equilibrium shifts to the right (fewer gas moles) → more NH₃ produced
• Decrease pressure → equilibrium shifts to the left (more gas moles) → less NH₃

Effect of Concentration

• Increase concentration of a reactant → equilibrium shifts to the right → more product
• Increase concentration of a product → equilibrium shifts to the left → more reactant
• Decrease concentration of a product (e.g. by removing it) → equilibrium shifts to the right → more product

Effect of a Catalyst

A catalyst does NOT change the position of equilibrium. It speeds up both the forward and reverse reactions equally, so equilibrium is reached faster, but the amounts of reactants and products at equilibrium remain the same.

The Haber Process

The Haber process manufactures ammonia (NH₃) from nitrogen and hydrogen:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward reaction is exothermic)

Conditions Used

Factor	Condition	Reason
Temperature	450°C	Compromise: lower temp gives higher yield but too slow
Pressure	200 atm	High pressure favours the right (fewer gas moles), giving more NH₃
Catalyst	Iron	Speeds up reaching equilibrium (does not change yield)

The Compromise

• A lower temperature would give a higher yield of ammonia (exothermic forward reaction favoured), but the rate would be too slow to be economical
• A higher pressure gives a better yield, but very high pressures are expensive and dangerous
• So 450°C and 200 atm are a compromise between yield and rate

Any unreacted nitrogen and hydrogen are recycled back through the reactor.

Exam Tips

• Equilibrium only occurs in a closed system — always state this
• At equilibrium, reactions have not stopped — the rates of forward and reverse are equal
• For Le Chatelier's: state what change is made, which direction the equilibrium shifts, and why (to oppose the change)
• A catalyst reaches equilibrium faster but does NOT change the yield
• In the Haber process, explain the compromise between rate and yield for both temperature and pressure