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Some Important Chemical Reactions you should know (O level chemistry)

Some Important Chemical Reactions you should know (O level chemistry)

Ionisation of elements

Ionisation of Group 1 elements (alkaline metals)

Atoms of Group 1 elements have 1 electron in the outer shell, so they ionise by losing 1 electron to form a positively charged ion of charge +1. This is oxidation because it involves loss of electrons.


Sodium → sodium ion + electron

Na → Na+ + e



Ionisation of Group 2 elements (alkaline earth metals)

Atoms of Group 2 elements have 2 electrons in the outer shell, so they ionise by losing 2 electrons to form a positively charged ion of charge +2. This is oxidation because it involves loss of electrons.


Magnesium → magnesium ion + 2 electron

Mg → Mg2+ + 2e


Ionisation of Group 7 elements (halogens)

Atoms of Group 7 elements have 7 electrons in the outer shell, so they ionise by gaining 1 electrons to form a negatively charged ion of charge -1. This is reduction because it involves gain of electrons.


Chlorine + electron → chlorine ion

Cl2 + 2e → 2Cl


Reactions involving acids

Reactions of metals with acids

Reactive metals react with dilute acids to form a salt and hydrogen gas.


Magnesium + sulphuric acid → magnesium sulphate + hydrogen

Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)


Zinc + hydrochloric acid → zinc chloride + hydrogen

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)


Magnesium + nitric acid → magnesium nitrate + hydrogen

Mg(s) + HNO3(aq) → Mg(NO3)2(aq) + H2(g)


Reactions of carbonates with acids

Carbonates react with acid to produce a salt, carbon dioxide and water.


Copper (ii) carbonate + hydrochloric acid → copper (ii) chloride + carbon dioxide + water

CuCO3(s) + 2HCl(aq) → CuCl2(aq) + CO2(aq) + H2O(l)


Neutralisation reactions of acids with alkalis

Alkalis neutralise acids to produce a salt and water.


hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)


Neutralisation reactions of acids with insoluble bases

Bases neutralise acids to produce a salt and water.


sulphuric acid + copper(ii) oxide → copper(ii) sulphate + water

H2SO4(aq) + CuO(s) → CuSO4(aq) + H2O(l)


Ionisation of acids

Ionisation of weak acids

Weak acids are only partially ionised in water and produce less hydrogen ions.


ethanoic acid → hydrogen ions + ethanoate ions

CH3COOH(l) → H+(aq) + CH3COO(aq)


Ionisation of strong acids

Strong acids fully ionise in water and produce more hydrogen ions than weak acids.


hydrochloric acid → hydrogen ions + chloride ions

HCl(aq) → H+(aq) + Cl(aq)


Ionisation of alkalis

Ionisation of weak alkalis

Weak alkalis partially ionise in water to produce fewer hydroxide ions.


ammonia + water → ammonium ions + hydroxide ions

NH3(g) + H2O(l) → NH4(aq) + OH(aq)


Ionisation of strong alkalis

Strong alkalis fully ionise in water to produce a lot of hydroxide ions.


sodium hydroxide → sodium ions + hydroxide ions

NaOH(s) → Na+(aq) + OH(aq)


Displacement reactions

Displacement of metals

A reactive metal displaces a less reactive metal from a compound of its ions.


Magnesium + Copper (ii) nitrate → Magnesium nitrate + Copper

Mg(s) + Cu(NO3)2(aq) → Mg(NO3)2(aq) + Cu(s)


Electrolysis reactions

Electrolysis of molten lead (ii) bromide

Molten lead (ii) bromide can be electrolysed using carbon electrodes to produce lead at the cathode and bromine gas at the anode.


Cathode reaction:

Pb2+(l) + 2e → Pb(l)

Anode reaction:

2Br(l) – 2e → Br2(g)

Overall reaction:

PbBr2(l) → Pb(l) + Br2(g)


Electrolysis of dilute sulphuric acid

Electrolysis of dilute sulphuric acid using inert electrodes produces hydrogen at the cathode and oxygen at the anode.


Anode reaction:

4OH(aq) → 2H2O(l) + O2(g) + 4e

Cathode reaction:

2H+(aq) + 2e → H2(g)


Electrolysis of concentrated hydrochloric acid

Electrolysis of concentrated hydrochloric acid using inert electrodes produces hydrogen at the cathode and chlorine at the anode.


Cathode reaction:

2H+(aq) + 2e → H2(g)

Anode reaction:

2Cl(l) – 2e → Cl2(g)


Electrolysis of aqueous copper (ii) sulphate using inert electrodes

Electrolysis of aqueous copper (ii) sulphate using inert electrodes produces copper at the cathode and oxygen at the anode.


Anode reaction:

4OH(aq) → 2H2O(l) + O2(g) + 4e

Cathode reaction:

Cu2+(aq) + 2e → Cu(s)


Electrolysis of aqueous copper (ii) sulphate using copper electrodes

During electrolysis of aqueous copper (ii) sulphate using copper electrodes, the anode dissolves and copper is produced at the cathode.


Anode reaction:

Cu(s) → Cu2+(aq) + 2e

Cathode reaction:

Cu2+(aq) + 2e → Cu(s)


Decomposition reactions

Decomposition by catalyst

Hydrogen peroxide decomposes in the presence of a manganese (iv) oxide catalyst to produce water and oxygen.


hydrogen peroxide → water + oxygen

2H2O2(aq) → 2H2O(l) + O2(g)


Combustion reactions

Combustion of hydrocarbons

Hydrocarbons burn in plentiful supply of air to produce carbon dioxide and water. The reaction is exothermic.


methane + oxygen → carbon + water + energy dioxide

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)


In limited oxygen supply, hydrocarbons burn to produce soot (unburnt carbon) and carbon monoxide. The reaction produces less heat energy.


methane + oxygen → carbon + carbon monoxide + water

2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(l)


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