acids, bases and buffers

what is a Bronsted-Lowry acid

a proton donor

Bronsted-Lowry base

proton acceptor

what are conjugate acid-base pairs

2 species that can be interconverted by transfer of a proton

what is an amphoteric substance

acts as an acid and a base

how is water an amphoteric substance

because it can accept a proton to form H30+ and act as a base or it can donate a proton to form OH- and act as an acid

conjugate acid

a base with a hydrogen ion added to it

conjugate base

an acid that has donated its hydrogen

monobasic acid

an acid with one H+ ions like HCL, CH3COOH

dibasic acid

an acid with 2 H+ ions eg H2SO4

example of a tribasic acid

H3PO4

metal carbonate + acid

salt + water + carbon dioxide

metal oxide +acid

salt + water

metal + acid

salt + hydrogen

alkali + acid

salt + water

how to calculate the acid dissociation constant, Ka

how to find pKa

-logKa

how to find Ka from pKa

what does the value of Ka indicate

the bigger the value, the stronger the acid

what does the value of pKa indicate

the smaller the value, the stronger the acid

how to find pH

how to find H+ concentration

how to find Kw, the ionic product of water

what is Kw at 298K

1.00 × 10^-14 mol² dm^-6

kw in pure water

[H+] = [OH-] so

Kw = [H+]²

what happens to Kw if you increase temperature

equilibrium will shift to the right and [H+] will increase meaning Kw will also increase —- pH of H20 decrease at temperature increases

what are 2 approximations for calculating pH of weal monobasic acids

approximation 1 : [H+(eqm)] = [A-(eqm)] —> assumption that all of the H+ comes from the acid and not the dissociation of water

approximation 2 : [HA(eqm)]= [HA(start)] —> assumption that the dissociation of weak acid is so strong that when eqm is reached the [HA] is unchanged

Ka for weak acids in aqueous solutions

what is a buffer solution

a system that minimises pH changed when small amounts of an acid or a base are added

what are the 2 components of a buffer solution

the weak acid, HA, removes added alkali

the conjugate base, A-, removes added acid

formation of buffer solution from a weak acid and its salt

mix a solution of ethanoic acid with a solution of one of its salts

when added to water, the salt will completely dissociate

formation of a buffer solution by partial neutralisation of the weak acid

add aq solution of an alkali like NaOH, to an excess of the weak acid

the weak acid is partially neutralised by the alkali forming the conjugate base, some weak acid is left unreacted

the solution now contains salt of the weak acid and any unreacted weak acid

how does a conjugate base remove added acid

[H+] increases

H+ ions react with conjugate base, A-

the eqm position shift to the left, removing most H+ ions

how does a weak acid remove added alkali

[OH-] increases

the small concentration of H+ ions react with the OH-

HA dissociates, shifting eqm position to the right, to restore most of the H+ ions lost

what does the pH of a buffer solution depend on

the pKa value of the weak acid

the ratio of conc of the weak acid and its conjugate base

how to find pH of a buffer solution

then sub into the Ph formula

what is the pH range of blood

7.35-7.45

what happens if the pH of blood falls/rises too much

falls below 7.35- develop acidosis

rises above 7.45 - develop alkalosis