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Types of Chemical Reactions

Key Concepts

  • A chemical change, or chemical reaction, involves the formation of atleast one new substance.

  • In a chemical reaction, atoms from reactants are rearranged to form products.

  • There are many ways to classify chemical reactions.
    In this tutorial we will use the following classifications:

          (a) synthesis reactions

          (b) decomposition reactions

          (c) single replacement reactions

          (d) double replacement reactions

Synthesis Reactions

Synthesis reactions are also known as direct combination reactions.

In a synthesis reaction, two or more reactants combine to form a product:

General word equationreactant 1 + reactant 2 product
General chemical equationA + B AB

The reactants may be elements or compounds, but the product must be a compound.

(i) Both reactants are elements

General word equationelement 1 + element 2 compound
General chemical equationA + B AB

General equation exampleactive metal + non-metal ionic compound
Word equation examplesodium + chlorine sodium chloride
Chemical equation example2Na + Cl2 2NaCl

General equation examplenon-metal 1 + non-metal 2 covalent compound
Word equation examplephosphorus + chlorine phosphorus trichloride
Chemical equation example2P + 3Cl2 2PCl3

(ii) Both reactants are compounds:

General word equationcompound 1 + compound 2 compound 3
General chemical equationAB + CD ABCD

General equation exampleactive metal oxide + water metal hydroxide
Word equation examplemagnesium oxide + water magnesium hydroxide
Chemical equation exampleMgO + H2O Mg(OH)2

General equation examplenon-metal oxide + water acid
Word equation examplecarbon dioxide + water carbonic acid
Chemical equation exampleCO2 + H2O H2CO3

Decomposition Reactions

In a decomposition reaction a compound is broken down into smaller chemical species (elements or compounds).

compound species 1 + species 2
AB A + B

Heat or electricity can be used to decompose a compound.
If heat is used to decompose a compound the reaction is known as a thermal decomposition reaction.
If electricity is used to decompose a compound the reaction is known as an electrolytic decomposition reaction.

(i) Decomposition using heat (thermal decomposition)

General word equationcompound heat
species 1 + species 2
General chemical equation AB heat
A + B

(a) Thermal decomposition of most metal carbonates.
General Word Equation metal carbonate heat
metal oxide + carbon dioxide
Word Equation Example calcium carbonate heat
calcium oxide + carbon dioxide
Chemical Equation Example CaCO3 heat
CaO + CO2

(b) Thermal decomposition of most metal hydroxides.
General Word Equation metal hydroxide heat
metal oxide + water
Word Equation Example calcium hydroxide heat
calcium oxide + water
Chemical Equation Example Ca(OH)2 heat
CaO + H2O

(c) Thermal decomposition of most metal chlorates.
General Word Equation metal chlorate heat
metal chloride + oxygen
Word Equation Example potassium chlorate heat
potassium chloride + oxygen
Chemical Equation Example 2KClO3 heat
2KCl + 3O2

(d) Thermal decomposition of some oxides.
General Word Equation metal oxide heat
metal + oxygen
Word Equation Example mercury(II) oxide heat
mercury + oxygen
Chemical Equation Example 2HgO heat
2Hg + O2

(e) Thermal decomposition of some acids.
General Word Equation acid heat
water + non-metal oxide
Word Equation Example sulfuric acid heat
water + sulfur trioxide
Chemical Equation Example 2H2SO4 heat
H2O + SO3

(ii) Decomposition using electricity (electrolyic decomposition)

General word equationcompound electricity
species 1 + species 2
General chemical equation AB electricity
A + B

(a) Molten salts can be decomposed using electricity.
General Word Equation salt electricity
metal + non-metal
Word Equation Example sodium chloride electricity
sodium + chlorine
Chemical Equation Example 2NaCl electricity
2Na + Cl2

(b) Some covalent compounds can be decomposed using electricity.
Word Equation Example water electricity
hydrogen + oxygen
Chemical Equation Example 2H2O electricity
2H2 + O2

Single Replacement Reactions

Single replacement reactions are also known as displacement reactions or substitution reactions.

In a single replacement reaction, one element in a compound is replaced by a different element to form two products.
One of the products is a compound and the other is an element.
The element in the compound that is to be replaced is known as the less active element.
The element that becomes part of the new product compound is known as the more active element.

General Word Equationelement 1
(more active element)
+ compound 1 compound 2 + element 2
(less activ element)
General Chemical EquationA + BC AC + B

The element to be replaced can be either a metal or a non-metal.

(i) Replacement of a metal in a compound by a more active metal.

General Word Equationmore active metal + compound 1 compound 2 + less active metal
General Chemical EquationA + BC AC + B
Word Equation Examplezinc + copper(II) sulfate zinc sulfate + copper
Chemical Equation ExampleZn + CuSO4 ZnSO4 + Cu

(ii) Replacement of a less active metal in a compound by a more active non-metal.

General Word Equationmore active non-metal + compound 1 compound 2 + less active metal
General Chemical EquationA + BC AC + B
Word Equation Examplecarbon + copper(II) oxide carbon dioxide + copper
Chemical Equation ExampleC + 2CuO CO2 + 2Cu

(iii) Replacement of a non-metal in a compound by a more active non-metal.

General Word Equationmore active non-metal + compound 1 compound 2 + less active non-metal
General Chemical EquationA + BC AC + B
Word Equation Examplechlorine + sodium bromide sodium chloride + bromine
Chemical Equation ExampleCl2 + 2NaBr 2NaCl + Br2

(iv) Replacement of hydrogen in water by an active metal.

General Word Equationactive metal + water ionic compound + hydrogen
General Chemical EquationA + H2O AOH + H2
Word Equation Examplesodium + water sodium hydroxide + hydrogen
Chemical Equation Example2Na + 2H2O 2NaOH + H2

(v) Replacement of hydrogen in acid by an active metal.

General Word Equationactive metal + acid ionic compound + hydrogen
General Chemical EquationA + HB AB + H2
Word Equation Examplezinc + hydrochloric acid zinc chloride + hydrogen
Chemical Equation ExampleZn + 2HCl ZnCl2 + H2

Double Replacement Reactions

Double replacement reactions are also known as double displacement reactions or metathesis reactions.

In a double replacement reaction, atoms or ions in both reacting compounds are exchanged to form products.

General Word Equationcompound 1 + compound 2 compound 3 + compound 4
General Chemical EquationAB + CD AD + CB

(i) Precipitation reactions

General Word Equationsoluble salt 1 + soluble salt 2 precipitate
(insoluble salt)
+ soluble salt 4
General Chemical EquationAB + CD AD + CB
Word Equation Example silver nitrate + sodium chloride silver chloride + sodium nitrate
Chemical Equation ExampleAgNO3 + NaCl AgCl + NaNO3

(ii) Neutralisation reactions

General Word Equationacid + base water + salt
General Chemical EquationHX + MOH H2O + MX
Word Equation Examplehydrochloric acid + sodium hydroxide water + sodium chloride
Chemical Equation ExampleHCl + NaOH H2O + NaCl

(iii) Double Decomposition reactions

The most common example of this type of reaction is the reaction between an acid and a carbonate or bicarbonate (hydrogen carbonate).
Initially, the acid reacts with the carbonate (or bicarbonate) to produce a salt and carbonic acid, H2CO3.
Then the carbonic acid spontaneously decomposes to produce carbon dioxide and water.
So, the overall reaction is represented as acid + carbonate (or bicarbonate) → salt + carbon dioxide + water

(a) acid + carbonate

General Word Equationacid + carbonate salt + carbon dioxide + water
General Chemical EquationHX + MCO3MX + CO2 + H2O
Word Equation Examplehydrochloric acid + calcium carbonatecalcium chloride + carbon dioxide + water
Chemical Equation Example2HCl + CaCO3CaCl2 + CO2 + H2O

(b) acid + bicarbonate (acid + hydrogen carbonate)

General Word Equationacid + bicarbonate salt + carbon dioxide + water
General Chemical EquationHX + MHCO3MX + CO2 + H2O
Word Equation Examplehydrochloric acid + sodium bicarbonatesodium chloride + carbon dioxide + water
Chemical Equation ExampleHCl + NaHCO3NaCl + CO2 + H2O


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