Phase changes

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chloewinnett1L
Posts: 55
Joined: Fri Sep 28, 2018 12:20 am

Phase changes

Postby chloewinnett1L » Thu Jan 31, 2019 12:24 am

Could someone explain why we need to include the "enthalpy of phase changes" when calculating the amount of heat in a reaction? Could you possibly give an example of how we would apply this concept?

gwynlu1L
Posts: 62
Joined: Fri Sep 28, 2018 12:19 am

Re: Phase changes

Postby gwynlu1L » Thu Jan 31, 2019 3:30 am

I think the easiest way to explain it is that a phase change requires heat to raise the energy of a substance to the melting point/vaporization point, and then once this temperature is reach, the phase change requires more heat to actually change the state of the molecules into the desired phase. Basically, if you try to melt water, first you have to add enough heat to raise its temperature to the melting point, and then you have to add more heat to change the solid into a liquid, since at this point some of the molecules are solid and some are liquid. Think about why vapor burns more than water when at the same temperature, bc vapor has more heat added to it in order to complete the phase change.

Tarika Gujral 1K
Posts: 62
Joined: Fri Sep 28, 2018 12:27 am

Re: Phase changes

Postby Tarika Gujral 1K » Thu Jan 31, 2019 7:29 am

^^ Like Gwyn said, heat is not only needed to raise the temperature of the substance, but it is needed to convert it from solid to liquid and liquid to gas. Heat is applied in order to break the intermolecular forces between molecules. Since more heat is required to completely sever bonds (liquid to gas):
Heat vaporization > Heat of Fusion


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