Acid Base Theory

Question 1

Why is the ionisation state important?

Answer 1

It is important for the absorption of drugs.

Question 2

How does the pH in different body fluids influence the ionisation state of drugs?

Answer 2

• Drugs with basic side chains may be protonated and become positively charged
• Drugs with acidic side chains may be deprotonated and become negatively charged.

Question 3

Based on the Arrhenius theory define an acid.

Answer 3

Any substance which produces a hydrogen ion in aqueous solution i.e. a proton donor.

Question 4

Based on the Arrhenius theory define base.

Answer 4

Any substance which delivers a hydroxide ion in an aqueous solution i.e. a hydroxyl donor.

Question 5

What are the limitations of the Arrhenius theory?

Answer 5

• No consideration of solvent effects
• Ammonia is a base with no hydroxyl group

Question 6

Based on the bronsted-lowry theory define an acid.

Answer 6

Any substance that donates a H+

Question 7

Based on the bronsted-lowry theory define base.

Answer 7

Any substance that accepts a H+

Question 8

Why is water described as amphoteric?

Answer 8

It can act as both acid and base.

Question 9

Define acidity

Answer 9

A measure of the tendency of a compound to give up H+

Question 10

Define basicity

Answer 10

A measure of a compound’s affinity for a H+

Question 11

What does bronsted-lowry theory fail to explain?

Answer 11

• Acid-base behaviour in other solvents
• Proton-less compounds like AlCl3

Question 12

Based on the Lewis theory define acid.

Answer 12

Electron pair acceptor

Question 13

Based on the Lewis theory define base.

Answer 13

Electron pair donor

Question 14

Give the equation to calculate pH.

Answer 14

pH = -log[H3O+]

Question 15

Give the equation to calculate pOH.

Answer 15

pOH = -log[OH-]

Question 16

Describe aqueous solutions of weak acids.

Answer 16

• Weak acids only partially ionise in aqueous solutions
• Equilibrium is setup favouring reactants

Question 17

Give the equation for pKa

Answer 17

pKa = -logKa

Question 18

What is meant by the common ion effect?

Answer 18

The shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.

Question 19

Describe the presence of a common ion.

Answer 19

Suppresses the ionisation of a weak acid or a weak base.

Question 20

What is the effect of having a weak acid HA and its salt in solution?

Answer 20

• Ionisation of HA is suppressed by the presence of A-
• Hydrolysis of A- is suppressed by the presence of HA
• The acid and conjugate base may react with one another

Question 21

What happens when a strong base is added to a buffer?

Answer 21

• The weak acid HA will give up its H+ to transform the base into water and the conjugate base
• OH- is being consumed so pH only changes slightly.

Question 22

What happens if a strong acid is added to a buffer?

Answer 22

• The weak base will react with the H+ from the strong acid to form the weak acid.
• The H+ gets absorbed by the A- instead instead of reacting with water to form H3O+ so the pH changes only slightly

Question 23

What is a buffer solution?

Answer 23

A solution which has the ability to resist changes inn pH upon the addition of small amounts of either acid or base.

Question 24

What do buffers contain?

Answer 24

Either a weak acid and its salt or a weak base and its salt.

Question 25

How does a buffer work?

Answer 25

- At the pKa HA = A- so the system is able to absorb the addition of OH- or H+ - Adding OH- near the pH where HA = A- then HA releases H+ to offset the OH- added so ratio of HA to A- does not change much - Adding H+ the A- can absorb H+ to form HA

Question 26

State the Henderson-Hasselbalch equation.

Answer 26

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

Question 27

What is the goal of a buffer?

Answer 27

- To keep the pH of a solution within a narrow range.

Question 28

What is the importance of the ratio of [A-]/[HA]?

Answer 28

It influences the pH of a solution.

Question 29

What is the importance of the concentrations of [A-] [HA]?

Answer 29

Controls the effectiveness of a buffer.

Question 30

Describe the effect of more A- and HA molecules.

Answer 30

- The less of an effect addition of a strong acid or base will have on the pH of a system

Question 31

The amount of strong base required to change the pH by one unit depends on what?

Answer 31

- The concentration of the components. - Higher concentration gives a higher beta.

Question 32

What happens to blood pH when you hyperventilate?

Answer 32

If blood pH drops due to metabolic production of H+ then [H2CO3] increases by protonation of HCO3-, H2CO3 rapidly loses water to form CO2(aq) which is expelled as CO2(g).

Question 33

What happens to blood pH when you hypoventilate?

Answer 33

If the blood pH rises [HCO3-] increases by deprotonation of H2CO3 then breathing changes and CO2(g) is converted to CO2(aq) and then to H2CO3 in the capillaries in the lungs.

Question 34

What is meant by the ionisation of drugs?

Answer 34

Protonation or deprotonation resulting in charged molecules.

Question 35

The acidity or basicity of a compound plays a major role in controlling what?

Answer 35

- Absorption and transport to site of action - Solubility, bioavailability, absorption and cell penetration, plasma binding, volume of distribution - Binding of a compound at its site of action - Un-ionised form involved in hydrogen bonding - Ionised form influences strength of salt bridges or H bonds - Elimination of compound - Biliary and renal excretion - CYP P450 metabolism

Question 36

Which form of the drug diffuses into the bloodstream?

Answer 36

The unionised form

Question 37

What is the strongest acid you can have in an aqueous solution?

Answer 37

H3O+

Question 38

What is the strongest base you can have in aqueous solution?

Answer 38

OH-

Question 39

What is meant by levelling effect?

Answer 39

- The effect of solvent on the property of acids and bases - Strength of strong acid is levelled by the basicity of the solvent - Strength of a strong base is levelled by the acidity of the solvent

Question 40

Give the equation to find pKa.

Answer 40

pKa = pH - log[A-]/[HA] therefore, [base]/[acid] = 10^(pH-pKa)