Acids and bases

what are the 3 main theories explaining acid base behaviour?

Arrhenius, Bronsted-Lowry, Lewis

give the main points of Arrhenius theory of acid base behaviour

acid: produces a H+ in (aq) (H3O+)

base: produces OH- in (aq)

doesn’t take solvent effects or bases with no OH- group into consideration eg, NH3

give the main points of Bronsted-Lowry theory of acid base behaviour

acid: donates a H+ in (aq)

base: accepts a H+ in (aq)

water can act as an acid or a base in this theory

give the main points of Lewis theory of acid base behaviour

acid: accepts a pair of electrons

base: donates a pair of electrons

give the equation used to calculate pH

pH = -log10 [H+]

what is the pKa of water?

16

how does pKa change as an acid gets stronger?

lowers

how does pKa change as a base gets stronger?

increases

give the equation to calculate Ka

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

give the equation to calculate pKa

-log10 Ka

when will a compound exist in a 50% ionised and 50% unionised state?

when pH = pKa

at what pH value compared to its pKa will an acid exist as 90, 99 and 99.9% ionised?

pKa + 1, pKa + 2 and pKa + 3

why does level of ionisation increase for an acid when you increase the pH value in comparison with its pKa?

an increase in pH = less H+ in solution so acid will release H+ more readily (ie be more ionised)

at what pH value compared to its pKa will a base exist as 90, 99 and 99.9% ionised?

pKa - 1, pKa - 2 and pKa - 3

how do drugs cross the phospholipid membrane from GI tract to blood?

in uncharged form, via passive/simple diffusion

why are basic drugs with pKa = 10 almost always fully ionised in the human body?

as will be 90% ionised at pH 9, 99% at pH 8 etc, and these pHs aren’t found in the body

list some factors that affect acid strength and explain overall why they do

electronegativity, size of atoms, resonance, hybridisation and inductive effects

determine the stability of the conjugate base formed when an acid gives up its proton

why is acetic acid a stronger acid than ethanol?

the conjugate base of acetic acid is stabilised by resonance

negative charge can be delocalised to stabilise the anion, making it a stronger acid (more readily loses its H+)

state two ways in which you can make a buffer solution

weak acid and its conjugate base/salt (eg CH3COOH + CH3COONa)

weak base and its conjugate acid/salt (eg NH3 + NH4Cl)

what is the effect of having a weak acid, HA, and its conjugate base, A-, in solution?

equilibrium is set up where the ionisation of HA is suppressed by the presence of A-, and the hydrolysis of A- is suppressed by the presence of HA - buffer set up

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

the weak base (A-) will react with the H+ added (instead of H+ reacting with water to form H3O+)

reduces base concentration so the pH decreases slightly

what happens if a strong base is added to a buffer?

the weak acid (HA) will give up its H+, transforming the base (OH-) into water

increases conjugate base (A-) concentration so the pH increases slightly

what is the Henderson-Hasselbach equation?

pH = pKa + log ([A-]/[HA])

what is buffer capacity?

the amount of acid or base a buffer solution can neutralise before its pH changes significantly

(or the ability of a buffer to resist change in pH)

how would you calculate buffer capacity?

n/delta pH

n = number of moles of acid or base per litre of buffer

delta pH = before and after addition

how do you decipher if a drug is an acid or a base?

look at its chemical structure (ie NOT pKa value)

what are antacids?

weakly basic drugs that work to neutralise excess acid in the stomach to relieve heartburn and indigestion

what is the common ion effect?

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

presence of common ion suppresses the ionisation of a weak acid or weak base

what is the importance of biological buffering?

pH needs to be controlled in living systems as large changes can affect enzyme functionality, cell membrane stability etc

give 2 examples of buffer systems in the body

phosphate and bicarbonate-carbonic acid

how does the phosphate biological buffer work?

two forms, a weak acid and a weak base which pick up and neutralise any strong acids or bases added

how does the carbonate-carbonic acid biological buffer work?

for maintaining a constant blood pH

carbonic acid/weak acid (H2CO3) - removes hydroxide ions (ie base) added to blood

bicarbonate ion/conjugate base (HCO3-) - removes any acidic substance which enters bloodstream

what happens to blood pH when you hyperventilate?

alkalosis - exhaling more CO2 so less H2CO3 formed so less H+ and increase in blood pH

what happens to blood pH when you hypoventilate?

acidosis - build up of CO2 (aq) leads to more H2CO3 so more H+ and blood pH decreases

how would you select the components of a buffer?

choose acid with pKa as close as possible to desired pH

calculate the ratio of base to acid using Henderson-Hasselbach equation (as now know the pKa and pH)

use at least 0.1M of the lower component

target both to be in range of 0.1-1.0M

[base]/[acid] in range 1:10 for ideal buffer capacity value

define ionisation

protonation or deprotonation resulting in charged molecules

name some things that the acidity or basicity of a compound plays a major role in controlling

absorption and transport to site of action

solubility, bioavailability, cell penetration, plasma binding

binding at site of action

elimination and biliary and renal excretion

CYP P450 metabolism

what is the approximate pH of the blood?

7.4

how does the pH of the duodenum and stomach change when fasting vs when fed?

increases by ~1.5

what is the levelling (or solvent) effect?

states that any acid stronger than H3O+ will react with water to form H3O+, making them all equally strong in aqueous solution

give the equation for calculating fraction of ionised acid or unionised base at a fixed pH

1/1+antilog (pKa-pH)

(same but pH-pKa for unionised acid or ionised base fraction)

how would you calculate % of unionised acid at fixed pH?

%acid = 100/1+10^(pH-pKa)

(for % ionised, 100-value)