consider the conversion of 2 moles of liquid water at 50 degrees C completely in
ID: 769524 • Letter: C
Question
consider the conversion of 2 moles of liquid water at 50 degrees C completely into steam at 150 degrees C in three steps: i) heating liquid water from 50 degrees C to 100 degrees C ii) conversion of liquid water to steam at 100 degrees C, iii) heating steam from 100 degrees C to 150 degrees C. Assume the pressure is 1 atm throughout. a) use the following info to calculate the enthalpy change for each step and the entire process: Cp,m(H2O,l)= 75.29 J/Kmol Cp,m (H2O,g)=33.58 J/Kmol Delta H vap=40.7 kJ/mol b) what is the work fone by the water during the conversion of liquid water to steam at 100 degrees C? (Hint: the colume occupied by the liquid is negligible compared to the volume occupied by gas) c) what is the change in internal energy, delta U, in the vaporization stage (step ii)Explanation / Answer
FOLLOW THIS any time you get "calorimetry" problems like this one think of these 3 equations 1) heat balance. heat gained by one thing = heat lost by another 2) heat = Q = m Cp dT for a substance undergoing a temperature change WITHOUT a phase change 3) heat = Q = m dHx.. (where x = vaporization, fusion, sublimation,etc) for a substance undergoing a phase change at constant temperature where heat = Q m = mass or moles depending on the units of Cp Cp = heat capacity dT = change in temperature dHvap = enthalpy of vaporization.. or fusion or sublimation or whatever. the idea is we break the process into parts. Steam cools as a vapor until it reaches it's boiling point.. m Cp dT.. Then it condenses at constant T.. m dHvap.. Then it cools as liquid water.. m Cp dT again.. Then at it's freezing point, it freezes at constant T.. m dHfus.. then it cools futher m Cp dT like this. ************ Qtotal = Qsteam cooling + Qwater condensing + Q liquid water cooling + Q water freezing + Q water ice cooling ie.. Qtotal = (m Cp dT) steam + (m dHvap) + (m Cp dT) liquid cooling + (m dHfus) + (m Cp dT) ice. where.. m = 50g Cp steam = 36.5 J/moleK x (1mole / 18.02g) x (1K/1
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