Explain the change in enthalpy related to a chemical reaction.

In a chemical reaction,reactants are converted into products,represented as: $\text{Reactants} \rightarrow \text{Products}$.
The enthalpy change accompanying a reaction is called the reaction enthalpy. The enthalpy change of a chemical reaction is denoted by the symbol $\Delta_{r} H$.
$\Delta_{r} H = (\sum \text{Enthalpies of products}) - (\sum \text{Enthalpies of reactants}) = \sum a_{i} H_{\text{products}} - \sum b_{i} H_{\text{reactants}}$
Here,the symbol $\sum$ represents summation,and $a_{i}$ and $b_{i}$ are the stoichiometric coefficients of the products and reactants,respectively,in the balanced chemical equation.
For example,for the reaction: $CH_{4(g)} + 2O_{2(g)} \rightarrow CO_{2(g)} + 2H_{2}O_{(l)}$
$\Delta_{r} H = [H_{m}(CO_{2}, g) + 2H_{m}(H_{2}O, l)] - [H_{m}(CH_{4}, g) + 2H_{m}(O_{2}, g)]$
where $H_{m}$ represents the molar enthalpy.