Chemical energy is the sum of a substance’s potential and kinetic energy. It is also denoted as H. Other synonyms for the term chemical energy include heat content and enthalpy.

For a reaction to occur, reactants must have the activation energy required for the reaction to take place. Activation energy is the energy required to break the bonds of reactants so that a reaction can proceed. Even if the energy of reactants is greater than that of its products and the result is an overall loss of energy, energy still needs to be input initially for the reaction to take place.

The heat of reaction is the energy that is either released or absorbed during the chemical reaction. The heat of reaction is represented as ΔH and has the formula of:

ΔH= H (Products) – H (Reactants)

Exothermic reaction

Exothermic reactions are reactions that involve the release of energy and the H (products) is lower than the H (reactants). The ΔH of an exothermic reaction is always less than 0.

An energy diagram for an exothermic reaction.¹

Endothermic reaction

Endothermic reactions are reactions that involve the absorption of energy and the H (Products) is greater than the H (Reactants). The ΔH of an endothermic reaction is always greater than 0.

An energy profile for an endothermic reaction.²

Thermochemical equations

Thermochemical equations are stoichiometric equations which also show the value of the energy that is either absorbed or released from a reaction as ΔH.

It is important to note that the ΔH doubles with the mole ratio as given by the stoichiometric equation.

If forward/backwards reactions are compared, their absolute values would be exactly the same however they will be the additive inverse/opposite sign (+/-) to each other.

Catalysed reaction pathway

By adding a catalyst to the system, we reduce the activation energy of the reaction which makes the reaction faster. The energy diagram is similar to that of a normal exo/endothermic reaction; the only difference being the peak which describes the activation energy is lowered.

Comparison between catalysed and non-catalysed reaction pathway.³

See Also

1. http://equilibriumchemistry11.blogspot.com.au/2011/02/endothermic-and-exothermic-reactions.html ↩

2. http://equilibriumchemistry11.blogspot.com.au/2011/02/endothermic-and-exothermic-reactions.html ↩

3. https://biochemix.wordpress.com/tag/activation-energy/ ↩