# Chapter 21: Buffers and Neutralisation (21.1-21.3) Flashcards

1
Q

what are buffer solutions used for?

A

to minimise changes in pH to a solution on addition of small amounts of an acid or base

2
Q

how does an acid buffer work

A

Acid buffer solutions contain two components, a weak acid HA and its conjugate base A-:
HA (aq) < – > H+ (aq) + A- (aq)

```When an acid (H+) is added, it reacts with the conjugate base (A-) to form HA and the equilibrium shifts to the left.
When alkali (OH-) is added, it reacts with the H+ to form H2O and the equilibrium shifts towards the right.p```
3
Q

preparing a buffer solution from a weak acid and a salt

A

a buffer solution can be prepared by mixing together a solution of a weak acid (eg CH3COOH) and a solution of a salt of that weak acid (eg CH3COONa).
Work out concentrations of acid and salt, assume the concentration of the salt is the same as the concentration of the conjugate base ( [A-] )

4
Q

preparing a buffer solution by partial neutralisation of a weak acid

A

a buffer solution can be prepared by mixing together an excess of a weak acid (eg CH3COOH) and a solution of an alkali (NaOH)

5
Q

Ka value for buffer solution

A

Ka = [H+][A-] / [HA]

6
Q

buffer system used in the blood

A

carbonic acid-hydrogencarbonate

The H2CO3/HCO3- buffer is present in blood plasma maintaining a pH between 7.35 and 7.45

7
Q

explain the shape of a pH titration curve

pH on y-axis, volume of base/acid added on x-axis

A

Initial shallow curve: pH changes slowly as there is still an excess of the original acid/base.

Vertical section: pH increases rapidly on addition of a very small amount of base/acid, acid and base concentrations are similar.

Equivalence point: the centre of the vertical section aka the point of neutralisation

Final shallow curve: pH changes slowly as it is now the base/acid that was added that is now in excess

8
Q

what is an indicator

A

An acid-base indicator is a weak acid, HA, that has a different colour from its conjugate base, A-.
The colour changes as a response to the shift in equillibrium

9
Q

at the end point of a titration

A
• There are equal concentrations of HA and A-

- The colour of the indicator is between the two extreme colours

10
Q

how to chose an indicator

A

Use an indicator where the vertical section goes completely through the range which the indicator would change colour.

No indicator is suitable for a weak acid/weak base titration

11
Q

What is the pH of a buffer where HA and A- have the same concentrations?

A

[H+] = Ka x [HA]/[A-] therefore [HA] and [A-] cancel so pH is equivalent to the pKa value and [H+] is equivalent to Ka

12
Q

How do you work out H+ in a buffer?

A

Ka x [HA]/[A-]