# Calculating the Specific Latent Heat of Fusion from a graph

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Thread starter 3 years ago
#1
I am trying to find the Specific Latent heat of fusion for Stearic Acid as an experiment for College.

I have googled a similar graph to what I have from my data:

I have to get an approximate value for the Specific Latent heat of fusion from this graph. The tutor said I can cut out the part where temperature remains constant and take a gradient just before it reaches this phase change.

Then use the gradient in: E = (m)(c)(T/t)(time spent at equilibrium phase)

then plug that energy value into the equation: E = m Lf

I don't really understand the first step, is the gradient multiplied by time somehow representing an approximation of temperature change?
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3 years ago
#2
Well if it's safe to assume that heat is being removed at a constant rate then the gradient in the non phase changing areas will be proportional to that rate.

the rate will be a number of Joules per second... which is also Watts

when the sample IS going through a phase change the temperature will stay constant while heat is being removed at the same rate as before

the amount of heat removed will be the rate of heat removal found above multiplied by time for which temperature is constant*

*(for the same reason that the distance travelled in one hour at a constant rate of 60MPH is 60 miles)
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Thread starter 3 years ago
#3
(Original post by Joinedup)
Well if it's safe to assume that heat is being removed at a constant rate then the gradient in the non phase changing areas will be proportional to that rate.

the rate will be a number of Joules per second... which is also Watts

when the sample IS going through a phase change the temperature will stay constant while heat is being removed at the same rate as before

the amount of heat removed will be the rate of heat removal found above multiplied by time for which temperature is constant*

*(for the same reason that the distance travelled in one hour at a constant rate of 60MPH is 60 miles)
Thankyou for the reply.

So even though heat isn't being removed at a constant rate, we are assuming it is nearly constant at the point just before the acid is changing phase?

So far this is what i'm visualising. The brown line being the gradient i'm taking and the red lines indicating the time of phase change?
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3 years ago
#4
(Original post by Sir_Malc)
Thankyou for the reply.

So even though heat isn't being removed at a constant rate, we are assuming it is nearly constant at the point just before the acid is changing phase?

So far this is what i'm visualising. The brown line being the gradient i'm taking and the red lines indicating the time of phase change?
I think you've drawn the tangent a bit to the right of where it should be, IMO you want something that represents the average gradient from 90 to 70 ish degrees and you seem to have gone on the 'elbow' into the phase change.

never done this experiment though tbh.
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