Topic 12 Acid-Base Equilibria
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Define an acid.
Proton donor.
Define a base.
Proton acceptor.
Conjugate acid-base pair
A base and its conjugate acid or an acid and its conjugate base.
Conjugate acid
When a base accepts a proton, the species formed is its conjugate acid.
Conjugate base.
When an acid donates a proton, the species formed is the conjugate base of the acid.
Amphoteric
A substance that can act as both an acid and a base.
What substances tend to be bases?
They must have a lone pair to accept a proton by forming a dative covalent bond with it. This means a base must contain an atom of the RHS of the periodic table, often oxygen.
What substances tend to be acids?
A substance containing a slightly positively-charged hydrogen, e.g., when it is bonded to a highly electronegative atom such as oxygen of a halogen.
Amphiprotic
A substance that can both donate and accept protons such as amino acids or HSO4^-. All amphiprotic substances are also amphoteric, but no the other way around.
Monoprotic acids
E.g., HCl, can donate one proton. (Also termed monobasic acids.)
Diprotic acids
E.g.,H2SO4, can donate 2 protons (also termed dibasic acids).
Diprotic base
E.g., a base that can accept 2 protons e.g., CO3^2-+ 2H+ –> H2CO3. Also termed diacidic bases.
Nitric acid as a base in the nitration of benzene.
Concentrated H2SO4 + concentrated HNO3 react: H2SO4 + HNO3<–> HSO4^- + H2NO3^+ H2SO4 is an acid; its conjugate base is HSO4-. HNO3 is a base; its conjugate acid is H2NO3^+.
When ammonia dissolves in water, this is an example of a conjugate acid-base pair.
NH3 (aq) + H2O (l) <–> NH4^+ (aq) + OH^- (aq) In the forward reaction: NH3 acts as a base & H2O acts as an acid. In the backward reaction: NH4^+ acts as an acid & OH- acts as a base.
Strong acid
Completely dissociates/ionises in aqueous solution. HCl(aq) –> H+(aq) + Cl-(aq)
Weak acid
Only partially dissocisates in aqueous solution, typically organic acids. CH3COOH(aq) <–> CH3COO-(aq) + H+(aq)
pH of an aqueous solution
The reciprocal of the logarithm to the base 10 of the hydrogen ion concentration measured in mol dm^-3. pH= -log[H+]
For strong acids, the [acid] is directly related to the [H+].
A solution of HCl of 0.100 mol dm^-3 produces a [H+] of 0.100 mol dm^-3.
How can you determine [H+] from pH?
[H+] = 10^-pH
Ka
Ka = [H+][A-]/[HA]
[A-]=[H+] at equilibrium. How?
Every time a molecule of HA dissociates, a H+ ion & A- ion are formed.
How can we find the pH of a weak acid?
Ka= [H+]^2/[HA], assuming the concentration of undissociated acid at equilibrium is equal to the concentration of acid initially as Ka is very small.
pKa
-log Ka
The larger the value of Ka…
… the stronger the acid.