Neutralisation Reactions
- A neutralization 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) → salt + water
- The proton of the acid reacts with the hydroxide of the base to form water:
H+ (aq) + OH– → H2O (l)
- The spectator ions which are not involved in the formation of water, form the salt
Diagram to show neutralisation between an acid and a base
The diagram shows a neutralisation reaction of HCl and NaOH and the two individual reactions that take place to form the water and salt
- The name of the salt produced can be predicted from the acid that has reacted
Acid Reacted & Salt Table
| Acid reacted | Salt produced |
| Hydrochloric acid | A chloride |
| Nitric acid | A nitrate |
| Sulfuric acid | A sulfate |
Exam Tip
- The enthalpy of neutralisation is the enthalpy change that occurs when an acid reacts with a base to form one mole of water
- Since the reaction between strong acids and strong bases is the same regardless of the acid or base, it should be no surprise the enthalpy change is the same and is approximately -57 kJ mol-1
Metals and acids
- The typical reaction of a metal and an acid can be summarised as
acid + metal → salt + hydrogen
- For example:
2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)
hydrochloric acid + zinc → zinc chloride + hydrogen
- Clearly, the extent of the reaction depends on the reactivity of the metal and the strength of the acid
- Very reactive metals would react dangerously with acids and these reactions are not usually carried out
- Metals low in reactivity do not react at all
- For instance, copper does not react with dilute acids
- Stronger acids will react more vigorously with metals than weak acid
- What signs of reaction would be expected to be different between the two?
- Faster reaction, seen as:
- More effervescence
- The metal dissolves faster
- More exothermic
- Faster reaction, seen as:
Metals and oxides
- The reaction of an acid with a metal oxide forms two products:
acid + metal oxide → salt + water
- For example:
2HCl (aq) + CaO (s) → CaCl2 (aq) + H2O (l)
hydrochloric acid + calcium oxide → calcium chloride + water
Metals and hydroxides
- The reaction with a metal hydroxide and an acid follows the same pattern as an oxide:
acid + metal hydroxide → salt + water
- A suitable example might be:
H2SO4 (aq) + Mg(OH)2 (s) → MgSO4 (aq) + 2H2O (l)
sulfuric acid + magnesium hydroxide → magnesium sulfate + water
Metals and carbonates
- The reaction between a metal carbonate and an acid produces three products:
acid + metal carbonate → salt + water + carbon dioxide
- For example:
2HNO3 (aq) + CuCO3 (s) → Cu(NO3)2 (aq) + H2O (l) + CO2 (g)
nitric acid + copper carbonate → copper nitrate + water + carbon dioxide
Metals and hydrogencarbonates
- The reaction between a metal hydrogencarbonate and an acid is the same as the carbonate reaction with a slight difference in stoichiometry:
acid + metal hydrogencarbonate → salt + water + carbon dioxide
- An example of this would be:
HCl (aq) + NaHCO3 (s) → NaCl (aq) + H2O (l) + CO2 (g)
hydrochloric acid + sodium hydrogencarbonate → sodium chloride + water + carbon dioxide
Exam Tip
Make sure you learn the formulae of the common acids and bases and that you can write examples of balanced equations of their characteristic reactions
- The acids and bases needed to make different salts can be deduced using the principles covered in the previous section
- The table below summarises these reactions
Making Salts Table
| Type of salt | Ion | Acid needed | Formula | Base needed |
| Sulfates | SO42– | sulfuric | H2SO4 | metal oxide, hydroxide, carbonate or hydrogen carbonate |
| Nitrates | NO3– | nitric | HNO3 | |
| Chlorides | Cl– | hydrochloric | HCl | |
| Ethanoates | CH3COO– | ethanoic | CH3COOH | |
| Ammonium | NH4+ | any | - | aqueous ammonia |
Note that although some metals can be used to make salts, they are not classified as bases as water is not a product of the reaction
Worked example
Which are the products of the reaction between zinc oxide and hydrochloric acid?
A. zinc chloride and carbon dioxide
B. zinc chloride, hydrogen gas and water
C. zinc, hydrogen gas and water
D. zinc chloride and water
Answer:
- The correct option is D.
- Metal oxides react with acids to produce a salt and water as the only products