Sciencemadness Discussion Board

Specific Enthalpy of Fusion to Molar Heat of Fusion?

tylerlopez - 15-12-2013 at 09:06

My brother has AP chemistry in high school and he recently performed a thermodynamics lab. In the lab he had to calculate heat of neutralization of NaOH and HCl and molar heat of fusion of ice.

I was able to successfully help him with all questions he had except for the very last segment of the molar heat of fusion of ice calculations.

For all other calculations an equation was given except for the molar heat of fusion.

14. Calculate the specific enthalpy of fusion in J/g°C
Of which he found the answer to be 1540 J/g°C.


15. Using your answer to #14 as your given, calculate the molar heat of fusion for ice. This is the amount of energy needed to raise 1 mole of ice 1°C. your answer should be expressed as J/mol°C.

This is where I am confused. He believes (as he found from an online search) that he should perform the following equation...

q=n(ΔH)

However I don't understand the units there.
q=(25.0g [mass of ice] x 1mol/18.02g [molar mass of water]) x (1540J/g°C)

Wouldn't the resulting answer be in J*mol/g°C?

Perhaps my understanding of units is sloppy, but his answer was vastly off of theoretical. (Which is expected to be about 6,000 J/mol°C.

Your help would be very much appreciated.

Thank you!

PS. I'd like to apologize that my first post on this forum was basic homework-help. I've recently purchased many home lab supplies and I'm very excited to participate with this forum!

smaerd - 15-12-2013 at 10:12

No it won't tyler lopez.

His units are slightly off. The issue is this once he has calculated for the specific enthalpy of fusion all he needs then to do is to convert it too the molar heat of fusion. This does not require the amount of ice melted :).

ΔHmolar = ΔHspecific x molar mass
ΔHmolar =1540J/g water °C x 18.02g water/1mol water

Notice grams cancel and mols in the denominator result.

ΔHmolar = 2780 J/mol °C
=2.780 kJ/mol °C

You're right in that this answer is off from the known value. I would ask him to check his algebra, or maybe the experiment was far from ideal. Both are likely.

tylerlopez - 15-12-2013 at 10:33

Thank you very much! It hit me a few moments earlier that the reason a calculation was not provided was that it was such a simple conversion as to be assumed. The true specific heat of enthalpy is something along the lines of 334J/g water degree C and when that is converted to mols gives the desired answer.

Thank you again and as I said I look forward to participating with this forum.

smaerd - 15-12-2013 at 10:39

Made a calculator error in my last post I'm sure you can figure it out from there though. No problem happy to help.

Magpie - 15-12-2013 at 11:24

Quote: Originally posted by tylerlopez  


For all other calculations an equation was given except for the molar heat of fusion.

14. Calculate the specific enthalpy of fusion in J/g°C
Of which he found the answer to be 1540 J/g°C.


15. Using your answer to #14 as your given, calculate the molar heat of fusion for ice. This is the amount of energy needed to raise 1 mole of ice 1°C. your answer should be expressed as J/mol°C.



The fusion takes place at a given constant temperature, such as O°C for water at 1 bar. But could take place at other temperatures and pressures.

The specific and molar heats of fusion do not include the unit °C. The units should be energy/mass.

Here's a value from Wiki: "The specific enthalpy of fusion of water is 333.55 kJ/kg at 0 °C."