CHM 2045C Module Seven Sample Exam Answers

Module Seven Part D: Enthalpy change with Phase Change/Ice Cube Problem    5 points

Phase Change (3 points):

1. Calculate the amount of heat necessary to melt 27.0 grams of ice at 0oC, if the heat of fusion, ΔHfus, of ice is 333 J/g.

qmelting = m x ΔHfus           qmelting = (27.0g)(333 J/g) = 8991 J

If I had the same amount of water at 100oC, calculate the amount of heat required to boil 27.0 grams of water if the heat of vaporization  ΔHvap  of water is 2256 J/g?

qboiling = m x ΔHvap           qboiling = (27.0g)(2256 J/g) = 60,912 J

How much heat is required to raise the temperature of the 27 grams of water at 0oC to 100oC, if the specific heat of water is 4.184 J/goC

qH2O = CH2O x m x ΔT

qH2O = (4.184 J/g oC) (27.0 g) (100 oC)  =  11,297 J

Ice Cube Problem (2 points):

If 27.0 grams of ice at 0oC is added to an insulated cup of water containing 123 grams of water at 50oC. What will be the final thermodynamic equilibrium temperature of the water/ice mixture assuming no heat is lost to the surroundings?

qmelting + qH2O warming   + qH2O cooling   = 0    or   Heatgain cool water  +  Heatloss warm water = 0

From the above examples

(27.0g ice)(333 J/g) + (27.0 g)(4.184 J/g oC)(T – 0) oC + (123g) (4.184 J/g oC )(T – 50) oC = 0

multiple equation by  oC; divide equation by J to simplify units:

8991 oC + 113.0T + 514.6T  25732 oC = 0

627.6 T – 16741 oC = 0

627.6 T = +16741 oC

T = 26.7 oC