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If you are calculating the change in enthalpy of a substance and that substance changes phases then it is important to add the energy required by the phase change in order to get an accurate calculation for ΔH.
When you are calculating the enthalpy of a reaction, sometimes the reactants or products will be in the liquid form. Since predetermined enthalpy values of elements are usually given in the gas phase, you must add/subtract the enthalpies of the phase changes to these values to determine what they are in a different state so that the overall enthalpy change of the reaction can be calculated.
I think also phase change helps orient what values will be positive and negative. You need to take phase changes into account when calculating the change of enthalpy in a reaction (the calculations we use assume that all the reactants and products are gases).
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