Two blocks of the same metal having same mass and at temperature T1 and T2 respectively, are brought in contact with each other and allowed to attain thermal equilibrium at constant pressure. The change in entropy, △S for this process is
A
2Cpln[T1+T22T1T2]
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B
Cpln[(T1+T2)24T1T2]
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C
2Cpln[(T1+T2)24T1T2]
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D
2Cpln⎡⎢
⎢
⎢
⎢⎣(T1+T2)124T1T2⎤⎥
⎥
⎥
⎥⎦
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Solution
The correct option is BCpln[(T1+T2)24T1T2] As the two boxes are identical, So Cp will be same for both and Tfinal=T1+T22
Entropy change in the first box will be given by △S1=CplnTfTisince,Ti=T1△S1=CplnTfT1
Entropy change in the second box will be given by △S2=CplnTfTisince,Ti=T2△S2=CplnTfT2
Total entropy change will be given by addition of above two. △S=△S1+△S2=CplnT2fT1T2 putting the value of Tf △S=Cpln[(T1+T2)24T1T2]