State Functions: Example Problems

Try to solve these problems before watching the solutions in the screencasts.

Example Problem 1

One kg of liquid water at 40°C is in a flask that is connected, through a valve, to a vacuum pump. Water evaporates when the valve is opened. The water vapor is removed fast enough that the flask can be considered adiabatic. As water evaporates, the remaining liquid water cools, and when it reaches 0°C, it starts to freeze. What fraction of the water will have evaporated when the rest is frozen?

Example Problem 2

Calculate the heat of reaction for ammonia formation at 175°C from the heat of reaction at 25°C. \[N_2 + 3H_2 \rightarrow 2NH_3 \hspace{5mm}  \Delta H_{f,25^\circ C} \]\[C_{P,N_2} = 29\, J/mol\cdot K\hspace{3mm} C_{P,H_2} = 29\, J/mol\cdot K \hspace{3mm} C_{P,NH_3} = 35.7\, J/mol\cdot K\]