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what is the definition of Enthalpy?

When we talk of enthalpy, we talk about enthalpy of reaction i.e. heat of reaction or heat changes.

 Enthalpy of the reaction is the amount of heat evolved or absorbed in a chemical reaction when the number of moles of reactants as represented by the chemical equation have completely reacted.

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Enthalpy just simply means energy

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Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

Enthalpy is a thermodynamic potential. It is a state function and an extensive quantity. The unit of measurement for enthalpy in the International System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie.

 

Enthalpy =A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume .

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Enthalpy is a thermodynamic state function that describes (in quantity) changes in heat under constant pressure.This is really neat because most reactions usually occur understand constant pressure because everyday we experience atmospheric pressure conditions which don't fluctuate wildly.Enthalpy is a state function which means that it is only dependent on change between initial and final state. That means' the reaction doesn't matter. Enthalpy change is what's important.Enthalpy is calculated by the difference between enthalpy of products and enthalpy of reactants. From enthalpy you can determine whether you're reaction releases heat (exothermic) or if it absorbs heat (endothermic). Exothermic is denoted by a negative enthalpy and Endothermic is denoted by a positive enthalpy.

Usually it is impossible to calculate enthalpy directly. However a change in enthalpy in reactions can be measured.

Change in enthalpy for a reaction = (sum of the products' standard heat of formation sum of the reactants' standard heat of formation)

Standard heat of formation (H(f)) is just the enthalpy change which would occur if 1 mol of the compound is formed from its elements.

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