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-]

titration curve

plots varying pH values of a reaction mixture over time

equivalence point 

the volume of a solution that completely reacts with the volume of another solution 

how to find equivalence point

-find centre of vertical section (range / 2)

-draw vertical line going down to identify corresponding volume

STRONG acid - STRONG base curve (PLOT + DRAW)

-large vertical section between pH 3-11

-equivalence point usually at pH 7

STRONG acid - WEAK base curve (PLOT + DRAW)

-vertical section between pH 3-7

-equivalence point is below pH 7 as the salt produced is slightly acidic

WEAK acid - STRONG base curve (PLOT + DRAW)

-vertical section between pH 5-12

-equivalence point is above pH 7 as the salt produced is slightly basic

WEAK acid - WEAK base curve (PLOT + DRAW)

-no vertical section as the pH change is more gradual due to low concentrations used

-instead use pH meter

end point

when there are equal concentrations of the weak acid and conjugate base - action of the indicator = where the pH rapidly changes

different indicators for different reactions

-phenolphthalein = pink in alkali, colourless in acid
strong acid-strong base, weak acid-strong base

-methyl orange = yellow in alkali, red in acid
strong acid-weak base

cannot use any for weak acid-weak base

METHOD for obtaining pH curve with titration

-calibrate pH meter using buffers (distilled water)

-measure pH using pH meter

-add alkali in known small portions/regular intervals

-swirl mixture after each addition

-measure pH too

-repeat until alkali in excess

-add in smaller increments near endpoint