# Thermodynamics - obtaining quenching entropy

I'm having a problem determining the total entropy change in a metal quenching problem. Here is the information I am given (subscript 'm' is for metal, 'w' is water):

mm = 20 kg
mw = 1000 kg
T1m = 800 C
T1w = 30 C
Cpm = .4 kJ/kg K
Cpw = 4.18 kJ/kg K

With this information, I found T2 to be 304.6 K.

How can I obtain the entropy change with this information? I have no pressure values, and all my entropy equations involve pressure, or specific volume which depends on pressure. Thanks for any help.

Mapes
Homework Helper
Gold Member
Do you know of an equation that connects dS to Cp that you might be able to integrate?

I did find a relationship, but it is returning negative values for me, so something is amiss...

s2 - s1 = Cp ln(T2/T1)

I'm trying to find the total entropy change. Would that mean that I sum the entropy change in the quenched metal and the water?

stewartcs
Would that mean that I sum the entropy change in the quenched metal and the water?
Yes.

CS

Mapes
Homework Helper
Gold Member
Yes. It's not unusual for entropy to decrease; it happens whenever a hot object cools down.

stewartcs
I did find a relationship, but it is returning negative values for me, so something is amiss...

s2 - s1 = Cp ln(T2/T1)
You'll need to consider the mass of each as well in your equation.

CS

stewartcs