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In: Physics

In each of the following cases, predict the sign of change in entropy by providing proper...

In each of the following cases, predict the sign of change in entropy by providing proper explanation.                                                                                                                                                            (i) Isothermal expansion.


      (ii) Liquefaction of a gas

Solutions

Expert Solution

(i) For isothermal process you can easily predict by using the entropy change formula

S = Q/T

For isothermal process Q = Work done and that can be calculated by

W = nRTdV/V = nRTln(Vf/Vi)

Now for expansion we have Vf/Vi > 1 Thus the W > 0

So, S > 0 (from above equations)

Also you can think it in terms of randomness, now as in expansion the volume will increase but the no. of molecules will still remain same so the randomness of the molecules to move here and there will increase which indicates that the entropy has increased which results in a positive entropy change.

(ii) Now for Liquefaction of gas, you can see that the gas is coming to liquid from gas which means that it has got compressed which means that the gas molecules are getting restricted to move randomly which indicates that the entropy has decreased which results in a negative entropy change.

Also from the formula you can guess that state change from gas to liquid, the gas gives out heat.

SO we have Q<0, => S < 0

genius_generous answered 2 years ago
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