Question

How does a substitution reaction differ from an addition reaction?

Answer 1

Difference between substitution reaction and addition reaction

Substitution reaction	Addition reaction
A substitution reaction is a reaction that involves the replacement of an atom or a group of atoms by another atom or a group of atoms.	The addition reaction is the combination of two or more atoms or molecules in order to form a large molecule.
This results in a byproduct named the leaving group.	Most addition reactions are limited to molecules with unsaturation that has either double bonds ($\mathbf{=}$) or triple bonds ($\mathbf{\equiv }$).
Example: Addition of Hydrogen to Ethene $$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\stackrel{\mathrm{Ni}/\mathrm{Pt}250°\mathrm{C}}{\to }{\mathrm{C}}_2{\mathrm{H}}_6\left(\mathrm{g}\right) \\ \mathrm{Ethene}\mathrm{Ethane} \end{gathered}$$	Example: Reaction of Methane with Chlorine in presence of light $$\begin{gathered} {\mathrm{CH}}_4\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\stackrel{\mathrm{sunlight}}{\to }{\mathrm{CH}}_3\mathrm{Cl}\left(\mathrm{g}\right)+\mathrm{HCl}\left(\mathrm{g}\right) \\ \mathrm{Methane}\mathrm{Chloro}\mathrm{Hydrogen} \\ \mathrm{methane}\mathrm{chloride} \end{gathered}$$