Q:

How do you calculate enthalpy?

A:

Enthalpy is calculated by summing up the energy changes taking place in individual steps of a chemical process. To arrive at the enthalpy changes, Hess’s law is employed. It states that the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.

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Enthalpy refers to the internal energy of a chemical reaction in addition to the product of pressure and volume of the system. Whatever routes are used to convert reactants into products, the enthalpy changes between those routes should be equal at constant pressure, irrespective of whether these changes are positive or negative. If extra energy is introduced at some stage of the reaction, it is compensated in another stage.

It does not matter how many steps are followed to arrive at the chemical change. When a liquid is heated and turns into vapor, it must absorb heat from the atmosphere to replace the energy taken by the vaporizing molecules. The heat needed to vaporize the liquid is referred to as enthalpy of vaporization. Similarly, when a solid melts, the energy required is called enthalpy of fusion. Other enthalpy energies are the enthalpy of sublimation, the enthalpy of freezing and the enthalpy of condensation. The SI unit for measurement of enthalpy is the joule.

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