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Postby LBacker_2E » Thu Feb 06, 2020 3:52 pm

When you are determining the change in enthalpy, I know you have to take into account any phase changes. I know with bond enthalpies you add the delta H of the phase change(s) (to/from gas form) to the delta H of bonds broken to find the delta H of the reaction. But, when you are trying to determine the delta H using the specific heat capacity (which is equal to q), are phase changes already accounted for.
I guess what I am asking is does q include phase changes, or are they added to q to find the total delta H?

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Re: equation

Postby SnehinRajkumar1L » Thu Feb 06, 2020 5:44 pm

No, you must add the heat of phase changes. q = mC(Tf - Ti) only calculates how much energy it takes to increase a mass of a substance by a certain amount of degrees.

Sanjana Munagala_1j
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Re: equation

Postby Sanjana Munagala_1j » Fri Feb 07, 2020 10:44 am

Like the previous commenter said, q is only the heat released or absorbed due to a temperature change. In order to account for a phase change you need to use the enthalpy of vaporization or fusion, depending on what a question is asking you. For example, if a question asked about the enthalpy for a reaction that involves ice at -3 degrees Celsius being melted to liquid water to the temperature at 25 degrees Celsius, then you have to first calculate the heat for the temperature change from -3 to 0(using q=mct) and then add it will with the enthalpy of fusion and then finally add the heat for the temperature change from 0 to 25 (using q=mct).

Hope that helps!

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