Entropy of a system water-reservoir

I’m studying for my exam for tomorrow and we solved an exercise in class , but a question was not answered and I don’t know how to solve it.
1. The problem statement, all variables and given/known data

1 Kg of water is heated at 0 degree C is brought into contact with a large heat reservoir at 100 degrees C. When the water has reached 100 degrees C, what has been the change in entropy of the water? And of the heat reservoir ? what has been the change in the entire system consisting of both water and the heat reservoir?

Show that we can heat water ( from 0°c to 100°C) without changing the entropy of the whole system.
2. Relevant equations

3. The attempt at a solution
For the first questions we have
ΔS(water)=4180*ln(373/273) J
ΔS(reservoir)=-418000/373 J

I don’t get it how we can heat the water without changing the entropy ..? :confused:


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