5.4 Buffers and Neutralisation

Question 1

What is a buffer solution ?

Answer 1

Is a system that minimises the pH change when small amounts of an acid or a base is added.

Question 2

What 2 components do buffer solutions contain ?

Answer 2

A weak acid and its conjugate base.

Question 3

What occurs when acids and alkalis are added to a buffer ?

Answer 3

2 components in buffer solution react and are eventually used up. As soon as one has all reacted, the solution loses its buffering ability towards an acid or alkali. As the buffer works the pH does change but only by a small amount.

Question 4

What happens when ethanoic acid is added to water ?

Answer 4

Ethnic acid partially dissociates and the amount of ethanol ions in solution is very small.
CH3COOH = H+ + CH3COO-

Question 5

What happens to ethanoate when added to water ?

Answer 5

Salt completely dissolves. Dissociation into ions is complete and so the salt is the source of the conjugate base component of the buffer solution.
CH3COONa + aq = CH3COO- + Na+

Question 6

How can a buffer solution be prepared by partial neutralisation of the weak acid ?

Answer 6

Can be prepared by adding an aqueous solution of an alkali, such as NaOH, to an excess of weak acid. The weak acid is partially neutralised by the alkali, forming the conjugate base. Some acid is left unreacted so resulting solution contains salt and unreacted weak acid.

Question 7

Where does the equilibrium position lie in ethanoic acid reaction ?

Answer 7

Lie well toward the ethanoic acid

Question 8

How does equilibrium position of ethanoic acid change when CH3COO- ions are added ?

Answer 8

Shifts position even further to the left, reducing the already small concentration of H+ ions, and leaving a solution containing mainly the 2 components CH3COOH and CH3COO-. They act as 2 reservoirs that are able to act independently to remove added acid and alkali. This is achieved by shifting the system to right or left.

Question 9

What control the pH in an acid buffer solution ?

Answer 9

The conjugate acid-base pair control the pH. Control of pH can also be explained using shifts in the equilibrium position.

Question 10

What happens to the equilibrium position when acid is added?

Answer 10

[H+] increases.
H+ ions react with the conjugate base A-.
Equilibrium position shifts to the left, removing most of the H+ ions, so conjugate base removes the added acid.
HA = H+ + A- (Swings to the left)

Question 11

What happens to equilibrium when alkali is added ?

Answer 11

[OH-] increases.
Small concentration of H+ ions react with OH- ions, forming water.
HA dissociates, shifting the equilibrium position to the right to restore most of the H+ ions, so the H+ ions in the acid removes the added alkali.
HA = H+ + A- (Swings to the right)

Question 12

When is a buffer solution most effective ?

Answer 12

Most effective at removing either added acid or alkali when there are equal concentrations of the weak acid and its conjugate base.

Question 13

What happens when [HA] = [A-] ?

Answer 13

-pH of buffer is same as pKa value of HA.
-Operating pH is typically over 2 units, centred at the pH of the pKa value.

Question 14

What occurs when you add acid to the carbonic acid-hydrogen carbonate buffer system ?

Answer 14

[H+] increases, and they react with the conjugate base HC03-. The equilibrium position shifts to the left, removing most of the H+ ions.

Question 15

What occurs when you add alkali to the carbonic acid-hydrogen carbonate buffer system ?

Answer 15

[OH-] increases, and this reacts with a small concentration of H+ ions.
H+ + OH- = H2O
H2CO3 dissociates, shifting the equilibrium position the the right to restore the H+ ions used up.

Question 16

What does a pH titration curve show ?

Answer 16

Shows when the base is added, the acid is in excess and pH increases slightly. The equivalence point is the vertical section that can be altered by a drop of base. This also shows when the pH increases slowly during addition of a small value of base.

Question 17

What is the equivalence point ?

Answer 17

It is the volume of one solution that exactly reacts with the volume of the other solution. The solutions have the reacted exactly with one another and the amounts match the stoichiometry of the reaction.

Question 18

What is an acid base indicator ?

Answer 18

It is a weak acid that has a different colour form its conjugate base.

Question 19

What is the end point of the acid-base indicator ?

Answer 19

This is when the indicator contains equal concentrations of HA and A- and the colour will be in between the 2 extreme colours.

Question 20

How does the equilibrium shift when indicator is added to a basic solution ?

Answer 20

OH- ions react with H+ ions in the indicator.
H+ + OH- = H2O
The weak acid dissociates, shifting the equilibrium to the right.
The colour changes first to orange at the end point and finally to yellow as equilibrium is shifted to the right.

Question 21

How does the equilibrium shift when the indicator is initially added to the basic solution and then the acid is added ?

Answer 21

H+ ions react with the conjugate base, causing the equilibrium to shift to the left. The colour chages, first to orange at the end point, then to red as position shifts to the left.

Question 22

How sensitive are indicators ?

Answer 22

Sensitivity depends on the indicator itself. Different indicators have different Ka values.

Question 23

What is Ka equal to at the end point ?

Answer 23

Ka = [H+]
pKa = pH

Question 24

How do you choose an indicator for different titrations ?

Answer 24

Must have an end point that coincides with the equivalence point.
There is no suitable indicator for a weak acid-weak base titration, as there is no vertical section.