3.1.12.6 buffer action

what are buffer solutions?

solutions that maintain a constant pH despite the addition of acid, base or water

give the two types of buffers

• acidic buffers which have a pH of below 7

• basic buffers which have a pH of above 7

what does an acidic buffer typically consist of?

of a weak acid (HA) + one of its salts (A-)

describe one of the two ways of making an acidic buffer

• a weak acid is added to one of its salts → eg ethanoic acid + sodium ethanoate

  • a solution of the salt is added to a weak acid or

  • a solid salt is added to the weak acid

• that salt dissociates completely + the acid dissociates only slightly

• there is a large [HA] + a large [A-] so [H⁺] ≠ [A-]

describe a second way of making an acidic buffer

• partial neutralisation of a weak acid

• in this case, the salt is made as a product of the reaction + some of the excess acid will still be present

• a strong base is added to the weak acid → eg ethanoic acid + sodium hydroxide

• there is a large [HA] + a large [A-] (all of the OH- is reacted)

explain how an acidic buffer works if H⁺ is added

• if more H⁺ ions are added by adding some acid, they react with the A- ions

• the acid equilibrium moves to the left to decrease the moles of H⁺

• so [H⁺] remains unchanged + so does the pH

explain how an acidic buffer works if OH- is added

• if OH- ions are added by adding some alkali, they react immediately with H⁺, forming H₂O

• therefore acid equilibrium moves to the right + more acid is dissociated to replace H⁺

• so [H⁺] remains unchanged + so does the pH

what does a basic buffer typically consist of?

of a weak base + one of its salts

describe the two ways of making a basic buffer using the example of ammonia as the weak acid

• a solution containing ammonium chloride + ammonia would make a basic buffer

• the salt dissociates completely + the base dissociates only partially

• or partial neutralisation of a weak base by a strong acid eg ammonia (NH₃) + HCl

explain how an basic buffer works if H⁺ is added

• if H⁺ ions are added by adding some acid, they react immediately with the OH- ions

• therefore the equilibrium moves to the right to replace the OH-

• so [H⁺] remains unchanged + so does the pH

explain how an basic buffer works if OH- is added

• if more OH- ions are added by adding some alkali, they react immediately with the base ions (eg in this case NH₄⁺ ions)

• so equilibrium moves to the left to decrease the moles of OH- ions

• so [OH-] remains unchanged, so does [H⁺] + so does pH

applications of buffers - fill in the blanks

give the template for answering buffer ‘explain’ questions

describe the method of calculating the pH of an acidic buffer thats been made using route 1: using a weak acid + its salt

• rearrange the Ka expression to work out [H⁺]

  • remember moles can just be used rather than working out concentration

• use -log to calculate pH

describe the method of calculating the pH of an acidic buffer thats been made using route 2: using an excess weak acid + strong base

1. work out moles of HA

2. work out moles of OH- which is the moles of A-

3. work out leftover/excess moles of HA

4. rearrange Ka expression to work out [H⁺]

5. use -log to work out pH

outline the effect of adding a strong base (OH-) to a buffer

it moves equilibrium moves to the right which:

• decreases moles of HA

• increases moles A-

outline the effect of adding a strong acid (H⁺) to a buffer

it moves equilibrium to the left which:

• increases moles of HA

• decreases moles of A-

describe the method of working out the new pH when a strong acid or base is added to a buffer solution

1. work out moles of strong acid/base being added + so the moles of H⁺/OH⁻

2. work out the new moles of the salt (A⁻) → ie does it decrease/increase

3. work out the new moles of the acid (HA)

   → ie does it decrease/increase

4. work out [H⁺] by rearranging Ka expression

5. use -log to work out pH

using the info of the buffer solution given, work out the change in pH

change = - 0.04

using the info of the buffer solution given, work out the change in pH

change = + 0.03

give the 3 possible calculations involving mixtures of weak acids + strong bases, and state when each one is used

1. weak acid in excess + strong base → BUFFER CALCULATION

2. weak acid + strong base in excess → MIXTURE CALCULATION

3. half equivalence point reached (ie when half of moles of acid/base have been neutralised)→ pH = pKa