Buffers and Neutralisation

Question 1

What is a buffer solution

Answer 1

a system that minimizes pH changes when small amounts of an acid or a base are added.

Question 2

What does a buffer solution contain

Answer 2

a weak acid (removes added alkali), and a conjugate base (removes added acid)

Question 3

Why does a buffer solution only work for small volumes

Answer 3

because there are only a small amount of reserves and once all of the reserves have been reacted then the solution loses its buffering ability

Question 4

preparation of a buffer from a weak acid and its salt

Answer 4

e. g.CH3COOH and CH3COONa
1. CH3COOH -> CH3COO- H+
2. CH3COONa(s) + (aq) -> CH3COO- + Na+

Question 5

preparation of a buffer by partial neutralization of the weak acid

Answer 5

an alkali (e.g. NaOH) is added to an excess of a weak acid The weak acid is partially neutralized forming a conjugate base. Resulting solution contains a mixture of the salt and weak acid

Question 6

How does the conjugate base remove added acid

Answer 6

H+ ions increase and react with the conjugate base. Equilibrium shifts left removing ost of H+

Question 7

How does the weak acid remove added alkali

Answer 7

[OH-] increases, the small concentration of H+ ions react with OH- ions. HA dissociates shifting equilibrium to the right to restore most of the H+ ions.

Question 8

When is the buffer most effective

Answer 8

when there are equal concentrations of the weak acid and the conjugate base ([HA]=[A-]

Question 9

When [HA(aq)] = [A-(aq)] :

Answer 9

1. pH of the buffer solution is the same as the pKa value of HA
2. the operating pH is typically over about 2 pH units, centered at the pH value of the pKa value

Question 10

How do you calculate the pH of a buffer solution

Answer 10

1. [H+] = Ka x [HA]/[A-]

2. -log[H+]

Question 11

How do you calculate the pH of a buffer solution made from a weak acid and its salt

Answer 11

1. Calculate the amount in mol of the acid and base
2. calculate the concentrations
3. [H+] = [HA]/[A-]
4. -log[H+]

Question 12

Where is the buffer solution in the body

Answer 12

blood plasma controlling blood pH between 7.35-7.45 for optimum enzyme activity.

Question 13

What happens if blood pH falls below 7.35

Answer 13

develop a condition called acidosis which may cause fatigue, shortness of breath and in extreme cases death

Question 14

What happens if blood pH rises above 7.45

Answer 14

a condition called alkalosis develops which can cause muscle spasms, light-headedness and nausea

Question 15

Equation for the carbonic acid buffer system

Answer 15

H2CO3(aq) -> H+(aq) + HCO3-(aq)

Question 16

How do you calculate the concentration ratio of HCO3-/H2CO3 in healthy blood

Answer 16

1. [HCO3-]/[H2CO3] = ka/[H+]
2. convert pH into [H+] and pKa into ka
3. Calculate ratio

Question 17

What is the Henderson - Hasselbalch equation

Answer 17

pH= pka + log([A-]/[HA])

Question 18

What does a pH meter consist of

Answer 18

an electrode that is dipped into a solution, and connected to a meter that displays a pH meter usually recorded to 2 decimal places

Question 19

Describe a procedure to monitor the change of pH as an aq base is added to an acidic solution

Answer 19

1. using a pipette, add a measured volume of acid to a conical flask
2. Place the electrode of the pH meter into the flask
3. Add the base to the burette and add to the acid on the conical flask 1cm3 at a time
4. After each addition, swirl the contents. Record the pH and the total volume of base added
5. repeats steps 3 nd 4 until pH starts to change more rapidly. Then add the base dropwise until pH starts changing less rapidly.
6. Add base 1 cm3 at a time again until an excess has been added and the pH has been basic, with little change, for several additions.

Question 20

Describe the first section of a pH titration curve

Answer 20

Excess of acid - pH increases slightly as basic solution is added

Question 21

Describe the equivalence point of a pH titration curve

Answer 21

Equivalence point (center of the vertical section) - The volume of one solution that exactly reacts with the volume of the other solution. The solutions have then exactly reacted with one another and the amounts used matching the stoichiometry of the reaction.

Question 22

Describe the vertical section of a pH titration curve

Answer 22

pH increases rapidly on addition of a very small volume of base. Acid and base concentrations similar.

Question 23

Describe the final section of a pH titration curve

Answer 23

excess of base - pH increases slowly as basic solution is added

Question 24

pH titration curve shape of adding an acid to a base

Answer 24

The shape is the same just the other way around

Question 25

What is an acid-base indicator

Answer 25

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

Question 26

Acid-base indicator end point

Answer 26

indicator contains equal concentrations of HA and A- and the colour will be in between the 2 extreme colors

Question 27

What happens to the indicator when a basic solution is added

Answer 27

OH- react with H+ in the indicator. The weak acid dissociates, shifting equilibrium to the right and causing the colour to change with it e.g. methyl orange turns yellow.

Question 28

What happens to the indicator when an acidic solution is added

Answer 28

H+ ions react with the conjugate base shifting equilibrium left. e.g. methyl orange turns red for the weak acid

Question 29

At the end point:

Answer 29

1. ka = [H+]

2. pka=pH

Question 30

How do you choose which indicator to use

Answer 30

The indicator must have a colour change which coincides with the vertical section of the pH titration curve. Ideally the end point and equivalence point would coincide. However, this may not be possible and the end point may give a volume that is slightly different to the equivalence point, but any difference will be small.