Chapter 21 - Buffers and Neutralisation Flashcards
define a buffer solution
“A buffer solution is a system that minimises pH change when small amounts of an acid or base are added”
what do buffer solutions generally contain
- a weak acid, HA, this helps to remove alkali
- the weak acid’s conjugate base, A-, this helps to remove acid
when might a buffer solution lose its buffering ability
when all of one component is used up
what are the two main ways to prepare a buffer solution
- Mixing solutions of a weak acid and a salt of that weak acid
- partial neutralisation of a weak acid
how can a buffer solution be formed by reacting a weak acid and its salt, and why does this create a buffer
- mix a weak acid solution with a solution of a salt of that weak acid
e. g. ethanoic acid and sodium ethanoate - the weak acid only slightly dissociates in solution so doesn’t provide many conjugate base ions, but does provide a large amount of the weak acid component
- the salt of the weak acid fully dissociates in solution and provides a source of the conjugate base ions
how can a buffer solution be formed by the partial neutralisation of weak acid, and why does this create a buffer
- add an aqueous solution of an alkali to an excess of a weak acid
- the weak acid is partially neutralised by the alkali, forming its conjugate base
- some of the weak acid is left over, providing a reservoir of the weak acid component
why does a buffer provide two ‘reservoirs’ of the acid and conjugate base components but not many H+ ions
- in a weak acid the position of equilibrium lies heavily to the left because weak acids only partially dissociate
- when the conjugate base is added, the equilibrium position shifts even further left as it reacts with some of the H+ ions
- this leaves very few H+ ions but large amounts of acid and its conjugate base
how does a buffer solution work to minimise the effect of added acid
if H+ ions are added:
- [H+] increases
- more H+ ions react with the conjugate base A-
- this shifts the equilibrium position to the left, removing most of the H+ ions
how does a buffer solution work to minimise the effect of added alkali
if alkali is added:
- [OH-] increases
- the OH- ions react with the small amounts of H+ ions
- this decreases [H+]
- equilibrium shifts right
- more of the acid dissociates
- most of the H+ ions are restored
when is a buffer most effective and what can we say about its pH at this point
a buffer solution is most effective when
[HA(aq)] = [A-(aq)]
at this point
pKa = pH
how can we calculate the concentrations of the conjugate base ions A- and the acid in a buffer solution
assumption 1:
- the weak acid only very slightly dissociates so [HA] can be taken to be the same as the acid originally
assumption 2:
- the conjugate base ions almost entirely come from the salt so [A-] can be taken to be the same as the conc. of the salt solution
how do these assumptions about buffers allow us to calculate pH
Ka = [H+] [A-] / [HA]
- this means if we know [HA] and [A-] from the original conc.’s of the salt and acid solutions we can calculate [H+] then we can calculate pH
what can we say about the pH of a buffer solution where [HA] = [A-]
pKa = pH
why are buffer solutions in the body necessary
- the well-being of many living things rely on precise pH control to prevent enzymes denaturing
what occurs if pH in the body becomes too low
If pH drops below 7.35, acidosis occurs, this includes:
- shortness of breath
- fatigue
- shock
- death (in extreme cases)
what occurs if the pH in the body becomes too high
If pH rises above 7.45, alkalosis occurs, this includes:
- muscle spasms
- light-headedness
- nausia
what is the buffer system generally used for blood pH control, what else can our body do to aid it
H2CO3(aq) —-(REVERSIBLE)—> HCO3-(aq) + H+(aq)
- it works to control acid and alkali as every other buffer system does
- to prevent H2CO3 build up, our bodies can break it down into H2O and CO2, these can be exhaled
what is the general shape of a pH curve where a base is being added to an acid
1) an initial flat section, acid is in great excess, pH increases very slightly, it starts to increase more quickly as the vertical section approaches and acid is used up quicker
2) pH increases rapidly for a small amount of base added - this is the vertical section
3) after the vertical section the base is in excess, the pH rises slowly
how can this shape change depending on conditions
- if you are using strong/weak acids/bases it can change shape
- the shape is reversed if you are adding an acid to a base
what is the equivalence point of a titration
the equivalence point of a titration means that the solutions have been added such that they have exactly react given the stoichiometry of the reaction
what is the pH curve for a strong acid/strong base
- starts at a very low pH, sharp and high range vertical section, finishes at a very high pH
what is the pH curve for a strong acid/weak base
starts at a very low pH, sharp but lower range vertical section, finishes at a slightly high pH
what is the pH curve for a weak acid with a strong base and what is the special case about this
- it rises, drops off, then vertical section and flattens out again
- starts at a slightly low pH then rises sharply and finishes at a very high pH
- it has a weird shape because initially when the strong base is added to the weak acid it neutralises and pH rises, it also forms a buffer solution meaning the pH rises more slowly again
what is the pH curve for a weak acid and a weak base
range of vertical section is small
limitations for indicators
what are indicators
Indicators are (generally) a weak acid (HA) where the acid has a distinctly different colour from its conjugate base (A-)
what occurs to an indicator if its added to an acidic solution and base is added
- it starts mostly in its acid form (HA)
- as OH- is added, it reacts with the H+ ions
- this moves the equilibrium position to the right
- we see the dissociated (conjugate base) colour once the base is in excess
what occurs when an indicator is added to a basic solution and acid is added
- it starts mostly in its dissociated form
- as H+ is added, [H+] increases, H+ reacts more with the conjugate base
- equilibrium shifts left and more of the undissociated acid forms
- we see the undissociated colour once acid is in excess
how can we calculate the pH of the end point of an indicator
we know that at the end point of a titration, [HA] = [A-] as Ka = [H+] [A-] / [HA] this means [H+] = Ka so pH = pKa
what is roughly the range for an indicator’s end point and how can a suitable indicator be chosen
- most indicators have an end point range of about 2 pH units
- for an indicator to be suitable, its end point must coincide with the vertical section of the pH curve
what are the names of the species in the blood buffer system
Carbonic acid = H2CO3
Hydrogen Carbonate = HCO3-
Hydronium ion = H3O+
How would you conduct an experiment to create a pH curve for a titration of an acid and a base
1) add a known volume of the acid to a conical flask using a pipette
2) measure initial pH using a pH meter
3) add the base to a burette
4) add approx. 1cm^3 at a time and swirl after adding each amount, then record volume added and pH
5) repeat until pH starts to change more rapidly then only add dropwise
6) once pH starts to change more slowly again, add base at 1cm^3 again
7) continue until pH stops changing
what to always remember when explaining forming a buffer by partial neutralisation of a weak acid
EXCESS WEAK ACID
if a solid weak acid salt is added to a weak acid to make a buffer and the volume increases, what will happen to the pH
- it will remain constant
- because both conc’s decrease by the same amount
- because weak acids are monobasic
