CHAPTER 21 - BUFFERS AND NEUTRALISATION

Question 1

What is a buffer solution

Answer 1

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

Question 2

What is a buffer comprised of

Answer 2

Weak acid (HA) and conjugate base (A-)

Question 3

What are the two methods to prepare a buffer solution

Answer 3

Preparation from a weak acid and its salts

Preparation by partial neutralisation of the weak acid

Question 4

How is a buffer solution prepared from a weak acid and its salts

Answer 4

Mixing a solution of (eg.) ethanoic acid - CH3COOH - with one of its salts, (eg) sodium ethanoate CH3COONa

Ethanoic acid is the source of the weak ass component of the buffer solution

Salts of weak acids are ionic compounds, and provide a convenient source of the conjugate base. When added to water, the salt completely dissolves, dissociating into ions, so that the source of the conjugate base component of the buffer solution.

CH3COOH <–> H+ + CH3COO- (all aq)

CH3COONa(s) + Aq –> CH3COO-(aq) + Na+ (aq)

Question 5

How is preparation of a buffer by partial neutralisation of the weak acid prepared

Answer 5

Adding aqueous solution of alkali (eg. NaOH) to an excess of weak acid.

Weak acid is partially neutralised by alkali, forming conjugate base, but some of acid remains untreated, leaving a mixture of salt of weak acid and unreacted weak acid

Question 6

How does an ethanoic acid equilibrium act as two reservoirs to remove added alkali and acid

Answer 6

In ethanolic acid equilibrium, the equilibrium position lies well towards ethanolic acid.

When CH3COO- ions are added to CH3COOH, the equilibrium position shift, even further to the left, reducing the already small concentration of H+ (aq) ions, and leaving a solution containing, mainly the two components, CH3COOH and CH3COO-

CH3COOH and CH3COO- act as to reservoirs that are able to act independently to remove added acid and alkali. This is achieved by shifting the buffers equilibrium system either to the right or left.

Question 7

What is the equilibrium equation in a buffer solution of an conjugate acid-base pair

Answer 7

HA (aq) <—> H+ (aq) + A- (aq)

Question 8

How does a conjugate base remove added acid in a buffer solution?

Answer 8

[H+] increases
H+ ions react with conjugate base - A-
The equilibrium position shits to the left, remain most of the H+ ions

HA (aq) <—> H+(aq) + A- (aq)

Question 9

How does a weak acid remove added alkali in a buffer solution?

Answer 9

[OH-] increases
The small concentration of H+ ions reacts with the OH- ions
H+(aq) + OH-(aq) —> H2O (l)
HA dissociates, shifting equilibrium position to the right to restore most of the H+ ions

Question 10

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

Answer 10

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

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

pH of buffer solution is the same as the pKa value of HA

The operating pH is typically over about two pH units, centred at the pH of the pKa value

Question 11

How do you calculate Ka

Answer 11

Ka = [H+][A-] / [HA] (all aq apart form Ka obvs)

(Pg 334)

Question 12

What pH range does blood plasma need to be maintained between

Answer 12

7.35-7.45

Question 13

What is one example of a buffer solution in the body

Answer 13

Carbonic acid-hydrogen carbonate (H2CO3/HCO3-) system in blood plasma

Question 14

What happens if Blood pH falls below or rises above the desired pH range

Answer 14

Falls below:
acidosis, causes fatigue, shortness of breath, shock or death

Rises above:
Alkalosis, , causes muscle spasms, light-headedness and nausea

Question 15

CALCULATE Ka EXAMPLE QUESTIONS

Answer 15

DO SOME

Question 16

How is a build up of H2CO3 prevented

Answer 16

Converting it into CO2 which is then exhaled by lungs

Question 17

What is the Henderson-Hasselbalch equation

Answer 17

An alternative way of calculating the pH of a buffer solution that allows you to really see how pKa and the base/acid ratio control the pH

pH=pKa+ log [A-] / [HA]

Question 18

Describe an experiment using a pH meter/probe to see the effect of neutralisation

Answer 18

Add measured volume of acid to a conical flask using a pipette

Place the electrode of the pH meter in the flask

Add the aqueous base to the burette, and add to the acid in the conical flask, 1 cm³ at a time

After each addition, spell the contents. Record the pH and the total volume of the aqueous base added.

Repeat steps three and four, until the pH starts to change more rapidly. Then add the aqueous base drop wise for each reading until the pH changes less rapidly.

Now, add the aqueous base 1 cm³ at a time until an excess has been added, and the pH has been basic, with little change, for several additions

Or reverse - acid added to base

Question 19

What is the equivalence point

Answer 19

The volume of solution that EXACTLY reacts with the volume of another solution

Question 20

What is he endpoint of a titration

Answer 20

When the indicator contains equal concentrations of HA and A- and the colour will be between the two extreme colours

Question 21

Read about indicator suitability

Answer 21

fairly straight forward, along with pH graphs