Buffers

Question 1

What is a buffer solution?

Answer 1

A system that minimises pH changes when small amounts of an acid or base are added.

Question 2

What are the 2 components of a buffer solution?

Answer 2

A weak acid (HA)

Its conjugate base (A-)

Question 3

What are the 2 ways to prepare weak acid buffer solutions?

Answer 3

Preparation from a weak acid and its salt.

Preparation by partial neutralisation of the weak acid.

Question 4

How do you prepare a buffer solution from a weak acid and it’s salt?

Answer 4

Mix a solution of weak acid (eg ethanoic acid) and one of its salts (eg CH3COONa).
When the acid is added to water, the acid partially dissociates. Weak acid component of the buffer.
CH3COOH (aq) H+(aq) + CH3COO- (aq)
Salt of the weak acid is the conjugate base part of the buffer. When added to water, they completely dissolve.
CH3COONa(s) + aq -> CH3COO-(aq) + Na+(aq)

Question 5

How do you prepare a buffer solution from partial neutralisation of the weak acid?

Answer 5

Adding an aqueous solution of alkali (eg NaOH) to an excess of weak acid. Weak acid partially neutralised by alkali forming the conjugate base. Some weak acid is left over unreacted. Resulting solution contains mixture of salt of the weak acid and any unreacted weak acid.

Question 6

How can a buffer act as 2 reservoirs to act independently to remove added acid or alkali?

Answer 6

HA(aq) H+(aq) + A-(aq)

Position of equilibrium can shift left to remove acid (H+) and right to remove alkali (OH-).

Question 7

How does changing the weak acid effect the buffer solution it produces?

Answer 7

It changes the pH ranges that buffer works at

Question 8

When is a buffer most effective?

Answer 8

When there are equal concentrations of weak acid and it’s conjugate base.
[HA] = [A-]

Question 9

When [HA] = [A-] what is true about the pH?

Answer 9

pH = pKa value of HA

Question 10

How many pH units is the operating pH usually over?

Answer 10

2, centred at the pH/pKa value (pH=pKa)

Question 11

How do you find the ratio of concentrations of the weak acid and it’s conjugate base?

Answer 11

[HA] / [A-]

Question 12

How do you work out Ka?

Answer 12

([H+] X [A-]) / [HA]

Question 13

How do you work out pH using the Ka expression?

Answer 13

Rearrange to [H+] = Ka X ([HA] / [A-])

pH = -log [H+(aq)]

Question 14

What pH does blood need to be at?

Answer 14

Between pH 7.35 and 7.45

Question 15

How is pH in the blood maintained?

Answer 15

A mixture of buffers such as the Carbonic acid - hydrogencarbonate buffer system.

Question 16

What happens if the pH of the blood drops below 7.35?

Answer 16

Called acidosis, causes shortness of breath, fatigue, maybe shock or death.

Question 17

What happens if the pH of blood rises above 7.45?

Answer 17

Called alkalosis, causes muscle spasms, lightheadedness and nausea.

Question 18

How does the carbonic acid - hydrogencarbonate buffer system work?
( H2CO3(aq) H+(aq) + HCO3-(aq) )

Answer 18

H+ added, react with conjugate base HCO3-(aq), equilibrium shifts left to remove most H+.
OH- added, small [H+] react to form H2O, H2CO3 dissociates shifting equilibrium right to replace H+ lost

Question 19

What are the 2 ways to measure changes in pH during an acid-base titration?

Answer 19

Using an indicator

pH meter

Question 20

What are the key features of a pH titration curve?

Answer 20

When the base is first added, the acid is in excess so pH rises very slightly. As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly.
pH increases quickly (vertically) only drops are needed for the whole vertical section.
After vertical section, pH only increases slowly again as base is in excess.

Question 21

How is the graph different if you add an acid to a base?

Answer 21

The shape of the curve is the same it just starts from a high pH and gets lower.

Question 22

What is the equivalence point?

Answer 22

The volume of one solution that exactly reacts with the volume of another solution.
The vertical section of the graph.

Question 23

What is the end point?

Answer 23

When the indicator contains equal concentrations of HA and A- and the colour will be in between the 2 extremes of colour.

Question 24

What is an acid-base indicator?

Answer 24

A weak acid, HA, that has a distinctly different colour from its conjugate base, A-.

Question 25

How do you know when methyl orange has reached the end point?

Answer 25

HA = red
A- = yellow
Therefore, the end point is orange

Question 26

What happens to the position of equilibrium of an indicator in acidic conditions?

Answer 26

Shifted towards the weak acid (HA), changing the colour as it does so.

Question 27

What happens to the position of equilibrium of an indicator in alkaline conditions?

Answer 27

Shifted towards the conjugate base (A-), changing the colour as it does so.

Question 28

Why are different indicators used?

Answer 28

They have different Ka values and change colour over different pH ranges.

Question 29

How do you work out Ka at the end point?

Answer 29

HA=A- therefore, Ka = [H+(aq)]

Question 30

How do you choose an indicator?

Answer 30

Must have a colour change that coincides with the vertical section of the graph.

Question 31

What indicator is used in strong acid - strong base titration?

Answer 31

Phenolphthalein / methyl orange / most

Question 32

What indicator is used in strong acid - weak base titration?

Answer 32

Methyl orange (work at lower pH’s)

Question 33

What indicator is used in weak acid - strong base titration?`

Answer 33

Phenolphthalein (work at higher pH’s)

Question 34

What indicator is used in weak acid - weak base titration?

Answer 34

None - no vertical section and even at it’s steepest, pH requires several cm cubed to pass through a typical pH indicator range of 2 pH units.