Chapter 21

• A system that minimises pH changes on addition of small amounts of an acid or a base

• Some undissociated acid (which is why WEAK acids must be used) • Its conjugate base

• By mixing a weak acid and a salt of the weak acid (e.g. ethanoic acid and sodium ethanoate) • By reacting excess weak acid with an alkali (e.g. ethanoic acid and sodium hydroxide)

• Method 1: the salt of the weak acid supplies the conjugate base (as it would break up in solution) • Method 2: • the acid and the alkali react to form the conjugate base (the salt formed would break up in solution) • an EXCESS of acid is used to supply the weak acid (otherwise it would all be neutralised)

• The weak acid dissociates, creating an equilibrium • If H+ is added, it reacts with the conjugate base to form the acid • Equilibrium shifts to the left, which removes H+ ions

• If alkali is added, OH- is added • The OH- ions react with H+ ions to form water • This decreases [H+], which shifts equilibrium to the right, and restores [H+]

• If too much alkali or acid is added, buffers cannot remove or replenish enough H+ ions

• [H+]= Ka × [HA] / [A-] • You can use the moles of HA and A- if it’s faster, since they would have the same volume

• For some, [HA] = [A-], so they would cancel out in the Ka expression, and [H+] = Ka, and pH = pKa • This is because they contain equal concentrations of a weak acid and its conjugate base (in the reaction method of making a buffer, you would add half as much as alkali as weak acid)

• [H+] = [A-] • (A- was added to the buffer, so this is no longer true)

• 7.35-7.45

• Using the carbonic acid-hydrogencarbonate buffer system • H2CO3 <-> H+ + HCO3-

• You plot volume of base added against pH • Twice as much base as needed is used • This causes the graph to be flat at first, have a steep rise, and then go flat again (s-shaped)

• At first, there is an excess of acid, so the pH slowly increases • Then there is vertical line called the vertical section, where pH increases rapidly on addition of a very small amount of base • The equivalence point is at the centre of the vertical section • At the end, there is an excess of base, so the pH slowly increases

• The point in a titration where the volume of one solution has reacted exactly with the volume of the other solution

• The graph starts at roughly pH 1 • It ends at pH 13 • The vertical section is from about pH 4 to 10 • The pH steadily increases at first

• The graph starts at roughly pH 1 • It ends at pH 11 • The vertical section is from about pH 4 to 7 • The pH steadily increases at first

• The graph starts at roughly pH 3 • It ends at pH 13 • The vertical section is from about pH 7 to 10 • The pH rapidly increases at first

• The graph starts at roughly pH 3 • It ends at pH 11 • There is no vertical section • The pH rapidly increases at first

• It makes the vertical region smaller

• They’re weak acids where the acid and conjugate base have different colours • When hydroxide ions are added, they react with H+ ions, which shifts equilibrium to the right, making the solution the colour of the conjugate base • If H+ ions are added, equilibrium shifts to the left, so the solution becomes the colour of the acid

• Each indicator has a pH range that it works for • Its range has to be within the vertical section for it to be able to show the end point of a titration

• When the indicator contains equal concentrations of acid and conjugate base; the point where the indicator changes colour

• Methyl orange: 3 to 4 • Any with a strong acid • Phenolphthalein: 9 to 10 • Any with a strong base