21) Buffers and neutralisation

Question 1

Define buffer solution

Answer 1

a system that minimises pH changes on addition of small amounts of an acid or base

Question 2

What are the 2 key components of a buffer solution and their roles?

Answer 2

weak acid HA - removes added alkali

conjugate base A- - removes added acid

Question 3

When is a buffer solution’s buffeting ability lost?

Answer 3

as soon as 1 component is all used up

Question 4

Give 2 ways of preparing a buffer solution

Answer 4

• a weak acid and a solution of its salt (salts of weak acids are ionic compounds which when added to water completely dissolve to provide the conjugate base)
• partial neutralisation of the weak acid (an aqueous solution of an alkali is added to an excess of weak acid, forming the conjugate base - some weak acid is unreacted and left over)

Question 5

Describe the action of a buffer solution on addition of acid, H+(aq)

Answer 5

[H+] increases
H+ react with conjugate base A-
position of equilibrium shifts to the LHS, removing H+ ions
HA(aq) <=> H+(aq) + A-(aq)

Question 6

Describe the action of a buffer solution on addition of alkali, OH-(aq)

Answer 6

[OH-] increases
small conc. of H+ reacts with OH- forming water
HA dissociates, position of equilibrium shifts to the RHS to restore H+ ions
HA(aq) <=> H+(aq) + A-(aq)

Question 7

When is a buffer most effective at removing either acid or alkali?

Answer 7

when there are equal concentrations of the weak acid and its conjugate base

Question 8

What is significant about when [HA] = [A-] for a buffer solution?

Answer 8

Ka = [H+]
pKa = pH

Question 9

What is the typical operating pH range of a buffer?

Answer 9

2 pH units centred at the pH of the pKa value

Question 10

What does the pH of a buffer solution dependent upon?

Answer 10

pKa value of weak acid

ratio of [H+] and [A-]

Question 11

Give the key equation for the pH of a buffer solution

Answer 11

[H+] = Ka x ( [HA] / [A-] )

Question 12

For a buffer solution why is [H+] not equal to [A-]

Answer 12

because A-(aq) has been added as one of the components

Question 13

What is the pH range blood plasma needs to maintained at?

Answer 13

7.35 - 7.45

Question 14

What happens if the pH of blood plasma < 7.35?

Answer 14

acidosis

fatigue, shortness of breath, shock or death

Question 15

What happens if the pH of blood plasma > 7.45?

Answer 15

alkalosis

muscle spasms, light-headedness, nausea

Question 16

How is blood plasma pH controlled?

Answer 16

carbonic acid - hydrogencarbonate (H2CO3 / HCO3 -) buffer system

Question 17

The body produces far more _ (acidic or alkaline?) products / materials which the _ converts to the _?

Answer 17

acidic
conjugate base HCO3 -
weak acid H2CO3

Question 18

How does the body prevent the build-up of H2CO3?

Answer 18

by converting it to CO2 which is then exhaled by the lungs

Question 19

What is a pH meter?

Answer 19

an electrode that is dipped into a solution and connected to a meter which displays the pH reading (typically 2dp - more accurate than indicator paper)

Question 20

Give a 6 step method for using a pH meter and an alternative

Answer 20

1. add a measured volume of acid to conical flask with a pipette
2. place electrode in flask
3. add base (aq) to burette and add to conical flask 1cm3 at a time, after each addition swirl contents and record both pH and total volume added
4. when pH starts to change more rapidly, add base dropwise for each reading until pH changes less rapidly
5. add base 1cm3 at a time again until an excess has been added and the pH has been basic, with little change for several additions
6. plot a graph of pH against total vol. of base added
   alternative: attach pH meter to data logger and use a magnetic stirrer in flask - the base would be added slowly and the pH titration curve could be plotted automatically

Question 21

Give and describe the 3 key stages of a pH titration curve

Answer 21

excess of acid - pH increases slowly as basic sol. is added
vertical section - pH increases rapidly on addition of a very small amount of base
excess of base - pH increases slowly

Question 22

What are the axis for a pH titration curve?

Answer 22

x axis = volume of base added

y axis = pH

Question 23

Define equivalence point

Answer 23

the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
found halfway up the vertical section

Question 24

When adding an acid to a base - how is the pH titration curve affected?

Answer 24

the curve is flipped - goes from a high pH to a low pH

Question 25

What is an acid-base indicator?

Answer 25

a weak acid that has a distinctly different colour from its conjugate base

Question 26

Define end point

Answer 26

the point in a titration where the indicator changes colour; indicates when the reaction is just complete

Question 27

What will an indicator contain?

Answer 27

equal concentrations of HA and A-

Question 28

What will the colour of an indicator at end point be?

Answer 28

in between two extreme colours

Question 29

What varies between different indicators?

Answer 29

different Ka values

change colour over different pH ranges

Question 30

What 3 pairs of values are equal at end point?

Answer 30

[HA] = [A-]
Ka = [H+]
pKa = pH

Question 31

In a titration, you must use an indicator that has…?

Answer 31

a colour change which coincides with the vertical section of the pH titration curve
ideally end point and equivalence point would coincide

Question 32

Is it true that there is no indicator suitable for a weak acid - weak base titration? Why?

Answer 32

yes

there is no vertical section and pH requires several cm3 to pass through a typical pH indicator range (2 pH units)

Question 33

Give 2 examples of indicators

Answer 33

phenolphthalein

methyl orange