Question

Name the types of chemical reactions and write an example of each.

Answer 1

Chemical reaction

A chemical reaction is a process that occurs when two or more molecules interact to form a new product. The chemical reactions are classified into the following types

• Combination reaction
• Decomposition reaction
• Displacement reaction
• Double displacement reaction

Combination reaction

• A reaction in which two or more elements or substances combine to form a single product is called a combination reaction. This type of reaction is also known as a synthesis reaction.

$\mathrm{A}+\mathrm{B}\to \mathrm{AB}$

• Example: When a piece of magnesium ribbon is burned or ignited in the air, it burns with a white dazzling light to form a white powder of magnesium oxide.

$$\begin{gathered} 2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{MgO}\left(\mathrm{s}\right) \\ \mathrm{magnesium}\mathrm{oxygen}\mathrm{magnesium}\mathrm{oxide} \end{gathered}$$

Decomposition reaction

• A reaction in which a single compound breaks into two or simpler substances is known as a decomposition reaction.

$\mathrm{AB}\hspace{0.17em}\to \mathrm{A}+\mathrm{B}$

• Example: When limestone is heated, it breaks up into calcium oxide and carbon dioxide.

$$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Calcium}\mathrm{carbonate}\mathrm{Calcium}\mathrm{oxide}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

• Decomposition reactions that take place when an electric current is passed through the compound in its molten state or its aqueous solution are called electrolytic decomposition reactions.
• Example: When electricity is passed through acidified water, it decomposes to give hydrogen and oxygen.

$$\begin{gathered} 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\mathrm{electricity}}{\to }2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \mathrm{Acidified}\mathrm{water}\mathrm{Hydrogen}\mathrm{Oxygen} \end{gathered}$$

• Decomposition reactions that take place during the absorption of light are called photochemical decomposition reactions.
• Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

$$\begin{gathered} 2\mathrm{AgCl}\left(\mathrm{s}\right)\stackrel{\mathrm{light}}{\to }2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right) \\ \mathrm{Silver}\mathrm{chloride}\mathrm{Silver}\mathrm{Chlorine} \end{gathered}$$

Displacement reaction

• A reaction in which a more reactive element displaces a less reactive element from its compound when reacted together is called a displacement reaction.

$\mathrm{A}+\mathrm{BC}\to \mathrm{AC}+\mathrm{B}$

• Example: When iron reacts with copper sulphate, iron being highly reactive displaces copper which is less reactive and forms ferrous sulphate and copper.

$$\begin{gathered} \mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{CuSO}}_4\left(\mathrm{s}\right)\to {\mathrm{FeSO}}_4\left(\mathrm{s}\right)+\mathrm{Cu}\left(\mathrm{s}\right) \\ \mathrm{Iron}\mathrm{Copper}\mathrm{sulphate}\mathrm{Ferrous}\mathrm{sulphate}\mathrm{Copper} \end{gathered}$$

Double displacement reaction

A chemical reaction in which two compounds in their aqueous solution interchange their ions to form new compounds is called a double displacement reaction.

$\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}\hspace{0.17em}+\mathrm{CB}$

These reactions are of two types:

(i) Precipitation reaction

• A double displacement reaction usually occurs in solution. When one of the products formed is an insoluble solid, the type of double displacement reaction is known as a precipitation reaction.
• Example: When solutions of silver nitrate and sodium chloride react, a white precipitate of silver chloride is produced along with sodium nitrate.

$$\begin{gathered} {\mathrm{AgNO}}_3\left(\mathrm{aq}\right)+\mathrm{NaCl}\left(\mathrm{aq}\right)\to \mathrm{AgCl}\downarrow \left(\mathrm{s}\right)+{\mathrm{NaNO}}_3\left(\mathrm{aq}\right) \\ \mathrm{Silver}\mathrm{nitrate}\mathrm{Sodium}\mathrm{chloride}\mathrm{Silver}\mathrm{chloride}\mathrm{Sodium}\mathrm{nitrate} \end{gathered}$$

(ii) Neutralization reaction

• A neutralization reaction is a type of double displacement reaction in which an acid and a base react together to form salt and water.
• Example: When a solution of sodium hydroxide reacts with dilute hydrochloric acid, sodium chloride and water are formed.

$$\begin{gathered} \mathrm{NaOH}\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)\to \mathrm{NaCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Sodium}\mathrm{hydroxide}\mathrm{Hydrochloric}\mathrm{acid}\mathrm{Sodium}\mathrm{chloride}\mathrm{Water} \end{gathered}$$