Entropy change when heat is transferred between two metal blocks You probably kn

Question

Entropy change when heat is transferred between two metal blocks

You probably know that when two objects at different temperatures are brought into contact, heat always travels from the hotter object to the colder object; never the reverse.

Imagine that you have two gold blocks, one with a mass 390 g at a temperature of 38.5 oC, and another with a mass 377 g at a temperature of 139 oC. The two blocks are brought into contact with each other in an insulated container.

Specific heat of gold = 0.129 JK-1g-1

1. What is the sign of the entropy change for the heat transfer (enter positive or negative) that takes place?

Sign of entropy change =  

2. What is the final temperature of the two gold blocks?

Final temperature =  oC

3. What is the entropy change for the heat transfer described in this question?

Entropy change =  JK-1

Explanation / Answer

1. positive

2. Finally both the blocks reaches to same final temperature = Tf

m1*s*DT = m2*s*DT

m1 = 390 grams , T1 = 38.5 C

m2 = 377 grams, T2 = 139 C

s = specificheat = 0.129 JK-1g-1

390*0.129*(Tf-38.5) = 377*0.129*(139-Tf)

Tf = final temperature = 87.89 C

3. q hot = (377)*0.129*(139-87.8983)

                   = 2485.23 j

Entropy change = 2485.23/412 = 6.03 j/k

Q cold = (390)*0.129*(87.8983-38.5) = 2485.23 j

Entropy change = 2485.23 / 311.5 = 7.97 j/k

net entropy change = 7.97-6.03 = 1.94 j/k

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