So the reaction in which there is release of heat with or without light is called exothermic reaction.
When the medium in which the reaction is taking place collects heat, the reaction is exothermic.
You can get enthalpy numbers from a chart in a textbook.
The numbers represent the strength of a chemical bond.
The enthalpy change, Δ, of a chemical reaction is much easier to work with.
The enthalpy change equals the change in internal energy of the system plus the work needed to change the volume of the system against constant ambient pressure.
Thus in an exothermic reaction, energy is transferred into the surroundings rather than taking energy from the surroundings as in an Therefore, it can be understood that the net amount of energy required to initiate an exothermic reaction is less than the net amount of energy released by the reaction.
When a calorimeter, a device used to measure the heat released by a chemical reaction, the net amount of heat energy that flows through the device is equal to the negative of the total energy change of the system.
Expressed in a chemical equation: reactants → products energy.
Exothermic Reaction means "exo" (derived from the greek word: "έξω", literally translated to "out") meaning releases and "thermic" means heat.