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Acids, bases & dissociation This builds on your GCSE knowledge of acids and alkalis
Acid: A substance that releases H+ ions (protons) when dissolved in water
Base: A substance that neutralises an acid
A specific type of base is an alkali, which is a soluble base that releases hydroxide (OH-) ions in water
Salt: An ionic compound formed when the H+ ion of an acid is replaced by a metal ion or an ammonium ion (NH4+)
Strong acids dissociating Strong acids fully dissociate in solution
For example, when hydrogen chloride (HCl) dissolves in water
100% of the substance dissociates into ions:
HCl (aq) โ H+ (aq) + Cl- (aq)
The dissociation of strong acids releases H+ (aq)
Strong bases dissociating Strong bases fully dissociate in solution
For example, when sodium hydroxide (NaOH) dissolves in water
100% of the substance dissociates into ions:
NaOH (aq) โ Na+ (aq) + OH- (aq)
The dissociation of strong bases releases OH- (aq)
Weak acids dissociating Unlike strong acids, weak acids do not fully ionise in water
Weak acids partially dissociate in solution
This means that only a small percentage of the products will be ions
In an equilibrium reaction, the products are formed at the same rate as the reactants are used
This means that at equilibrium, both reactants and products are present in the solution
For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
CH3COOH (aq) โ CH3COO- (aq) + H+ (aq)
The dissociation of weak acids releases some H+ (aq)
Weak bases dissociating Unlike strong bases, weak bases do not fully ionise in water
Weak bases partially dissociate in solution
Only a small percentage of the products will be ions
An equilibrium is established containing reactants and products
For example, ethylamine (CH3CH2NH2) is a weak base and will partially dissociate in solution and produce hydroxide ions:
CH3CH2NH2 (aq) + H2O (l) โ CH3CH2NH3+ (aq) + OH- (aq)
The dissociation of weak bases releases some OH- (aq)
Neutralisation What happens during neutralisation? A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt:
acid + base (alkali) โ salt + water
Specifically, neutralisation is the reaction between H+ ions from the acid and OH- ions from the base or alkali to form water:
H+ (aq) + OH- (aq) โ H2O (l)
The remaining spectator ions form the salt
Example: HCl neutralisation by NaOH The overall equation for this reaction is:
Acid + base โ salt + water
HCl + NaOH โ NaCl + H2O
The two individual reactions taking place are:
H+ + OH- โ H2O (neutralisation)
Na+ + Cl- โ NaCl (salt formation)
Cause the metal to dissolve faster
Release more heat during the reaction
Acid reactions with metal oxides Acids react with metal oxides to form a salt and water:
metal oxide + acid โ salt + water
For example:
CaO (s) + 2HCl (aq) โ CaCl2 (aq) + H2O (l)
calcium oxide + hydrochloric acid โ calcium chloride + water
Acid reactions with metal hydroxides Metal hydroxides react with acids in the same way as metal oxides, producing a salt and water:
metal hydroxide + acid โ salt + water
For example
Mg(OH)2 (s) + H2SO4 (aq) โ MgSO4 (aq) + 2H2O (l)
magnesium hydroxide + sulfuric acid โ magnesium sulfate + water
Acid reactions with metal carbonates Metal carbonates react with acids to produce a salt, water, and carbon dioxide gas:
metal carbonate + acid โ salt + water + carbon dioxide
For example:
CuCO3 (s) + 2HNO3 (aq) โ Cu(NO3)2 (aq) + H2O (l) + CO2 (g)
copper carbonate + nitric acid โ copper nitrate + water + carbon dioxide
