At GCSE we defined acids as substances that ionised in water to give hydrogen ions, and an alkali dissolved in water to give hydroxide ions. This is essentially the definition proposed by the Swedish chemist Svante Arrhenius at the end of the 19th century.
Acids dissociate or ionise in water to give H+ and an anion
HCl(aq) + (aq) ⇾ H+(aq) + Cl–(aq)
Alkalis dissociate or ionise in water to give OH– and a cation
NaOH(aq) + (aq) ⇾ Na+(aq) + OH–(aq)
This definition limits us to thinking about acids and alkalis only in the context of aqueous solutions, but there are many bases that are able to neutralise an acid but don’t produce hydroxide ions in solution (these are not alkalis by definition) such as magnesium oxide and calcium carbonate.
In 1923 Johannes Brønsted and Martin Lowry independently proposed that since a hydrogen ion was a proton …
Acids are proton donors
Bases are proton acceptors
In an acid-base reaction a proton is transferred from an acid to a base
The true nature of H+ ions
A hydrogen ion is simply a proton.
A proton has a diameter of ∼10-15m which is 100000 times smaller than a lithium ion (the next largest cation).
This means that a H+ ion has a very high charge density. We are never going to find H+ ions floating around in solution as they are always bonded to a water molecule through a dative covalent bond using the lone pair on one of the oxygen.
H+(g) + H2O(l) ⇾ H3O+(aq)
H3O+ is the oxonium / hydronium / hydroxonium ion and is always hydrated.
- water molecules form a hydration shell around the ion
- in acidic solution the ion is more accurately represented as [(H3O)(H2O)3]+
- the formation of hydration shells is a dynamic process with water molecules coming and going, and so it is easier to describe all this as (aq)
The special case of water
Water can behave as both an acid and a base – remember that if it is donating H+ it is behaving as an acid and if it is accepting H+ it is behaving as a base.
Next we need to consider the nature of acid-base pairs in chemical reactions.
Practice questions
- Write equations for the following reactions (in each case there is only one product). Decide whether the product of each reaction will be acidic, basic or neutral.
(i) magnesium oxide reacting with water
(ii) phosphorus(V) oxide reacting with water to form phosphoric acid
(iii) magnesium oxide reacting with phosphorus(V) oxide to form magnesium phosphate
- Preparation of an aqueous solution of hydrochloric acid shows that the dissociation of one molecule of HCl requires 4 molecules of water. Assuming that 1.00 dm3 of water contains 55.6 mol of H2O, determine the maximum mass of HCl that will be able to dissociate in 1.00 dm3 of water.
Answers
- (i) MgO + H2O ⇾ Mg(OH)2 BASIC
(ii) P2O5 + 3H2O ⇾ 2H3PO4 or P5O10 + 6H2O ⇾ 4H3PO4 ACIDIC
(iii) 3MgO + P2O5 ⇾ Mg3(PO4)2 NEUTRAL
2. Ratio of HCl : H2O is 1:4 so 55.6 mol of H2O will allow 13.9 mol HCl to dissociate.
Mass of HCl = n x Mr = 13.9 mol x 36.5 g mol-1 = 507g