Very sensitive instrumentation can be used to show that even the purest water has some ability to conduct an electric current. This is due to the presence of ions, formed as a result of water undergoing an acid-base reaction with itself. Here, water is behaving as both an acid (proton donor) and a base (proton acceptor), according to Brönsted-Lowry definitions.
H2O(l) + H2O(l) 
H3O+(aq) + OH-(aq)
H3O+(aq) + OH-(aq)H2O(l) H+(aq) + OH-(aq)
H+(aq) + OH-(aq)At a temperature of 298 K, the concentration of H+(aq) is found to be 1 x 10-7 mol dm-3. The [OH-(aq)] is also 1 x 10-7 mol dm-3.
The pH Scale
pH was introduced as an abbreviation for 'power of Hydrogen'. The pH scale is used to measure how acidic (or alkaline) a solution is. It is often shown ranging from pH 1 to pH 14.
pH is related to the [H+(aq)].
pH = -log10 [H+(aq)] or pH = log10 (1/[H+(aq)])
A ten times increase in [H+(aq)] results in a decrease in pH of one unit.
Universal Indicator is a mixture of different indicators giving a series of colours across the pH range. The illustration below shows the colours of Universal Indicator.
Universal Indicator | |||||||||||||||||
pH | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | |||
increasing acidity neutral increasing alkalinity | |||||||||||||||||
Bronsted-Lowry definitions
An acid is a proton donor.
A base is a proton acceptor.
A base is a proton acceptor.
An acid is a substance that contains hydrogen which it can give up as H+ ions. The Bronsted-Lowry definition of an acid is that it is a proton donor. A base is the opposite of an acid; it is a proton acceptor.
The concentration and strength of an acid or base are different.
By the strength of an acid we mean how readily it will give up its hydrogen as H+ ions. By the strength of a base we mean how readily it will accept H+ ions. The stronger an acid the greater is its tendency to donate H+ ions; the stronger a base the greater is its tendency to accepts H+ ions. Concentration refers to the amount of a substance that is dissolved in unit volume of a solution. An acid, may be concentrated but nonetheless weak, or in dilute solution but still a strong acid. The same is the case for bases.
A strong acid is fully ionised in aqueous solution.
When a strong acid, for example hydrochloric acid, is added to water it becomes completely ionised.
HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)
The [H+(aq)] is considered to be due entirely to the ionisation of strong acid since the ionisation of water contributes only negligibly.
A weak acid is only partially ionised in aqueous solution.
A weak acid, for example ethanoic acid, becomes only partially ionised in water. In this case a chemical equilibrium is established in aqueous solution. That is, at any instant of time only a small proportion of the original number of acid molecules have become ionised.
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO-(aq)
H3O+(aq) + CH3COO-(aq)HCl(aq) and Cl-(aq), and CH3COOH(aq) and CH3COO-(aq), form what is called a conjugate acid-base pair. It follows that a strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.
An equilibrium expression, equal to Ka, can be written for the equilibrium established by the acid in aqueous solution. Ka is called the acid dissociation constant (or acid ionisation constant). Notice that water (H2O) is not included in this expression since it is present in vast excess and its concentration changes negligibly on equilibrium being reached.
Comparing the strengths of acids
We now want to compare the strengths of different acids, say ethanoic acid and benzoic acid. We allow each acid separately to react with the same base. The base chosen is water. Each acid is allowed to establish equilibrium in aqueous solution, and the equilibrium concentrations of the acid, H+(aq) ions, and conjugate base are measured. Ka for each acid can now be calculated. The more readily the acid donates H+ ions to water molecules, the more the equilibrium position will favour the products. The stronger the acid, the bigger the value of Ka.
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO-(aq)
Ka = 1.75 x 10-5 mol dm-3
H3O+(aq) + CH3COO-(aq)Ka = 1.75 x 10-5 mol dm-3
C6H5COOH(aq) + H2O(l) H3O+(aq) + C6H5COO-(aq)
Ka = 6.46 x 10-5 mol dm-3
H3O+(aq) + C6H5COO-(aq)Ka = 6.46 x 10-5 mol dm-3
pKa = -log10 Ka.
The smaller the value of pKa the stronger the acid.
The smaller the value of pKa the stronger the acid.
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