Exothermic and endothermic reactions can be described in chemical equations based on the energy changes that occur during the reaction.

Exothermic Reaction:
- In an exothermic reaction, energy is released in the form of heat to the surroundings.
- This can be represented in a chemical equation by showing heat as a product.
- For example, the combustion of methane can be represented as:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat

Endothermic Reaction:
- In an endothermic reaction, energy is absorbed from the surroundings in the form of heat.
- This can be represented in a chemical equation by showing heat as a reactant.
- For example, the decomposition of ammonium chloride can be represented as:
NH4Cl(s) + heat → NH3(g) + HCl(g)
In both cases, the presence of heat as a product or reactant indicates whether the reaction is exothermic or endothermic. This information is crucial in understanding the direction and energy changes that occur during chemical reactions.