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Exothermic and endothermic reactions

A reaction that gives off heat when it occurs is said to be exothermic. The change in enthalpy, ΔH, of an exothermic reaction is negative. This may seem counter-intuitive, but remember that the properties of a system are seen from the systems point of view. Since the system looses heat (by giving it off to the surroundings) the change in enthalpy is negative. A negative ΔH value means that the enthalpy of the products is lower than the enthalpy of the reactants.

The image shows graphs of potential energy change versus reaction progress for exothermic reaction on the left and endothermic reaction on the right side. The energy change for exothermic reaction starts at the horizontal line of the energy of reactants, which increases to its peak and decreases again to another horizontal line indicating the energy of products. The initial energy of reactants is higher than the energy of the products. The vertical line from the energy of reactants and the peak indicates activation energy, whereas the vertical line from the energy reactants to energy of products indicates the energy released in the reaction. The energy change for endothermic reaction starts similarly on the horizontal line of the energy of reactants, increases to the peak and decreases again to energy of the products, but this time this energy is higher than the energy of the reactants. The vertical line from the energy of reactants and the peak indicates activation energy, whereas the vertical line from the energy reactants to energy of products indicates the energy absorbed in the reaction.

Figure 1. The energy diagram of endothermic and exothermic reactions.

The opposite is the case for a reaction that consumes heat when it occurs. Such a reaction is said to be endothermic. The enthalpy change, ΔH, of an endothermic reaction is positive, because heat is applied to the system. A positive ΔH value means that the enthalpy of the products is higher than the enthalpy of the reactants.

If a chemical reaction is exothermic going in one direction, it will be endothermic going in the opposite direction. The magnitude of the change in enthalpy, ΔH, will be the same, but with reversed number sign.

Referred from:

Article
Enthalpy
Article
Potential energy diagram