Chapter 21 - Buffers and Neutralisation

Question 1

define a buffer solution

Answer 1

“A buffer solution is a system that minimises pH change when small amounts of an acid or base are added”

Question 2

what do buffer solutions generally contain

Answer 2

• a weak acid, HA, this helps to remove alkali

- the weak acid’s conjugate base, A-, this helps to remove acid

Question 3

when might a buffer solution lose its buffering ability

Answer 3

when all of one component is used up

Question 4

what are the two main ways to prepare a buffer solution

Answer 4

• Mixing solutions of a weak acid and a salt of that weak acid
• partial neutralisation of a weak acid

Question 5

how can a buffer solution be formed by reacting a weak acid and its salt, and why does this create a buffer

Answer 5

• 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

Question 6

how can a buffer solution be formed by the partial neutralisation of weak acid, and why does this create a buffer

Answer 6

• 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

Question 7

why does a buffer provide two ‘reservoirs’ of the acid and conjugate base components but not many H+ ions

Answer 7

• 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

Question 8

how does a buffer solution work to minimise the effect of added acid

Answer 8

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

Question 9

how does a buffer solution work to minimise the effect of added alkali

Answer 9

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

Question 10

when is a buffer most effective and what can we say about its pH at this point

Answer 10

a buffer solution is most effective when
[HA(aq)] = [A-(aq)]

at this point
pKa = pH

Question 11

how can we calculate the concentrations of the conjugate base ions A- and the acid in a buffer solution

Answer 11

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

Question 12

how do these assumptions about buffers allow us to calculate pH

Answer 12

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

Question 13

what can we say about the pH of a buffer solution where [HA] = [A-]

Answer 13

pKa = pH

Question 14

why are buffer solutions in the body necessary

Answer 14

• the well-being of many living things rely on precise pH control to prevent enzymes denaturing

Question 15

what occurs if pH in the body becomes too low

Answer 15

If pH drops below 7.35, acidosis occurs, this includes:

• shortness of breath
• fatigue
• shock
• death (in extreme cases)

Question 16

what occurs if the pH in the body becomes too high

Answer 16

If pH rises above 7.45, alkalosis occurs, this includes:

• muscle spasms
• light-headedness
• nausia

Question 17

what is the buffer system generally used for blood pH control, what else can our body do to aid it

Answer 17

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

Question 18

what is the general shape of a pH curve where a base is being added to an acid

Answer 18

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

Question 19

how can this shape change depending on conditions

Answer 19

• 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

Question 20

what is the equivalence point of a titration

Answer 20

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

Question 21

what is the pH curve for a strong acid/strong base

Answer 21

• starts at a very low pH, sharp and high range vertical section, finishes at a very high pH

Question 22

what is the pH curve for a strong acid/weak base

Answer 22

starts at a very low pH, sharp but lower range vertical section, finishes at a slightly high pH

Question 23

what is the pH curve for a weak acid with a strong base and what is the special case about this

Answer 23

• 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

Question 24

what is the pH curve for a weak acid and a weak base

Answer 24

range of vertical section is small

limitations for indicators

Question 25

what are indicators

Answer 25

Indicators are (generally) a weak acid (HA) where the acid has a distinctly different colour from its conjugate base (A-)

Question 26

what occurs to an indicator if its added to an acidic solution and base is added

Answer 26

- 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

Question 27

what occurs when an indicator is added to a basic solution and acid is added

Answer 27

- 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

Question 28

how can we calculate the pH of the end point of an indicator

Answer 28

``` we know that at the end point of a titration, [HA] = [A-] as Ka = [H+] [A-] / [HA] this means [H+] = Ka so pH = pKa ```

Question 29

what is roughly the range for an indicator's end point and how can a suitable indicator be chosen

Answer 29

- 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

Question 30

what are the names of the species in the blood buffer system

Answer 30

Carbonic acid = H2CO3 Hydrogen Carbonate = HCO3- Hydronium ion = H3O+

Question 31

How would you conduct an experiment to create a pH curve for a titration of an acid and a base

Answer 31

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

Question 32

what to always remember when explaining forming a buffer by partial neutralisation of a weak acid

Answer 32

EXCESS WEAK ACID

Question 33

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

Answer 33

- it will remain constant - because both conc's decrease by the same amount - because weak acids are monobasic