A2: Buffers and neutralisation

A2: Buffers and neutralisation

1. Buffer solutions

   • • A solution that minimises the change in pH on addition of small amounts of an acid or base. • Made when an acid is added to a solution of its conjugate base. • [HA] and [A-] are both high. • [H+] is very low.
   • Make an acidic buffer: 1. Weak acid + salt of the weak acid (i.e. CH3COONa) -CH3COOH ↔ H+ + CH3COO- -CH3COOH is a reservoir of undissociated acid. - CH3COONa fully dissociates, increases CH3COO-. 2. Weak acid (in excess) and strong base/alkali – partial neutralisation (more likely) -CH3COOH + NaOH → CH3COONa + H2O -Conjugate base made in situ.
   • Model answers • When an acid is added to a solution, the concentration of [H+] increases. The conjugate base reacts with the [H+]. • This causes more product to be formed so the equilibrium shifts to the left to minimise the changes. • When an alkali is added to a solution, the concentration of [OH-] increases. • [OH-] reacts with [H+]. • H+ + OH- → H2O. • Equilibrium shifts right.

2. Buffer solutions in the body

   • • Blood plasma needs to be maintained at a pH between 7.35 and 7.45. • Normal healthy blood has a pH around 7.40. • If the pH falls below 7.35, acidosis occurs, which causes fatigue, shortness of breath, shock or death. • If the pH rises above 7.45, alkalosis occurs, which causes muscle spasms, light-headedness and nausea.
   • • H2CO3 ↔ H+ + HCO3- Adding an acid - [H+] increases. - H+ ions react with the conjugate base (HCO3-). - EQM shifts to the left, removing H+ ions. Adding an alkali - [OH-] increases. - H+ + OH- → H2O. - H2CO3 dissociates so EQM shifts to the right, restoring H+ ions.

3. Neutralisation

   • • Equivalence point – the volume of one solution that exactly reacts with the volume of the other solution. It is the centre of the vertical section of the pH titration curve. • The end point contains equal concentrations of HA and A- and the colour will be between the two extreme colours. At the end point: • [HA] = [A-] • Ka = [H+(aq)] and pKa = pH.
   • Indicator colour changes model answers: Adding a basic solution containing OH- ions: - OH- ions react with H+ ions in the indicator (equation for H2O). - The weak acid dissociates, shifting the EQM to the right. - Colour change: orange to yellow. If methyl orange is added initially to a basic solution and acid is added: - H+ ions react with the conjugate base. - EQM shifts to the left. - Colour change: orange to red. A suitable indicator must have a colour change that coincides with the vertical section of the pH titration curve i.e. the end point and equivalence point would coincide.