This page explains what an enthalpy change is, and then gives a definition and brief comment for three of the various kinds of enthalpy change that you will come across.
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Standard enthalpy changes
Standard enthalpy changes refer to reactions done under standard conditions, and with everything present in their standard states. Standard states are sometimes referred to as "reference states".
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Standard enthalpy change of reaction, ΔH°r
Remember that an enthalpy change is the heat evolved or absorbed when a reaction takes place at constant pressure. The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state.
That needs exploring a bit. Here is a simple reaction between hydrogen and oxygen to make water:
When you are writing one of these equations for enthalpy change of formation, you must end up with 1 mole of the compound. If that needs you to write fractions on the left-hand side of the equation, that is OK. (In fact, it is not just OK, it is essential, because otherwise you will end up with more than 1 mole of compound, or else the equation won"t balance!)
The equation shows that 286 kJ of heat energy is given out when 1 mole of liquid water is formed from its elements under standard conditions.
Standard enthalpy changes of formation can be written for any compound, even if you can"t make it directly from the elements. For example, the standard enthalpy change of formation for liquid benzene is +49 kJ mol-1. The equation is: