Enthalpy equation used in solutions manual

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Hannah Meacham 2A
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Joined: Wed Sep 21, 2016 2:58 pm

Enthalpy equation used in solutions manual

Postby Hannah Meacham 2A » Sun Jan 22, 2017 6:40 pm

For 8.37 in the solutions manual, the equation used to find the enthalpy of vaporization is delta H = q (4.76 kJ) divided by n (.579 mol), but I was wondering which equation this was/where this equation came from to calculate enthalpy? Is this some form of the equation delta H = n*molar heat capacity*delta T?

Jenna_Hakel_2A
Posts: 34
Joined: Fri Jul 15, 2016 3:00 am

Re: Enthalpy equation used in solutions manual

Postby Jenna_Hakel_2A » Mon Jan 23, 2017 12:18 pm

For the enthalpies associated with changes in phase, they are all just given in terms of kJ/mole, so in 8.37, they are simply dividing the number of kJ required to vaporize the .579 moles of CH4 to achieve this value.
The equation q=moles x molar heat capacity x change in temperature wouldn't apply during phase changes because during this time, the sample does not change temperature. The heat that is added to the sample is used to overcome the bonds between molecules, rather than to heat the sample. That's why in a phase change diagram, this part of the diagram is a flat line, rather than having increasing temperature.
Therefore, the enthalpy of vaporization for CH4 doesn't depend on any change in temperature, but rather just how much of the sample there is to be vaporized. That's why it is just kJ/mole.
Hopefully this helped you to figure out the units and the difference between the two equations! :)

Hannah Meacham 2A
Posts: 10
Joined: Wed Sep 21, 2016 2:58 pm

Re: Enthalpy equation used in solutions manual

Postby Hannah Meacham 2A » Tue Jan 24, 2017 1:08 am

Thank you!!


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