Energetics

Enthalpy Changes

Enthalpy Changes

Exothermic and endothermic reactions

The products and reactants of chemical reactions possess different stabilities. Therefore, a change in potential energy takes place. The total amount of energy is always conserved so any change that occurs in potential energy has to be balanced by an opposite yet equal change in kinetic energy. The lower the chemical potential energy of a chemical species the more stability it has.

In some reactions, the products have more stability than the reactants. In other words, the products possess less potential energy than the reactants and there is a decrease in the potential energy of the reacting species. Due to the fact that total energy is conserved, there must be an increase in the kinetic energy of the species. The particles move faster and there is an increase in temperature.

PE ? KE

A reaction which gives out heat is exothermic.

In other reactions, the reactants have more stability than the products. In other words, the reactants possess less potential energy than the products and there is an increase in the potential energy of the reacting species. Due to the fact that total energy is conserved, there must be a decrease in the kinetic energy of the species. Thus the temperature decreases.

KE ? PE

A reaction which absorbs heat is endothermic.

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Standard enthalpy changes

The enthalpy change for a reaction is the change in chemical potential energy which occurs. It has the symbol ?H.

If the reaction is exothermic the enthalpy change is negative: ?H = -ve
If the reaction is endothermic the enthalpy change is positive: ?H = +ve

Enthalpy change is dependent on reaction conditions. Therefore, standard conditions need to be specified. These are:

  • atmospheric pressure = 1 atm
  • room temperature = 298K

When measured under these conditions, the enthalpy changes measured are called standard enthalpy changes and have the symbol ?H