Buffers and neutralisation

Question 1

Describe titration curve?

Answer 1

-show pH against volume of base added from a titration
-they show different combinations of weak and strong acids and bases

Question 2

Describe strong acid/ strong base titration curve?

Answer 2

-graph starts at pH 1 as there is excess strong acid
-graph ends around pH 13 as we now have excess strong base

Question 3

Describe strong acid/ weak base titration curve?

Answer 3

-graph starts at pH 1 as there is excess strong acid
-ends at around pH 9 as we now have excess weak base

Question 4

Describe weak acid/ strong base titration curve?

Answer 4

-graph starts at around pH 5 as there is excess weak acid
-ends at around pH 13 as we now have excess strong base

Question 5

Describe weak acid/ weak base titration curve?

Answer 5

-graph starts at around pH 5 as there is excess weak acid
-ends at around pH 9 as we now have excess weak base

Question 6

Describe the equivalence point in a titration curve?

Answer 6

The equivalence point or end point is the point in which the acid has been neutralised fully by the base. The sharp vertical rise shows a rapid change in pH. At this point [H+] =[OH-]

Question 7

Describe indicators and the most common types?

Answer 7

indicator can be used to determine the end point of a titration. A suitable indicator must change colour entirely within the vertical part of the titration curve. Two most common indicators: Methyl orange and phenolphthalein

Question 8

Describe methyl orange?

Answer 8

-colour change: pH 3- 4.5
-is red at low pH and yellow at high pH values
-It can be used for strong acid/ strong base and strong acid/weak base

Question 9

Describe phenolphthalein?

Answer 9

-colour change: pH 8.2- 10
-is colourless at low pH and pink at high pH values
-It can be used for weak acid/ strong base

Question 10

Why is no indicator suitable for weak acid/ weak base titrations?

Answer 10

-they have no sharp pH change so we have to use a pH meter
-no vertical section

Question 11

What is a buffer?

Answer 11

chemical that resists the change in pH when small amounts of acid or base are added

Question 12

What are the two types of buffer?

Answer 12

acidic and basic

Question 13

Describe acidic buffers?

Answer 13

Acidic buffers resist the change in pH in order to keep the solution below pH 7. They are made from a weak acid and its salt.

Question 14

Explain the acidic buffer using ethanoic acid (weak acid) and sodium ethanoate (it’s salt)?

Answer 14

In any buffer solution there are two equilibrium equations which co-exist in the same beaker.
1) CH3COOH(aq) (reversible arrow) CH3COO-(aq) + H+(aq)
equilibrium lies to the left as weak acids dissociate weakly
2) CH3COO-Na+(aq) (reversible arrow) CH3COO-(aq) + Na+(aq)
Salts dissociate strongly so equilibrium lies to the right.

Question 15

What happens when we add an acid (H+) to this acidic buffer using ethanoic acid (weak acid) and sodium ethanoate (it’s salt)?

Answer 15

• H+ ions react with the CH3COO- ions in solution
  -there is a high concentration of these from the salt so more CH3COOH is produced which means equilibrium shifts to the left

Question 16

What happens when we add a base (OH-) to this acidic buffer using ethanoic acid (weak acid) and sodium ethanoate (it’s salt)?

Answer 16

• The OH- ions react with the H+ ions in solution
  -There is a low concentration of H+ however they can be reproduced from a high concentration of CH3COOH to counteract the change. Equilibrium shifts to the right to replace the reacted H+ ions.

Question 17

How do you calculate the pH of a buffer?

Answer 17

need to know Ka value and the concentration of weak acid and its salt.

Question 18

Describe how buffers are needed in the blood?

Answer 18

blood pH must be maintained at pH 7.4- buffer is present in our blood to help maintain this pH

Question 19

Describe how buffers are needed in the blood?

Answer 19

blood pH must be maintained at pH 7.4- buffer is present in our blood to help maintain this pH

Question 20

What are the two equilibrium reactions that co-exist in the buffer of blood?

Answer 20

H2CO3(aq) (reversible arrow) H+(aq) + HCO3-(aq)
and
H2CO3(aq) (reversible arrow) H2O(l) + CO2(aq)

Question 21

Describe how the level of carbonic acid (H2CO3) is controlled?

Answer 21

It is controlled by respiration in your cells. When we breath out CO2 the level of carbonic acid reduces as equilibrium shifts to the right to attempt to replace them

Question 22

Describe how the level of hydrogen carbonate (HCO3-) is controlled?

Answer 22

excess is removed via kidneys