calculating phase change
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calculating phase change
Whats the general rule/equation for calculating heat required for a phase change? I think in a UA review session they added ndeltaH to q, is that what you guys are doing to or is there another way?
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Re: calculating phase change
I'm pretty sure that's generally how you would do it. I was watching some Khan Academy review videos today and in one of them the guy said you need to calculate how much heat is required to first bring the substance up to the boiling point (or melting point), and then calculate how much heat you need for the phase change itself, which would be n*delta H, as you said. And then, if you wanna heat it even more, like if you wanna vaporize water and then also bring it up to 120 degrees celsius for example, then you need to calculate the heat required to raise it to that temperature and add them all together.
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Re: calculating phase change
Yes, ndeltaH represents the heat transferred for the phase change and q would represent the heat transferred as a result of the increase or decrease in temperature.
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Re: calculating phase change
Yes adding ndelta H to q is the way to find the heat required for phase change.
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Re: calculating phase change
Correct add delta H to q. I don't think that's really easy to remember or understand though so I would explain it this way. Its the sum of the heat required to reach the temperature of the phase change and the enthalpy of the phase change. This makes complex problems like one of our homework problems easier to complete. In that problem you had to take the heat + the enthalpy and then you had to subtract the heat to cool the gaseous phase of the substance. If you split that into steps it becomes really easy.
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Re: calculating phase change
you need to know whether you are vaporizing, or fusing the molecule. If vaporizing you need the (heat of vaporization)(moles of the molecule)=q, and for fusing use the same equation, but you need the heat of fusion.
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Re: calculating phase change
Make sure the enthalpy given is the enthalpy of that phase change at some constant temperature, then just add it to your other "q" values for your total heat transfer value.
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