Buffer Solutions HL Only

Specification Reference R3.1.16

Quick Notes

A buffer resists changes in pH when small amounts of acid or base are added.
Buffers are made from:
- A weak acid and its conjugate base (acidic buffer), or
- A weak base and its conjugate acid (basic buffer).
Buffers work through equilibrium shifts that neutralize added H⁺ or OH⁻.
Strong acids or bases cannot form buffers – they fully dissociate and don’t establish reversible equilibria.

Full Notes

What is a Buffer?

A buffer solution maintains a relatively constant pH despite the addition of small amounts of acid or base.

It contains a weak acid or base and its conjugate in a reversible equilibrium. Because the concentrations of the acid/base pair are much higher than [H⁺], the equilibrium can shift to absorb added H⁺ or OH⁻, helping maintain a stable pH.

Acidic Buffers:
Made from a weak acid and its salt (that contains the acids conjugate base).

For example The weak acid ethanoic acid (CH₃COOH) and its salt sodium ethanoate (CH₃COONa).

IB HL Chemistry diagram showing acidic buffer prepared from CH3COOH(aq) and CH3COONa(aq) producing a mixture containing CH3COOH and CH3COO−

When added to a solution of the ethanoic acid, the CH₃COONa would dissociate and release CH₃COO⁻ ions, which is the conjugate base (A⁻) of the ethanoic acid.

Basic Buffers:
Made from a weak base and its salt (that contains the conjugate acid of the base).

For example The weak base ammonia (NH₃) and its salt ammonium chloride (NH₄Cl).

IB HL Chemistry diagram showing basic buffer prepared from NH3(aq) and NH4Cl(aq) producing a mixture containing NH3 and NH4+

When added to a solution of ammonia, the NH₄Cl would dissociate and release NH₄⁺ ions, which are the conjugate acid ions of the ammonia.

How Acidic Buffers Work

An equilibrium is established in the buffer system between HA, A⁻ and H⁺.

IB HL Chemistry equilibrium HA(aq) ⇌ H+(aq) + A−(aq) for acidic buffer

The concentration of HA and A⁻ in the mixture must be much greater than the concentration of H⁺. This ensures the position of equilibrium is sensitive to changes in H⁺ concentration change more than changes to HA and A⁻ concentration. Equilibrium position can shift to keep H⁺ ion concentration nearly constant.

Example: Ethanoic Acid/Sodium Ethanoate Buffer
CH₃COOH ⇌ H⁺ + CH₃COO⁻

When an acid (H⁺) is added:

IB HL Chemistry diagram showing A− reacting with added H+ to form HA in an acidic buffer

CH₃COO⁻ combines with added H⁺ to form CH₃COOH.
Equilibrium shifts left, reducing the increase in H⁺. [HA] increases and [A⁻] decreases.

When a base (OH⁻) is added:

IB HL Chemistry diagram showing added OH− reacting with H+; HA dissociates to replace H+ in an acidic buffer

Added OH⁻ reacts with H⁺ to form H₂O.
CH₃COOH dissociates more to replace lost H⁺.
Equilibrium shifts right, replacing H⁺ ions that reacted with the added OH⁻, resisting pH increase. [HA] decreases and [A⁻] increases.

Matt’s exam tip

Remember that the concentration of HA and A^- will change when H⁺ or OH^- ions are added. When H⁺ ions are added to the mixture - the moles of HA will increase by the same as the moles of H⁺ added and moles of A^- decrease by the same amount. When OH^- ions are added - the moles of HA will decrease by the same as moles of OH^- added and the moles of A^- increase by the same amount.

As long as there are significant amounts of HA and A⁻ present, this system can buffer against pH changes.

Why Weak Acids and Bases Are Needed

Weak acids and bases only partially dissociate, setting up an equilibrium with their conjugates. This equilibrium helps them resist changes in pH by reacting with added H⁺ or OH⁻.

Strong acids and bases fully dissociate, so they cannot maintain this balance and therefore do not act as buffers.

Summary

Buffers resist changes in pH when small amounts of acid or base are added.
They are made from weak acids with conjugate bases or weak bases with conjugate acids.
Equilibrium shifts allow added H⁺ or OH⁻ to be neutralised.
Strong acids and bases cannot act as buffers.