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I am a little bit confused about when the phase change needs to be taken into account in enthalpy change calculations. I have written in my notes that it is only necessary to include the phase change when there are gases involved. Why is this the case?
Phase change is taken into account when the matter is changing phase. For examples if a problem asks to find the energy from boiling water when you started at a certain temperature. You would calculate q=mc(Tf-Ti) and use the specific heat and then add the heat of vaporization energy which does not have a temperature change and uses heat of vaporization rather than specific heat.
it is taken into account when energy is first required to change the state of matter and then heat the new phase. therefore for example the heating of water from ice takes into account the energy required to melt the ice and then raise the temperature of water.
Also, in addition to what is being said above, something to remember: whenever you're dealing with phase changes, you have to calculate the heat required in 2 different steps. For example, if you're trying to go from a block of ice at 0 degrees to water at 50 degrees, you first need to find the heat of fusion for the 0 degrees ice to melt to 0 degrees water and then in the next step, find the heat to get from 0 degrees water to 50 degrees water.
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