You make tea with 0.25 kg of 85 C water and let it cool to room temp (20C) a) Th

Question

You make tea with 0.25 kg of 85 C water and let it cool to room temp (20C) a) The water does not change its volume significantly during the cooling process. Calculate the change in entropy of the water during the process.b )The cooling of the tea is essentially an isothermal process for the air in the room. Assuming the heat lost by the water goes into the air, calculate the change in entropy of the air during the process. C) What is the total change in entropy for the tea room together? The specific heat of the water is 4190 L/kgK.

Explanation / Answer

a.

Change in entropy is.   

dS = mcpln(T2/T1) = 0.25*4190*ln(358./293)=-209.88 J!

The heat loss is

Q=mcpdT =0.25*4190*(358-293)=68087.5 J

Change in entropy is

ds= Q/T2= 232.38 J/K

Total entropy is

dS = 22.5 J/K

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