21.3 Neutralisation

Question 1

What are the 4 key parts of an acid-base titration pH curve?

Answer 1

1- Excess of acid
2- Verticle section
3- Equivalence point
4- Excess of base

Question 2

Describe what is occurring during the excess of acid part of the acid-base titration.

Answer 2

pH increases slowly as base is added

Question 3

Describe what is occurring during the verticle section part of the acid-base titration.

Answer 3

pH increase rapidly on addition of a small amount of base

Question 4

Describe what is occurring during the equivalence point part of the acid-base titration.

Answer 4

Centre of the verticle section. When both solutions have fully reacted with eachother

Question 5

Describe what is occurring during the excess of base part of the acid-base titration.

Answer 5

pH increases slowly as base is added in excess

Question 6

What is the equivalence point?

Answer 6

The volume of one solution that exactly reacts with the volume of the other.

Question 7

When a weak acid and weak base are titrated against eachother, why is there no vertical section?

Answer 7

Because the pH change is very gradual, as a result of buffer action that occurs throughout the reaction

Question 8

Why is the pH sometimes not 7 at equivalence?

Answer 8

Because when a weak acid/weak base is present, it only partially dissociates, so there are conjugate bases/acids present which increase/decrease the pH.

Question 9

What is an acid-base indicator?

Answer 9

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

Question 10

What happens to the indicator at the end-point of a titration?

Answer 10

The indicator contains equal concentrations of HA and A-, and the colour will be between 2 extreme colours.

Question 11

When does the end point occur?

Answer 11

Just before the equivalence point

Question 12

Why is the end point not the same as the equivalence point?

Answer 12

Endpoint is the observed point where the indicator changes colour, equivalence point is the theoretical point where moles of acid = moles of base

Question 13

What is the pH range of methyl orange?

Answer 13

3.1-4.4

Question 14

What is the pH range of phenolphthalein?

Answer 14

8.3-10.0

Question 15

How do you decide which indicator to use in an acid-base titration?

Answer 15

Indicator must have a colour change that coincides with the verticle section of the pH titration curve (runs through it)

Question 16

Why is no indicator suitable for a weak acid-weak base titration?

Answer 16

There is no verticle section, so no indicator can be used