# Molar Heat of Fusion for Water

Determination of the Molar Heat of Fusion for Water and Heat of Solution of a Solid

Introduction to Thermochemistry & Heat:

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1) Thermochemistry is one area of chemistry dealing with the quantitative analysis of heat absorbed or evolved by physical and chemical processes.

2) Heat (q) can be defined as the flow of energy into and out of a system due to a temperature difference between the thermodynamic system and its surroundings.

3) Heat energy will flow from the region of higher temperature to the region of lower temperature until thermal equilibrium has been reached.

a. At thermal equilibrium, the temperature of the two regions will be equal at any one time.

4) Heat & temperature are both measures of kinetic energy but…

a. Heat measures total energy.

b. Temperature measures the average energy.

c. Example to distinguish these two terms: If you had a teaspoonful of boiling water versus a cup of boiling water (both at 100oC), the cup of boiling water would be more painful to spill on your hand because it has more heat (total energy).

5) The formula for heat is as follows: q = s ( m ( Δt

a. q = heat, s = specific heat, m = mass, Δt = change in temperature.

i. Specific heat for water = 4.184 J/g ( oC.

Introduction to Heat of Fusion:

1) The heat needed to melt 1 mole of a pure substance is called the heat of fusion and denoted as ΔHfus.

2) To melt a pure substance at its melting point requires an extra boost of energy to overcome lattice energies.

3) The heat of fusion (ΔHfus) can be calculated.

b. At constant pressure, the heat exchanged is equal to the heat of fusion:

i. qp = ΔHfus.

Introduction to Heat of Solution:

1) A solution is a homogeneous mixture of two or more substances, consisting of ions or molecules.

2) The solute is the dissolved substance for gases or solids in a liquid solution.

c. In other cases, it is the component that is in a smaller amount.

3) The solvent is the liquid substance in which the solids or gases are dissolved.

d. In other cases, it is the component that is in a larger amount.

4) When a solid dissolves in water, an energy change will always result.

e. Heat can be evolved or absorbed when an ionic compound dissolves in water.

5) The dissolving process occurs in two distinct steps:

f. The first step is the breaking down of the solid crystal lattice.

i. This process is endothermic (absorbing energy).

g. The second step is the hydration of the individual particles by the solvent.

i. This process is exothermic (releasing energy).

6) If the first step is dominant in the course of dissolving, then the process will be endothermic.

7) If the second step is dominant in the course of dissolving, then the process will be exothermic.

8) The heat of solution (ΔHsol) can be calculated.

h. At constant pressure, the heat exchanged is equal to the heat of solution:

i. qp = ΔHsol

Substances to include in the Table of Chemical and Physical Properties:

Include the molar mass of the following substances ammonium nitrate, calcium chloride, and water.

Procedure for the Molar Heat of Fusion of Water:

1) Place about 400 – 500 mL of deionized water into a 600 mL beaker and place onto a hotplate.

a. Heat up the water to between 60oC and 70oC.

2) Obtain the mass of two clean and dried empty Styrofoam cups with the top.

a. These two cups should be stacked on top of one another and the top should have a hole in it to facilitate the use of your thermometer.

3) Add enough warm water to the pre–weighed set of cups to fill between 65–75% full.

4) Obtain the mass of the set of cups containing the warm water with the cap on top.

a. Ensure that no water is clinging to the outside walls of the Styrofoam cups.

5) Measure the temperature of the warm water to the nearest tenth to hundredth of a degree depending on the thermometer calibrations (0.1oC – 0.01oC).

6) Immediately following this measurement, add enough ice to cover the surface of the water.

a. Ensure that you do not allow any of the warm water to splash out.

7) Replace the top and gently stir the mixture with the thermometer.

8) Once the ice has melted and the temperature has stabilized, record the final temperature of the cool water to the nearest tenth to hundredth of a degree depending on the thermometer calibrations (0.1oC – 0.01oC).

9) Following this measurement, remove the thermometer and record the mass of the cool water in the set of cups with the top.

10) Repeat this experiment (steps 1 – 9) for a second trial.

Procedure for Heat of Solution of a Solid:

1) Obtain the mass of two clean and dried empty Styrofoam cups with the top.

2) Measure out approximately 10 – 15 grams of solid ammonium nitrate. Record the mass.

3) Add between 150 – 200 mL of deionized water to the Styrofoam cups.

4) Obtain the mass of the set of cups containing the water with the cap on top.

5) Measure the temperature of the water to the nearest tenth to hundredth of a degree depending on the thermometer calibrations (0.1oC – 0.01oC).

6) Immediately following this measurement, add the ammonium nitrate to the set of cups.

7) Replace the top and gently stir the mixture with the thermometer.

8) Once the solid has completely dissolved and the temperature has stabilized, record the final temperature of the cool water to the nearest tenth to hundredth of a degree depending on the thermometer calibrations (0.1oC – 0.01oC).

9) Following this measurement, rinse out and completely dry the set of cups.

10) Repeat this experiment with 10 – 15 grams of anhydrous calcium chloride.

Clean – Up:

1) All water and salt solutions can be disposed of down the sink.

Sample Data Table for the Molar Heat of Fusion of Water:

a) Mass of Styrofoam cups with top in grams:

Trial 1: ________ Trial 2: ________

b) Mass of Styrofoam cups with top plus warm water in grams:

Trial 1: ________ Trial 2: ________

c) Mass of warm water in grams (b – a):

Trial 1: ________ Trial 2: ________

d) Stable temperature of warm water in oC:

Trial 1: ________ Trial 2: ________

e) Stable temperature of warm water in Kelvin:

Trial 1: ________ Trial 2: ________

f) Stable temperature of cool water in oC:

Trial 1: ________ Trial 2: ________

g) Stable temperature of cool water in Kelvin:

Trial 1: ________ Trial 2: ________

h) Temperature change of warm water in oC (f – d):

Trial 1: ________ Trial 2: ________

i) Mass of Styrofoam cups with top plus cool water in grams:

Trial 1: ________ Trial 2: ________

j) Mass of cool water in grams (i – a):

Trial 1: ________ Trial 2: ________

k) Mass of ice in grams (j – c):

Trial 1: ________ Trial 2: ________

l) Amount of heat lost (in Joules) by the warm water†:

Trial 1: ________ Trial 2: ________

m) Amount of heat gained (in Joules) by the ice‡:

Trial 1: ________ Trial 2: ________

n) Amount of heat (in Joules) that melted ice (‌‌‌‌‌‌‌‌‌‌‌|l| – |m|):

Trial 1: ________ Trial 2: ________

o) Experimental heat of fusion (in Joules/gram) of water (n/k):

Trial 1: ________ Trial 2: ________

p) Experimental molar heat of fusion (in Joules/mole) of water (o•Mm):

Trial 1: ________ Trial 2: ________

q) Experimental molar heat of fusion (in kilojoules/mole) of water (p/1000):

Trial 1: ________ Trial 2: ________ Average: ________

r) True molar heat of fusion (in kilojoules/mole) of water *: _____________

s) Percent error for molar heat of fusion of water: _____________

Sample Data Table for Heat of Solution of NH4NO3:

a) Mass of Styrofoam cups with top in grams: _____________

b) Mass of NH4NO3 in grams: _____________

c) Mass of Styrofoam cups with top plus water in grams: _____________

d) Mass of water in grams (c – a): _____________

e) Stable temperature of water in oC: _____________

f) Stable temperature of in Kelvin: _____________

g) Stable temperature of water after solid addition in oC: ____________

h) Stable temperature of water after solid addition in Kelvin: ____________

i) Temperature difference in oC (g – e): _____________

j) Temperature difference in Kelvin (h – f): _____________

k) Amount of heat lost or gained (in Joules) by the water†: _____________

†q = (b+d)( 4.184 J/g ( oC)(i)

l) Amount of heat lost or gained (in Joules) by NH4NO3††: _____________

††qH2O = –qNH4NO3

m) Number of moles of NH4NO3 used (b/Mm): _____________

n) Experimental heat of solution (in J/mol) of NH4NO3 (l/m): ____________

o) Experimental heat of solution (in kJ/mol) of NH4NO3(n/1000):_________

p) True heat of solution (in kJ/mol) of NH4NO3*: _____________

q) Percent error for heat of solution of NH4NO3: _____________

Sample Data Table for Heat of Solution of CaCl2:

a) Mass of Styrofoam cups with top in grams: _____________

b) Mass of anhydrous CaCl2 in grams: _____________

c) Mass of Styrofoam cups with top plus water in grams: _____________

d) Mass of water in grams (c – a): _____________

e) Stable temperature of water in oC: _____________

f) Stable temperature of in Kelvin: _____________

g) Stable temperature of water after solid addition in oC: ____________

h) Stable temperature of water after solid addition in Kelvin: ____________

i) Temperature difference in oC (g – e): _____________

j) Temperature difference in Kelvin (h – f): _____________

k) Amount of heat lost or gained (in Joules) by the water†: _____________

†q = (b+d)(4.184 J/g ( oC)(i)

l) Amount of heat lost or gained (in Joules) by CaCl2††: _____________

††qH2O = –qCaCl2

m) Number of moles of CaCl2 used (b/Mm): _____________

n) Experimental heat of solution (in J/mol) of CaCl2 (l/m): ____________

o) Experimental heat of solution (in kJ/mol) of CaCl2 (n/1000):_________

p) True heat of solution (in kJ/mol) of CaCl2*: _____________

q) Percent error for heat of solution of CaCl2: _____________

Barnett & Jones

General Chemistry-I Lab

†q = (c)(4.184 J/g ( oC)(h)

‡q = (k)(4.184 J/g ( oC)(f–0.00oC)