CL 21- buffers and neutralisation Flashcards
what is a buffer solution?
definition
- a mixture that minimises pH changes on addition of small amounts of an acid or a base
what does a buffer solution contain?
- a weak acid
- a salt of the weak acid (a conjugate base)
dissociation of ethanoic acid
equation
CH₃COOH ₍ₐ₎ ⇌ CH₃COO⁻ ₍ₐ₎ + H⁺ ₍ₐ₎
dissociation of solid sodium ethanoate
equation
CH₃COONa ₍ₐ₎ ⇌ CH₃COO⁻ ₍ₐ₎ + Na⁺ ₍ₐ₎
how can you use ethanoic acid to form a buffer solution?
- ethanoic acid is the source of the weak acid component of the buffer solution
- when dissociated, a very small amount of CH₃COO⁻ is made
- CH₃COONa provides a convenient source of the conjugate base
- when CH₃COONa is added, it will produce more CH₃COO⁻ as seen in it’s dissociation
- this shifts the equilibrium of ethanoic acid to the left
HA ⇌ H⁺ + A⁻
how does a buffer solution control pH?
(a) when acid is added
use equation to illustrate
if some H⁺ is added
* H⁺ + A⁻ ⇌ HA
* added acid reacts with conjugate base, A, to produce more of the weak acid HA
* this shifts the equilbrium of the buffer solution to the left (as [H⁺] decreased)
HA ⇌ H⁺ + A⁻
how does a buffer solution control pH?
(b) when alkali is added
use equation to illustrate
if NaOH (eg) is added
* H⁺ + OH⁻ ⇌ H₂O
* added alkali reacts with H⁺ to make water
* therefore [H⁺] decreases, so equilbrium of buffer solution shifts to the right to replace H⁺ ions and oppose the change
Ka for buffer solution
calculation
Ka = [H⁺][A⁻]
……..[HA]
assumptions made for Ka of buffers
two assumptions
- [A⁻] comes from salt
- [HA] = initial conc of HA
explain how blood pH is controlled by the carbonic acid-hydrogencarbonate buffer system
H₂CO₃ ⇌ H⁺ + HCO₃⁻
on addition of an acid,
* [H⁺] increases
* H⁺ ions react with the conjugate base, HCO₃⁻
* equilbrium shifts to the left
on addition of an alkali,
* [OH⁻] increases
* small conc of H⁺ ions reacts with the OH⁻ ions
* H₂CO₃ dissociates and equilbrium shifts to the right
what is the equivalence point?
- the point in a titration at which the volume of one solution has reacted completely
general shape of a pH curve
- s-shaped
- vertical section length depends on strength
strong acid-strong base curve
features
- longest vertical section
- starts at 1-2 pH
- ends at 13-14 pH
- equivalence point = 7
weak acid-strong base curve
features
- starts at 4-5 pH
- ends at 13-14 pH
- equivalence point > 7
strong acid-weak base curve
- starts at 1-2 pH
- ends at 9-10 pH
- equivalence point < 7
weak base-weak acid curve
- no vertical section
- equivalence point = 7
- cant use an indicator
features of a good indicator
- change colour over a narrow pH range
- two definitive identifiable colour changes
- one of the colours should be intense
how to choose a suitable indicator
- choose an indicator which changes colour as close as possible to equivalence point
- pH range of indicator must lie within vertical portion of graoh
e.g strong acid-strong base reactions are easier to find indicators for, as they have such a large vertical section
but weak acid-weak base reactions can’t use indicators due to no vertical section
