Heat of vaporization and specific heat?

Question

I'm stuck here....

it takes 1440kJ to complete vaporize 10kg of a substance, it occurs at 100 degrees C, and takes 24 minutes. I also have a graph that shows the various changing states. How do I calculate both the specific heat and heat of vapoization?? there may be other info from the graph that could be used..but i think this is the most important stuff...thanks!

Answer 1

kirchwey is correct. The information like so much heat during so much time and so on is of little use if you do not know at what stage one begun to heat the material and when it began to boil. By the way I wonder how does time play role in the statement of the problem.

Let’s look at the equations first

The amount of heat required melting this substance

Q=m Cp(T2-T1) +m Cl
Q - total heat (given)
m –mass of the sample (given)
Cp –specific heat (?)
Cl – latent heat of vaporization (?)
T2 – boiling temperature ( given )
T1 – starting temperature (unknown)

So we have one equations and 3 unknowns. Darn!
Wait a minute didn’t you say you have a graph?

Can you read two point in the middle of this graph I hope it is temperature vs. heat like (T1, Q1) and (T2,Q2)

Because Cp=(Q2-Q1)/(m(T2-T1)) (per unit mass don’t forget)

And the Cp you read right off where you have a sharp rise in heat as temperature remains constant.

Cp= (Q4-Q3)/m

I hope it was useful

Answer 2

This information isn't sufficient to isolate the heat of vaporization from the heat needed to raise the material's temperature by a given amount, from which you could deduce the specific heat. All we know is that the vaporization occurs at 100C but we don't know the starting temperature of the material.

Answer 3

We first need to get the temperature up to 78.3 and then vaporize the ethanol. Get the number of moles of ethanol in 21.9 g of C2H5OH. Call this number N moles. Now lets heat up the ethanol. 21.9 g * (78.3-15.0)oK * 2.44 J/(oK * g) and add the vaporization energy N mol * 38.56 kJ/mol Note that the top part is in J and the bottom is in kJ. Multiply the top equation by 1 kJ / 1000 J to get it into kJ and add.