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 0^oC, if the heat of fusion, ΔH_fus, of ice is 333 J/g.

q_melting = m x ΔH_fus q_melting = (27.0g)(333 J/g) = 8991 J

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

q_boiling = m x ΔH_vap q_boiling = (27.0g)(2256 J/g) = 60,912 J

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

q_H2O = C_H2O x m x ΔT

q_H2O = (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 0^oC is added to an insulated cup of water containing 123 grams of water at 50^oC. What will be the final thermodynamic equilibrium temperature of the water/ice mixture assuming no heat is lost to the surroundings?

q_{melting +} q_{H2O warming +} q_{H2O cooling} = 0 or Heat_{gain cool water} + Heat_{loss 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