The theory behind enthalpy changes

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The theory behind enthalpy changes

Exothermic reactions are most common, however, an important example of an endothermic reaction is photosynthesis in plants, where the energy supplied is from sunlight.

Law of conservation of energy: Energy cannot be destroyed or created but only transferred from one form to another. The total energy of a system of reacting chemicals and surroundings remains constant.

Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol DH.

Enthalpy is the total energy content of the reacting materials. It is given the symbol, H.

DH = DH products - DH reactants

The units are kilojoules per mole (kJmol-1)

An exothermic enthalpy change is always given a negative value, as energy is lost to the surroundings.
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DH = -xkJmol-1

An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings.

DH = + ykJmol-1.

Standard enthalpy changes: standard conditions

If we are to compare the enthalpy changes of a various reactions we must use standard conditions, such as known temperatures, pressures, amounts and concentrations of reactants or products.

The standard conditions are:

A pressure of 100kilopascals (102kPa)

A temperature of 298K (25oC)

Reactants and products in physical states, normal for the above conditions.

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