# Homework Help: Heat and Entropy Change HELPPPP

1. Jan 16, 2009

### Discostu19

Heat and Entropy Change HELPPPP!!

Calculate change in entropy of the universe for:
1] A can of coke, at initial temperature of 5 degrees C, with heat capacity 2500JK-1, is allowed to reach thermal equilibrium with its surroundings at 40 degrees C

2] Two Cans of coke at 5 degrees C and 40 degrees C are placed in thermal contact with each other inside a cool bag where they are thermally insulated from their surroundings and allowed to equilibrate

3]One of the above coke cans at 40 degrees C weighing 250g is dropped from a height of 50m into a shallow pond of negligible depth which is also at 40 degrees C

Attempt

1] using delta S = Cpln(Tf/Ti)
giving 2500xln(40/5)
=5198.6JK-1

ii]
using Tf=(C1T1+C2T2)/C1C2

[2500x40]+[2500x5]/2500+2500
=22.5 degrees C

therefore delta S = C1ln(Tf/T1)+C2ln(Tf/T2)

=2500ln(22.5/40)+2500ln(22.5/5)
=2321.8

so far so gd?

for iii] do i use delta S = P.E / temp of surrounding ?

Last edited: Jan 17, 2009
2. Jan 18, 2009

### MathematicalPhysicist

Re: Heat and Entropy Change HELPPPP!!

For 1), you need to change the tempratures to kelvin units.

For 3), if I understand the question correctly, you use the connection between energy and heat transfer, Q=U+W, U=mgh, the work that being done by the bodies, W, not sure how to get it.
Anyway, dQ=SdT+TdS, there isn't a change in the tempratures, and the change is entropy is what you're after, so dQ=TdS, dS=dQ/T, find the change in the heat transfer and I believe your'e done, not sure though.

3. Jan 18, 2009

### Discostu19

Re: Heat and Entropy Change HELPPPP!!

cheers :)