Boiling point and Delta H of fusion

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Vera Ong 3H
Posts: 26
Joined: Fri Jun 17, 2016 11:28 am

Boiling point and Delta H of fusion

Postby Vera Ong 3H » Fri Mar 17, 2017 1:32 pm

Do we generally take account for the enthalpy of fusion when boiling a liquid? In one of the practice questions, it doesn't include the enthalpy of fusion to calculate the total amount of heat. The solutions only calculated for the change of temperature. why is this?

Pauline Tze 3B
Posts: 57
Joined: Sat Jul 09, 2016 3:00 am
Been upvoted: 1 time

Re: Boiling point and Delta H of fusion

Postby Pauline Tze 3B » Fri Mar 17, 2017 2:09 pm

Hi Vera,

You account for the enthalpy of fusion when the state of your system is going from a solid to a liquid. I assume you mean enthalpy of vaporization, which you would account for when the state is changing from a liquid to a gas. I don't know what question you are referring to, but I suggest looking carefully to see if the solutions are actually changing state. For example, are we heating the liquid just to boiling point? It hasn't started changing state yet!

I hope that helps. If not, I suggest posting your specific question.

Vera Ong 3H
Posts: 26
Joined: Fri Jun 17, 2016 11:28 am

Re: Boiling point and Delta H of fusion

Postby Vera Ong 3H » Fri Mar 17, 2017 3:12 pm

"suppose you want to use methane gabs (delta h of combustion = -890. kJ/mol^-1) to boil 50.0 mL of water at 25.0 degrees celsius. how many grams would need to be combusted to generate the required heat?"

when it says boiled, are we supposed to interpret this as all of the water turning into gas? because if this were the case, we would use delta h of vaporization. the solutions say we just account for the temperature change though

Pauline Tze 3B
Posts: 57
Joined: Sat Jul 09, 2016 3:00 am
Been upvoted: 1 time

Re: Boiling point and Delta H of fusion

Postby Pauline Tze 3B » Sat Mar 18, 2017 2:07 pm

Based on this question, it looks like the grams of gas are being combusted so you would need to use delta h to get the heat generated from that. It can't be simply a temperature change.


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