Chemical energy

In chemistry, chemical energy can be defined as  the energy associated with transfer of energy between chemical bonds. For example the formation of chemical bonds and the breaking of chemical bonds. 

In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it is invariably accompanied by an increase or decrease of energy of the substances involved. 

Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants.

Exergonic vs endergonic energy releases

A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. 

A reaction is said to be exothermic if the reaction releases heat to the surroundings; in the case of endothermic reactions, the reaction absorbs heat from the surroundings.

Energy for reactions

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor e-E/kT  - that is the probability of a molecule to have energy greater than or equal to E at the given temperature T. 

This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation. 

The activation energy necessary for a chemical reaction to occur can be in the form of heat, light, electricity or mechanical force in the form of ultrasound.

A diagram of activation energy activation energy (Ea) and enthalpy of formation (ΔH. image: wikpedia

A related concept free energy, which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in chemical thermodynamics. 

A reaction is feasible only if the total change in the Gibbs free energy is negative,  deltaG ,; if it is equal to zero the chemical reaction is said to be at equilibrium.


For some good examples of calculating energy released in chemical reactions see