Phase change equations

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Joined: Fri Sep 28, 2018 12:19 am

Phase change equations

Postby JeremyPark14B » Wed Jan 30, 2019 9:19 pm

Why is it that when calculating for the enthalpy of a phase change, it is (moles)(standard enthalpy of phase change) but when calculating a change in temperature, it becomes (grams)(specific heat)?

For example, how much heat is needed to convert 80.0 g of ice at 0.0 C into liquid water at 20.0 C:

the calculations are delta H = (80.0g / 18.02g/mol)(6.01 kJ/mol)

while the the temperature change portion is: delta H = (80g)(4.18 J/C g)(20.0 C - 0.0 C)

why is moles used in the first bit, but not all throughout?

Vincent Li 4L
Posts: 48
Joined: Fri Sep 28, 2018 12:19 am

Re: Phase change equations

Postby Vincent Li 4L » Wed Jan 30, 2019 9:53 pm

The difference really is just a matter of definition. Standard enthalpy of phase change (of formation, bond enthalpy, etc.) or any enthalpy really, is the measurement of energy required to do something to one mole of something, be it breaking a bond, forming the substance from its purest constituents, and so on. Specific heat capacity is defined to be the amount of energy required to raise the temperature of one gram of a substance one degree Celsius. If molar heat capacity were used instead, then it would be per mole instead of per gram.

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