Question

Name and define the four important types of chemical reaction.

Answer 1

Chemical reaction:

• When one or more substances (elements or compounds) undergo a chemical change with the absorption or evolution of energy to give new substances (called products). This change is called a chemical reaction.
• For example when sodium chloride$\left(\mathrm{NaCl}\right)$ reacts with silver nitrate$\left({\mathrm{AgNO}}_3\right)$ , it forms silver chloride$\left(\mathrm{AgCl}\right)$ and sodium nitrate$\left({\mathrm{NaNO}}_3\right)$.
• $\underset{\mathrm{sodium}\mathrm{chloride}}{\mathrm{NaCl}\left(\mathrm{aq}\right)}+\underset{\mathrm{silver}\mathrm{nitrate}}{{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)}\stackrel{}{\to }\underset{\mathrm{silver}\mathrm{chloride}}{\mathrm{AgCl}\left(\mathrm{s}\right)}+\underset{\mathrm{sodium}\mathrm{nitrate}}{{\mathrm{NaNO}}_3\left(\mathrm{aq}\right)}$

Types of chemical reaction: There are four important types of chemical reactions which are as follows:

1. Combination reaction:

• Combination is a chemical reaction in which two simpler reactants combine to form a single product.
• Such reactions can be brought by the action of heat, light, pressure, electricity, etc.
• A general combination reaction is represented as
• $\mathrm{A}+\mathrm{B}\stackrel{}{\to }\mathrm{AB}$
• For example when calcium oxide$\left(\mathrm{CaO}\right)$ reacts with water resulting in the formation of calcium hydroxide$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ or quick lime and releasing a tremendous amount of energy.
• $\underset{\mathrm{calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\stackrel{}{\to }\underset{\mathrm{calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}$

2. Decomposition reaction:

• It is a chemical reaction in which a chemical compound decomposes to give two or more new substances.
• Usually decomposition reactions require external agents like heat, light, electricity, etc.
• For example decomposition of mercuric oxide$\left(\mathrm{HgO}\right)$ is thermal decomposition i.e it is the decomposition reaction that is brought about by heat, resulting in the formation of mercury$\left(\mathrm{Hg}\right)$ and oxygen gas$\left({\mathrm{O}}_2\right)$
• $\underset{\mathrm{mercuric}\mathrm{oxide}}{2\mathrm{HgO}\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{mercury}}{2\mathrm{Hg}\left(\mathrm{l}\right)}+\underset{\mathrm{oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$

3. Displacement reaction:

• It is a chemical reaction in which an element replaces another element in a compound and takes its place.
• The most reactive metal occupies the top position in the series whereas the least reactive metal is at the bottom of the series.
• When iron$\left(\mathrm{Fe}\right)$ reacts with copper sulphate$\left({\mathrm{CuSO}}_4\right)$, it forms ferrous sulphate$\left({\mathrm{FeSO}}_4\right)$ and copper$\left(\mathrm{Cu}\right)$.
• $\underset{\mathrm{iron}}{\mathrm{Fe}(\mathrm{s}})+\underset{\mathrm{ferrous}\mathrm{sulpphate}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}\stackrel{}{\to }\underset{\mathrm{copper}\mathrm{sulphate}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{copper}}{\mathrm{Cu}\left(\mathrm{s}\right)}$
• The iron $\left(\mathrm{Fe}\right)$being higher reactive displaces the copper$\left(\mathrm{Cu}\right)$ which is lesser reactive from its compound$\left({\mathrm{CuSO}}_4\right)$.

4. Double displacement or double decomposition reaction

• It is a chemical reaction in which two compounds in their aqueous solutions react to form two new compounds by the mutual exchange of their radicals.
• In general double decomposition reaction can be represented as
• $\mathrm{AB}+\mathrm{CD}\stackrel{}{\to }\mathrm{AD}+\mathrm{BC}$
• For example when barium chloride $\left({\mathrm{BaCl}}_2\right)$ reacts with sulphuric acid$\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$, they undergo mutual exchange of their radicals resulting in the formation of barium sulphate$\left({\mathrm{BaSO}}_4\right)$ and hydrochloric acid$\left(\mathrm{HCl}\right)$.
• $\underset{\mathrm{barium}\mathrm{chloride}}{{\mathrm{BaCl}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{sulphuric}\mathrm{acid}}{{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}\stackrel{}{\to }\underset{\mathrm{barium}\mathrm{sulphate}}{{\mathrm{BaSO}}_4\left(\mathrm{s}\right)⇣}+\underset{\mathrm{hydrochloric}\mathrm{acid}}{2\mathrm{HCl}\left(\mathrm{aq}\right)}$