Types of Chemical Reactions

Learning Objective

In this lesson we will learn how the products of chemical reactions can often be predicted, by classifying reactions into different types, based on the way atoms are rearranged.

Learning Outcomes

By the end of this lesson you will be able to:

  • Identify the following types of chemical reactions:

    Combination, decomposition, single displacement, double displacement (precipitation and neutralisation), combustion (complete and incomplete), redox.

  • Predict the products of the following types of chemical reactions:

    Single displacement, precipitation, neutralisation, complete combustion.

  • Identify the reducing agent and the oxidising agent in a redox reaction.

 

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Introduction

  • There are many different chemical reactions that exist.
    These can be categorised into types, based on the way atoms and molecules are rearranged during the reaction.
    By classifying chemical reactions, it is possible to predict products, based on generalised formula equations.

 

Combination (Synthesis) Reactions

  • Combination reactions involve two or more reactants combining to form a single product:

 
combination synthesis reaction diagram

 

  • They can be generalised by the following equation:

 
combination synthesis reaction general formula

 

  • Examples of combination reactions:
    Two elements combining to form a compound:
    2 H2 (g)  +  O2 (g)    2 H2O (l)
    An element and a compound combining to form a new compound:
    2 CO (g)  +  O2 (g)    2 CO2 (g)
    Two compounds combining to form a new compound:
    Li2O (s)  +  CO2 (g)    Li2CO3 (s)

 

Decomposition Reactions

  • Decomposition reactions involve a single reactant breaking down to form two or more products:

 
decomposition reaction diagram

 

  • They can be generalised by the following equation:

 
decomposition reaction formula

 

  • Examples of decomposition reactions:
    A compound decomposing to form two elements:
    Mg3N2 (s)    3 Mg (s)  +  N2 (g)
    A compound decomposing to form a new compound and an element:
    2 KClO3 (s)    2 KCl (s)  +  3 O2 (g)
    A compound decomposing to form two new compounds:
    NH4NO3 (s)    N2O (g)  +  2 H2O (l)

 

Displacement (Replacement) Reactions

  • Displacement reactions involve the displacing or swapping of one type of atom with another.
  • There are two types of types of displacement reactions: single displacement and double displacement.

 

Single Displacement (Single Replacement) Reactions

  • Single displacement reactions involve an element and a compound reacting to form a different element and compound:
    element-1  +  compound-1    compound-2  +  element-2

 
single displacement reaction diagram

 

  • They can be generalised by the following equation:

 
single displacement reaction general formula

 

  • Examples of single displacement reactions:
    A metal displaces another metal in a compound:
    Zn (s)  +  Cu(NO3)2 (aq)    Zn(NO3)2 (aq)  +  Cu (s)
    A metal displaces hydrogen in an acid or water:
    Zn (s)  +  2 HBr (aq)    ZnBr2 (aq)  +  H2 (g)
    2 Na (s)  +  2 H2O (l)    2 NaOH (aq)  +  H2 (g)
    A non-metal displaces another non-metal in a compound:
    Cl2 (g)  +  2KI (aq)    2KCl (aq)  +  I2 (s)

 

Double Displacement (Double Replacement) Reactions

  • Double displacement reactions involve two compounds reacting to form two different compounds:
    compound-1  +  compound-2    compound-3  +  compound-4

 
double displacement reaction diagram

 

  • They can be generalised by the following equation:

 
double displacement reaction general formula

 

  • Examples of double displacement reactions:
    Precipitation reactions – two aqueous compounds combine to form two compounds where one is insoluble:
    AgNO3 (aq)  +  NaCl (aq)    AgCl (s)  +  NaNO3 (aq)
    Neutralisation reactions – an acid and a base combine to form a salt and water:
    H2SO4 (aq)  +  2 NaOH (aq)    Na2SO4 (aq)  +  2 H2O (l)

 

Combustion Reactions

  • Combustion reactions involve the burning of hydrocarbons (compounds containing carbon and hydrogen) with oxygen.
  • The products of combustion reactions vary, depending on the supply of oxygen.

 

Complete Combustion

  • Complete combustion involves the burning of hydrocarbons where the supply of oxygen is not limited.
  • The products of complete combustion reactions are carbon dioxide and water:

 
complete combustion reaction diagram

 

  • They can be generalised by the following equation:

 
complete combustion reaction general formula

 

  • Examples of complete combustion reactions:
    Complete combustion of methane:
    CH4 (g)  +  2 O2 (g)    CO2 (g)  +  2 H2O (g)
    Complete combustion of butane:
    2 C4H10 (g)  +  13 O2 (g)    8 CO2 (g)  +  10 H2O (g)
    Complete combustion of ethylene:
    C2H4 (g)  +  3 O2 (g)    2 CO2 (g)  +  2 H2O (g)

 

Incomplete Combustion

  • Incomplete combustion involves the burning of hydrocarbons where the supply of oxygen is limited.
  • The products of incomplete complete combustion reactions can be carbon monoxide or carbon, as well as water:

 
incomplete combustion reaction diagram 1

 
incomplete combustion reaction diagram 2

 

  • They can be generalised by the following equations:

 
incomplete combustion reaction general formula 1

 
incomplete combustion reaction general formula 2

 

  • Examples of incomplete combustion reactions:
    Incomplete combustion of methane:
    2 CH4 (g) + 3 O2 (g) 2 CO (g) + 4 H2O (g)
    CH4 (g) + O2 (g) C (s) + 2 H2O (g)
    Incomplete combustion of butane:
    2 C4H10 (g)  +  9 O2 (g)    8 CO (g)  +  10 H2O (g)
    2 C4H10 (g)  +  5 O2 (g)    8 C (s)  +  10 H2O (g)
    Incomplete combustion of ethylene:
    2 C2H4 (g)  +  4 O2 (g)    4 CO (g)  +  4 H2O (g)
    2 C2H4 (g)  +  2 O2 (g)    4 C (s)  +  4 H2O (g)
  • In reality, most combustion reactions will produce a mixture of carbon dioxide, carbon monoxide or carbon, as well as water.
    The ratio of these products will depend on the supply of oxygen.

 

Redox Reactions

  • There are different ways that chemical reactions can be classified.
    Therefore, a particular reaction may be classified in more than one way.
    For example, combustion reactions and some combination reactions, such as corrosion reactions, can be classified as oxidation reactions.
  • An oxidation reaction can be considered as a reaction where a substance gains oxygen.
    The opposite of an oxidation reaction – where a substance loses oxygen – is a reduction reaction.
  • Oxidation and reduction reactions always occur in pairs.
    As one substance is oxidised, the other is reduced.
    These oxidation-reduction reactions are commonly referred to as redox reactions.
  • The substance that is oxidised is known as the reducing agent, as it causes the other substance to be reduced.
    Similarly, the substance that is reduced is known as the oxidising agent, as it causes the other substance to be oxidised.
  • For example, in the following redox reaction:

 
redox reaction loss gain oxygen

 

    Hydrogen is oxidised, as it gains oxygen to form water; it is the reducing agent.
    Simultaneously, carbon dioxide is reduced, as it loses oxygen to form carbon monoxide; it is the oxidising agent.

 

Redox Reactions That Do Not Involve Oxygen

  • Not all redox reactions involve oxygen.
  • A broader definition of a redox reaction is any reaction that involves the transfer of electrons from one substance to another.
    Redox reactions therefore include combustion reactions, single displacement reactions, and most combination and decomposition reactions.
  • The substance that is oxidised is the one which loses electrons.
    The substance that is reduced is the one which gains electrons.
  • For example, in the following redox reaction:

 
redox reaction loss gain electrons

 

    Chloride (Cl) is oxidised, as it loses electrons to form chlorine (Cl2); it is the reducing agent.
    Simultaneously, fluorine (F) is reduced, as it gains electrons to form fluoride (F); it is the oxidising agent.
    Sodium ions (Na+) do not change; therefore they do not actually form part of the reaction.

 

Summary

  • Chemical reactions can be classified in different ways, based on the way atoms and molecules are rearranged during the reaction.
  • Classifying chemical reactions can make it possible to predict their products.
  • Combination reactions involve two or more reactants combining to form one product.
    A  +  B    AB
  • Decomposition reactions involve one reactant breaking down to form two or more products.
    AB    A  +  B
  • Single displacement reactions involve an element and a compound reacting to form a different element and compound.
    A  +  BC    AC  +  B
  • Double displacement reactions involve two compounds reacting to form two different compounds.
    AB  +  CD    AD  +  CB
  • Complete combustion reactions involve the burning of hydrocarbons, where the supply of oxygen is not limited.
    CxHy  +  O2    CO2  +  H2O
    Incomplete combustion reactions involve the burning of hydrocarbons, where the supply of oxygen is limited.
    CxHy  +  O2    CO  +  H2O
    CxHy  +  O2    C  +  H2O
  • Redox reactions involve simultaneous oxidation and reduction reactions.
    An oxidation reaction involves the loss of oxygen or the gain of electrons; a reduction reaction involves the gain of oxygen or the loss of electrons.
    A substance that is oxidised during a redox reaction is know as the reducing agent; a substance that is reduced during a redox reaction is know as the oxidising agent.
    Combustion reactions, single displacement reactions, and most combination and decomposition reactions are examples of redox reactions.

 
camp fire combustion oxidation

(Image: FotoshopTofs, Pixabay)

 

(Header image: vadimborkin, Adobe Stock)

 

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