Chapter 21 - Buffers + Neutralisation

buffer solution

a solution that minimises changes in pH on addition of small amounts of an acid or base

2 ways to make buffer system 1) a weak acid + its salt/conjugate base -ethanoic acid + sodium ethanoate

-ethanoic acid partially dissociates in aqueous solution so little ethanoate ions produced
CH₃COOH ⇌ H⁺ + CH₃COO^-

-the salt sodium ethanoate completely dissolves in aqueous solution so high concentration of ethanoate ions
CH₃COONa + aq ⇌ CH₃COO^- + Na⁺

-ethanoic acid is a weak acid so there is a low concentration of H⁺ ions

ACTION of buffer system 1 -addition of acid

-concentration of H⁺ increases

-equilibrium will shift to the left 

-H⁺ ions will react with CH₃COO^- ions to form CH₃COOH

-this decreases concentration of H⁺

2 ways to make buffer system 2) excess weak acid + strong alkali -ethanoic acid + sodium hydroxide

-ethanoic acid partially dissociates in aqueous solution so little ethanoate ions produced
CH₃COOH ⇌ H⁺ + CH₃COO^-

-sodium hydroxide completely dissociates in aqueous solution
NaOH + aq → Na⁺ + OH^- -Na⁺ then reacts with  CH₃COO^- to form salt 

-high concentration of CH₃COO^- as it is produced from 2 sources

• partial dissociation of CH₃COOH

• complete dissociation of CH₃COONa

ACTION of buffer system 2 -addition of alkali

-concentration of OH^- increases

-causes concentration of H⁺ to decrease as they react to form H₂O

-equilibrium will shift to the right

-causes more CH₃COOH to dissociate in aqueous solution

-concentration of H⁺ increases

carbonic acid-hydrogencarbonate buffer system

-carbonic acid (weak acid) + hydrogencarbonate ion (conjugate base) = type 1

H₂CO₃ ⇌ H⁺ + HCO₃^-

-addition of acid = increases H⁺ conc, equilibrium position shifts to the left, extra H⁺ in the blood reacts with HCO₃^- to form H₂CO₃, so conc of H⁺ decreases

-addition of alkali = increases OH^- conc so causes H⁺ conc to decrease, OH- reacts with H+ to form H2O, equilibrium shifts to the right, conc of H⁺ increases

pH of buffer solutions

type 1: [H+] = Ka x [HA] / [A-]

type 2: [HA] = n of HA in excess / new total v
[A-] = n of strong base / new total v

[H+] = Ka x [HA]_unreacted / [A-]