5.3 Acids, Bases and Buffers Flashcards
Brønsted–Lowry acid
a species that can donate a proton
HCl(aq) → H+(aq) + Cl-(aq)
When an acid is added to water, it releases H+ ions (protons) into solution
Brønsted–Lowry base
a species that can accept a proton
NH3 (aq) + H+(aq) → NH4+(aq)
common base
Common bases are metal oxides and metal hydroxides
e.g. MgO NaOH
Ammonia is also a base
monoprotic acids
Many acids are called monoprotic acids. This means that they only donate one mole of protons per mole of acid;
e.g. HCl, HNO3, CH3COOH
diprotic acids
Some acids are diprotic acids. This means that they can donate two moles of protons per mole of acid; e.g. H2SO4, HOOCCOOH.
H2SO4(aq) + 2H2O(l) -> 2H3O+(aq) + SO42-(aq
triprotic acids
Triprotic acids donate three moles of protons per mole of acid; e.g. H3PO4.
ammonia
Ammonia is a gas that dissolves in water to form a weak alkaline solution.
Dissolved NH3 reacts with water
NH3(aq) + H2O ⇌ NH4+(aq) + OH-(aq)
acts as a base because N in NH3 is accepting a proton
alkali
An alkali is any chemical compound that gives a solution of pH greater than 7.0 when dissolved in water.
Sodium hydroxide NaOH
Potassium hydroxide KOH
Sodium hydroxide Mg(OH)2
is a soluble base that releases OH- ions in aqueous solutions
conjugate acid-base pairs
Two species differing by H
conjugate acid
is a species formed from a Brønsted-Lowry base by the addition of a proton.
conjugate base
is a species formed from a Brønsted-Lowry acid by the loss of a proton
amphoteric compounds
is a molecule or ion that can react both as an acid as well as a base.
e.g. amino acids, which have amine and carboxylic acid groups, and self-ionisable compounds such as water
HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq)
base 1 acid 1
Kw
constant called the ionic product for water, Kw, is defined as:
Kw = [H+][OH–]
units= mol2 dm–6
what is meant by the strength of an acid?
the extent of dissociation
strong acids
A strong acid has a low pH (usually 0 or 1). This means that the concentration of H+ is high. This is because the acid is fully dissociated into its ions.
weak acid
A weak acid has a higher pH (but still less than 7). This means that the concentration of H+ is lower than for a strong acid. This is because the acid is not fully dissociated into its ions.
find pH of 250 cm3 of an aqueous solution containing 3.65 g hydrochloric acid
n(HCl)= 3.65g / (1 + 35.5) = 0.1 mols
conc(HCl)= 0.1 / (250/1000) = 0.4 mols dm-3
pH = –log10 [0.4]
pH=0.398
weak acid and Ka
HA(aq) + H2O(l) H3O+(aq) + A–(aq)
HA(aq) ⇌ H+(aq) + A–(aq)
Ka= [H+][A-]
[HA]
unit= mol dm–3
approximation 1
[H+(aq)] = [A−(aq)] [H+] = square root of Ka X [HA]
approximation 2
The equilibrium [HA] is smaller than the undissociated [HA]
The dissociation of weak acids is small you can assume [HA]start »_space; [H+] and you can neglect decrease in concentration of the HA dissociation
assume [HA]start»_space; [H+] and equilibrium [HA] = undissociated [HA]
limitations to approximation 1
At 25 C [H+] from dissociation of water = 10-7. If the pH > 6 then [H+] from the dissociation of water will be significant compared with the dissociation of HA.
Approximation breaks down for very weak acids or diluted solutions
limitations to approximation 2
This approximation holds for weak acids with small Ka values. It breaks down when [H+] becomes significant and there is a real difference between [HA]eqm and [HA]start – [H+]eqm
This approximation is not justified for stronger weak acids with Ka> 10-2 mol dm-3 and for very dilute solutions
Ka and pKa
The extent to which an acid dissociates is shown by the value of Ka. The larger the value of Ka, the stronger the acid.
if Ka is less than 1, the acid is weak
if Ka is greater than 1, the acid is strong
The values of Ka span a wide range. To make them easier to interpret, a new term pKa is used
pKa = –log10 Ka
calculating Ka from pKa
pKa = –log10 Ka
so
Ka = 10–pka