Buffers + neutralisation

OCR A

What is a buffer solution?

A buffer solution minimises changes in pH when small amounts of acid or alkali are added.

How is an acidic buffer made?

An acidic buffer is made from a weak acid and its salt (e.g. CH₃COOH/CH₃COONa) or by partial neutralisation of a weak acid with a strong alkali leaving excess weak acid.

What conjugate acid–base pair forms an ethanoic acid buffer?

The conjugate pair is CH₃COOH/CH₃COO⁻ linked by the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻.

How does an ethanoic acid buffer respond to added acid?

Added H⁺ is removed as the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ shifts to the left forming more CH₃COOH.

How does an ethanoic acid buffer respond to added alkali?

Added OH⁻ removes H⁺ so the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ shifts to the right to replace H⁺.

What equation is used to calculate the pH of a buffer?

Ka = [H⁺][A⁻]/[HA] rearranged to [H⁺] = Ka × [HA]/[A⁻].

How is buffer pH calculated when given amounts of acid and salt?

Convert concentrations to moles then substitute directly into [H⁺] = Ka × HA/A⁻ since both are in the same final volume.

How is buffer pH calculated after partial neutralisation?

Calculate moles of weak acid and alkali subtract reacted moles (1:1) then use remaining HA and formed A⁻ in [H⁺] = Ka × HA/A⁻.

What buffer system controls blood pH?

Blood pH is controlled by the carbonic acid–hydrogencarbonate buffer H₂CO₃/HCO₃⁻ in blood plasma.

What equilibrium controls blood pH?

H₂CO₃ ⇌ H⁺ + HCO₃⁻.

What is the normal pH range of blood?

Normal blood pH is maintained between 7.35 and 7.45.

Describe the pH curve for a strong acid–strong base titration.

Very low initial pH a steep vertical section and an equivalence point at pH 7.

Describe the pH curve for a weak acid–strong base titration.

Higher initial pH a buffer region an equivalence point above pH 7 and pH = pKa at half-neutralisation.

Describe the pH curve for a strong acid–weak base titration.

Low initial pH a steep section below pH 7 and an equivalence point below pH 7.

Describe the pH curve for a weak acid–weak base titration.

No steep vertical section and no suitable indicator.

What is an indicator?

An indicator is a weak acid whose acid form and conjugate base form have different colours.

How does an indicator change colour?

In acidic solution the equilibrium shifts towards the acid form and in alkaline solution it shifts towards the conjugate base form causing a colour change.

When is methyl orange used and what are its colours?

Methyl orange is used for strong acid titrations and changes from red in acidic solution to yellow in alkaline solution.

When is phenolphthalein used and what are its colours?

Phenolphthalein is used for strong base titrations and changes from colourless in acidic solution to pink in alkaline solution.

How is pH measured using a pH meter?

Calibrate the pH meter with a standard buffer rinse the probe with distilled water immerse the probe stir gently wait for a stable reading then record the pH.

What formula is used for dilution calculations?

New concentration = old concentration × old volume ÷ new volume.

What is the equivalence point in a titration?

The equivalence point is where stoichiometrically equivalent amounts of acid and base have reacted according to the balanced equation.

What is the end point in a titration?

The end point is the point at which the indicator changes colour and is chosen to be as close as possible to the equivalence point.