# Heat of fusion?

1. Feb 26, 2006

### metalmagik

I have a graph here separated into 5 different parts (A-B, B-C, C-D, D-E, E-F) It is a curve going upwards...I just need to know how to calculate the heat of fusion of the substance using the curve in the graph...I can use the formula and calculate the heat of fusion for each little piece but...do I add them after that? I'm just not really sure...any help is once again appreciated, if you need me to clarify or draw the graph I will, gladly. Thank you.

2. Feb 26, 2006

### Staff: Mentor

What is the curve?

http://en.wikipedia.org/wiki/Heat_of_fusion

and perhaps better - http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase2.html#c1

3. Feb 26, 2006

### metalmagik

It's a positive curve going upwards but its not really a curve it has constants in temperature every now and then but its going positive. I still dont understand how to interpret graphs from those links haha im sorry please help me

4. Feb 26, 2006

### Staff: Mentor

During a phase change, temperature is essentially constant. The energy goes into transforming from solid to liquid.

What are the abscissa (x or horizontal scale) and ordinate (y or vertical scale) of the graph?

If the graphs show rise - constant - rise - constant - rise, then it may correspond to solid - melting - liquid - vaporizing - vapor (gas). The first constant temperature interval may coincide with the heat of fusion.

5. Feb 26, 2006

### metalmagik

ah yes thats how it is exactly...thank you...how do i calcualte the heat of fusion of the unknown substance by simply using that? There is also a chart which asks me for the change on kinetic energy and the change in potential energy how do i do that!

6. Feb 26, 2006

### metalmagik

By the heat of fusion i mean for the ENTIRE graph (the whole substance and not just for the different segments

7. Feb 26, 2006

### Staff: Mentor

Answer this question - What are the abscissa (x or horizontal scale) and ordinate (y or vertical scale) of the graph? - and I will try to explain it. If the constant parts are horizontal, then the ordinate is temperature. If the constant parts are vertical then the abscissa is temperature.

Kinetic or potential of what?

Increasing a temperature of a substance increases the kinetic energy of the molecules, more so for vapor than liquid, and more so for liquid than solid. Increasing temperature also increases the potential energy or potential to do work.

8. Feb 26, 2006

### metalmagik

ordinate is temperature yes and abscissa is time in minutes. it is asking for the delta KE and delta PE for each line segment. How would I calculate this?

9. Feb 26, 2006

### Staff: Mentor

Heat of fusion applies only to the energy absorbed when a substance melts, i.e. changes from solid to liquid or liquid to solid.

Heat in to a material would imply solid to liquid transformation. Heat out (removal) would imply 'freezing' or transformation from liquid to solid.

Unless, one has two different substances which melt at two different temperatures.

10. Feb 26, 2006

### metalmagik

So does that mean that I can only calculate heat of fusion for those parts?

11. Feb 26, 2006

### Staff: Mentor

What other information is given.

If one is given a heat rate (or power, which = energy/time) then simply integrate the area of power * time to get energy, and the energy in the constant period would be heat of fusion for the solid to liquid transformation.

Is there a discussion in your text on the change in kinetic energy or potential energy as a function of temperature in a liquid or vapor?

Are you looking at the kinetic or potential energy from a molecular perspective?

See this - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html#c2

12. Feb 26, 2006

### metalmagik

I do not know how I am looking at the KE or PE...i have only looked through packets and sheets to understand this topic becuse I Was given no instruction. but so heat rate = energy/time...and if I find heat of fusion for each segment is THAT the energy? if so then I will be able to do this

13. Feb 26, 2006

### Staff: Mentor

If there are two constant temperature parts, then I expect one is for melting (heat of fusion) and the other is for boiling or vaporization (heat of vaporization).

Yes, power (heat rate) = energy/time, so power * time = energy. Time is the time interval (e.g. t2-t1) over which heat is added (or subtracted if heat is removed).

A temperature increase would result in a change in potential energy or possibly kinetic energy, but I would need further information.

Last edited: Feb 26, 2006
14. Feb 26, 2006

### metalmagik

Oh ok thank you, yes, I understand the heat of fusion and vaporization now.

15. Feb 26, 2006

### metalmagik

oh wait im sorry it tells me the energy here, 200 J/minute

16. Feb 26, 2006

### metalmagik

I can figure out the PE and KE thank you very much...there is another question concerning the Average KE of molecules in a larger solid compared to average KE of molecules in a smaller solid. It also says to compare their internal energies. Block A is 1 kg, Block B is 1 Gram, both temperatures are at 300 K

17. Feb 26, 2006

### Staff: Mentor

18. Feb 26, 2006

### Staff: Mentor

200 Joules/minute is the heat rate or power going into the substance. Make sure the time is compatible, i.e. in minutes.

The energy is just the area under the curve.

If the temperature is constant for 10 minutes, then at 200 J/min, 2000 J would be put into the substance.

Do you have a mass or number of moles into which the heat is input? Usually one works with energy/unit mass.

19. Feb 26, 2006

### metalmagik

the mass is 10 kg, i dont have a number of moles...im still trying to find the KE of this substance, what exactly was the formula again? I remember you told me power = energy/time and i found all of that but how do I get the KE and PE?

20. Feb 26, 2006

### Staff: Mentor

Just use the mass. Heat of fusion is often given by energy/unit mass.

See examples - http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase2.html#c1

How are potential and kinetic energy defined? Certainly increasing the temperature of a substance increases the molecular kinetic energy, as well as the potential energy.