Acid-Base equilibria

Question 1

What is an acid?

Answer 1

An acid is a substance that releases or provides H+ ions (proton donor).

Question 2

What is a base?

Answer 2

A base is a substance that removes or accepts H+ ions (proton acceptor).

Question 3

What is a strong acid?

Answer 3

One that almost totally dissociates into H+ ions and negative ions in solution in water (e.g HCl).

Question 4

What is a weak acid?

Answer 4

One that only partially dissociates into H+ ions and negative ions in solution in water. The free ions are in equilibrium with the undissociated acid molecule (e.g CH3COOH).

Question 5

What does concentrated mean?

Answer 5

There is a large amount of an acid or base dissolved in a small volume of water.

Question 6

What does dilute mean?

Answer 6

There is a small amount of acid or base dissolved in a large volume of water.

Question 7

What can be said about a substance which has a low pH?

Answer 7

It is a strong acid.

Question 8

What can be said about a substance which has a high pH?

Answer 8

It is a strong alkali.

Question 9

What can be said about a substance with pH 7?

Answer 9

It is neutral.

Question 10

What is the general equation to find pH?

Answer 10

pH= -log10[H+]

Question 11

What is the general equation to find [H+]?

Answer 11

[H+]= 10^-pH

Question 12

What is Ka?

Answer 12

The acid dissociation constant.

Question 13

How can you calculate Ka?

Answer 13

Ka = [H+][A-] / [HA]

Question 14

What is the Ka value of a weak acid?

Answer 14

Ka is low.

Question 15

What is the Ka value of a strong acid?

Answer 15

Ka is high.

Question 16

How can you calculate pKa?

Answer 16

pKa= -log10(Ka)

Question 17

How can we calculate the pH of water?

Answer 17

1.Kc = [H+][OH-] / [H2O] the amount of water that dissociates is so small we use Kw
2. Kw- the ionic product of water
Kw = [H+][OH-]
3. Kw= [H+]^2 as [H+]=[OH-] in pure water
4. Kw= [H+]^2 = 10^-14
5. [H+]= 10^-7 which is pH7

Question 18

What is [H+] for strong acids?

Answer 18

In strong acids, all the H+ ions are released from the acid molecule, therefore [H+]=[acid].

Question 19

What is [H+] for weak acids?

Answer 19

In weak acids, very little dissociation occurs, so the concentration of acid molecules is the same that we initially put in, therefore [H+]= √Ka x [acid].

Question 20

What is [H+] for a strong base?

Answer 20

In strong bases, the concentration of the base gives the concentration of OH- ions, so we rearrange Kw, therefore [H+]= Kw/[OH-].

Question 21

What is a buffer?

Answer 21

A buffer resists changes in pH as small amount of acid or alkali are added.

Question 22

What is a buffer typically made from?

Answer 22

• A weak acid (e.g CH3COOH)
• A salt of the same acid with a strong base (e.g CH3COONa)

Question 23

How does a buffer work?

Answer 23

In the buffer, there is a high concentration of the anion as the salt completely dissociates and the weak acid dissociates partly.

Question 24

What happens when acid is added to a buffer?

Answer 24

The amount of H+ is increased, and this causes the equilibrium to shift to the left, removing the H+ ions by reaction with the dissociated weak acid.

Question 25

What happens when alkali is added to a buffer?

Answer 25

The amount of OH- is increased, which removes some of the H+ ions present. This causes the equilibrium to shift to the right, producing H+ ions and the dissociated weak acid. The H+ ions replace those removed by reaction with the OH- ions.

Question 26

What are some uses for buffers?

Answer 26

-Using or storing enzymes to ensure pH remains at the optimum value -Storage of biological molecules, such as pharmaceuticals, which denature at the incorrect pH

Question 27

How do we calculate the pH of a buffer?

Answer 27

We need to know the Ka of the weak acid and the ratio of the concentrations of the acid and the salt. We use the expression Ka= [H+][A-] / [HA]

Question 28

What is the Henderson-Hasselbalch equation?

Answer 28

pH= pKa + log([salt] / [acid])

Question 29

What is an example of a buffer which maintains alkaline pH?

Answer 29

Ammonium chloride and ammonia solution.

Question 30

What salt will be produced from a strong acid and a strong alkali?

Answer 30

A neutral salt, such as NaCl.

Question 31

What salt will be produced from a strong acid and a weak alkali?

Answer 31

An acidic salt, such as NH4Cl.

Question 32

What salt will be produced from a weak acid and a strong alkali?

Answer 32

A basic salt, such as CH3COONa.

Question 33

Why is ammonium chloride an acidic salt?

Answer 33

It dissolves to produce free Cl- ions and NH4+ ions, which is a weak base. NH4+ dissolves further to NH3 and H+, releasing free H+ ions decreasing the pH to below 7.

Question 34

Why is sodium ethanoate a basic salt?

Answer 34

It dissolves to produce free Na+ ions and CH3COO- ions, which is a weak acid. CH3COO- reacts with and removes H+ from the water to make CH3COOH, increasing the pH to above 7.n

Question 35

What is the neutralisation reaction that occurs during acid-base titrations?

Answer 35

H+(aq) + OH-(aq) --> H2O(l)

Question 36

Describe a strong acid- strong base titration curve

Answer 36

The graph starts at pH 1, the value for 0.1moldm-3 of a strong acid. There is a slow, gradual increase in pH until there is a sudden gradual increase from pH 2-12 as the volume of base is around 25cm3. Then there is a slow gradual increase in pH as the last of the base is added, with the graph ending below pH 13.

Question 37

Describe a weak acid- strong base titration curve

Answer 37

The pH increases gradually to about 4, as base is added. When the volume of base is about half the volume of acid, the pH levels off as a salt is formed, acting as a buffer. The pH then gradually increases towards pH 7 before increasing vertically to pH 12. The pH then gradually increases upwards to between 13-14.

Question 38

Describe a strong acid- weak base titration curve

Answer 38

The pH up to 7 is the same as the strong acid- strong base titration curve. After pH 7, the line becomes smoother and then levels off at a volume of about 35-45cm3 due to the buffer effect. The pH then gradually increases to around 12.

Question 39

Why cant we analyse a weak acid- weak base curve sufficiently?

Answer 39

The graph lacks a clear vertical region, making it harder to study using an indicator in titration.

Question 40

How do we analyse a weak acid- weak base curve without an indicator?

Answer 40

A pH probe is used instead of an indicator to measure the pH throughout the titration. A plot of the data is then used to identify the equivalence point.

Question 41

How can we identify the equivalence point of a titration?

Answer 41

It is the vertical region of the curve, where the moles of alkali added equals the number of moles of acid in the original solution.

Question 42

What data can we obtain from titration curves?

Answer 42

Equivalence points pH of the salt formed pKa of the weak acid

Question 43

What is an indicator?

Answer 43

Substances which change colour as the pH changes.

Question 44

What is indicator made of?

Answer 44

A weak acid, with the original molecule and dissociated ions having different colours.

Question 45

How is indicator present in different solutions?

Answer 45

In acid, most of the indicator exists as the neutral form In alkali, most of the indicator exists as the anion

Question 46

What indicators can be used for strong acid- strong base titrations?

Answer 46

Phenolphthalein, bromothymol blue, litmus and methyl orange.

Question 47

What indicators can be used for weak acid- strong base titrations?

Answer 47

Phenolphthalein and bromothymol blue.

Question 48

What indicators can be used for strong acid- weak base titrations?

Answer 48

Methyl orange, litmus and bromothymol blue.