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What is enthalpy?
The enthalpy, H, of a system is a measure of the energy
stored in (or heat content of) a system. It cannot be
measured directly.
During reactions, the
enthalpy of the reactants and
the products is not the same.
This results in energy being
either given out or taken in
during the reaction. This
energy is the enthalpy
change, H (`delta H').
The enthalpy change for a reaction is usually observed as a
change in temperature, which can be measured or calculated.
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Enthalpy changes
The enthalpy change of a reaction is the heat energy
exchange with its surroundings at constant pressure.
Enthalpy is the energy content of the reactants and is
given the symbol H.
In science, change is represented by the upper case
Greek letter delta, .
Therefore, enthalpy change is represented by H. It has
the units kilojoules per mole (kJ mol-1).
Standard enthalpy changes are measured at a standard
pressure of 100 kPa and temperature of 298 K. Standard
enthalpy changes are represented by H298 but this is
usually shortened to H.
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Exothermic reactions
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Endothermic reactions
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