HEAT OF SOLUTION
The energy change associated with the process in which a solute dissolves in a solvent is called
the Heat of Solution. This energy change is the net result of two processes, the energy required
to break the solute-solute bonds, called the crystal lattice energy, and the energy released when
the solute particles bond with the solvent molecules, called the heat of hydration. For example,
the heat of solution for KCl in water can be considered the sum of two heat effects:
The lattice energy of KCl, the energy necessary to break apart the KCl crystal lattice and form
free ions, is represented by equation 1:
KCl(s) 6 K+(g) + Cl-(g)
[1]
)H = +701.2 kJ
The heat of hydration of KCl, the energy released when the free ions are hydrated, is represented
by equation 2:
[2]
K+(g) + Cl-(g) 6 K+(aq) + Cl-(aq)
)H = -684.1 KJ
In this example, the overall reaction is endothermic, and the heat of solution is positive since
more energy is required in step 1 than is released in step 2. Where
Step 1
Step 2
Overall reaction:
KCl(s) 6 K+(g) + Cl-(g)
K (g) + Cl-(g) 6 K+(aq) + Cl-(aq)
KCl(s) 6 K+(aq) + Cl-(aq)
+
)H = +701.2 kJ
)H = -684.1 kJ
)H = +17.1 kJ
Unfortunately, we can not determine the energy changes in each of the steps of the solution
process using the simple calorimeter techniques in this experiment, but we can determine the
overall energy change observed when a substance dissolves in water.
In an experiment similar to this experiment, a student added 1.01 grams of lithium bromide,
LiBr, to 12.21 grams of water at 20.1°C. After stirring for awhile, the temperature of the mixture
rose to 30.3°C. Given this information, we can calculate the heat of solution for calcium
chloride.
Step #1: Calculate the moles of the solute, lithium bromide, LiBr (86.84 grams/mole)
Step #2: Determine the change in the temperature of the water/solution.
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The initial temperature of the water was 20.1°C and the final temperature was 30.3°C.
Therefore, the change in the temperature was 30.3°C -20.1°C = 10.2°C.
There is a slight error in this calculation as the actual temperature of the lithium bromide and
water before the reaction are most likely not the same. That is, the temperature of the lithium
bromide was at room temperature and the water may not have been at exactly room temperature.
Therefore, we can expect a slight change in the temperature by simply mixing these two
substances even without a reaction. However, assuming the temperature of the water was close
to room temperature and the mass of the salt small, the error is most likely not important in our
overall problem.
Step #3: What is the total mass of the material?
The lithium bromide had a mass of 1.01 grams and the water had a mass of 12.21 grams.
Therefore the total mass of materials would be 1.01 grams + 12.21 grams or 13.22 grams.
Step #4: Calculate the change in energy, )q, from the mass of solution, the change in the
temperature, and the heat capacity of the solution, assumed to be 4.184 J/gC°C.
Step #5: Calculate the heat of solution for calcium chloride in water.
Step #6: Convert the heat of solution in joules per mole to kilojoules per mol.
Step #7: Determine the percent error between your calculated heat of solution at the theoretical
value given in Table I on your Reference Tables for Chemistry. According to Table I
on the Reference tables for Chemistry, the heat of solution for lithium bromide, LiBr, is
-48.83 kJ/mol. Given this information, we can calculate the percent error in our
experiment.
We discarded the negative sign for the theoretical heat of solution as it’s not helpful when
calculating the percent error in this case.
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The purpose of this experiment is to determine the heat of solution for some selected compounds
from the energy change associated with their solution process. By using the data we obtain in
our calculations, we can compare our experimental value for the heat of solution with the
theoretical value given in Table I on the Reference tables for Chemistry.
MATERIALS:
Calorimeter, 30 ml or 50 ml beaker, thermometer, vial containing the substance to be studied,
magnetic stirrer and small stir bar (optional).
PROCEDURE:
1. Tare a small clean, dry beaker. Add 1.00 to 1.2 grams of the assigned compound to the
beaker and re-mass to the nearest 0.01 gram.
2. Fill a 100 ml beaker with tap water. Adjust the temperature of the tap water until it is within
a degree or two of room temperature. Tare a clean, dry calorimeter. Add roughly 14-15 ml of
the tap water in the beaker to the calorimeter, and mass the calorimeter with the water to the
nearest 0.01 gram. Stir and record initial temperature of the water to the nearest 0.1°C.
3. Add the measured solid salt to the calorimeter. Stir the mixture until the compound is
completely dissolved and the temperature remains constant over several readings. Record
this constant temperature to the nearest 0.1°C.
4. Rinse and dry the both the thermometer and calorimeter, repeating the experiment with the
same solid as time permits.
QUESTIONS AND CALCULATIONS:
Q1. Calculate the number of moles of your assigned solid consumed in each of your trials.
Q2. (a) Calculate the total mass of material in the calorimeter from the mass of water and the
mass of dissolved salt.
(b) Calculate the heat required or liberated when your assigned solid dissolves in water
from the total mass of water plus the salt, the change in the temperature of the water,
and the specific heat capacity for water, 4.18 joules/gramA°C.
Q3. Calculate the heat of solution from the number of moles of salt dissolved and the amount of
heat liberated or absorbed when the salt dissolves. Convert this value from joules/mole to
kilojoules/mole. NOTE: 1000 joules = 1 kilojoule (kJ)
Q4. Calculate the average heat of solution for all of your trials in kilojoules/mole.
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Q5. Calculate the percent of error between your average heat of solution and the theoretical
value given in Table I on your Reference Table for Chemistry.
Q6. (a) According to your data, was your reaction an endothermic or exothermic process?
(b) How could you use Table I to predict if the process of dissolving a compound in water
is an exothermic or endothermic process?
Q7. Briefly describe the difference between the nature of the particles in the solid substance
before the reaction, and the nature of the particles in the resulting water solution.
Q8. Briefly describe how the entropy changes as the substance dissolves in water.
DATA:
Formula of substance studied: ____________________________ Formula mass:____________g.
Trial #1
Trial #2
Trial #3
Mass of substance dissolved
g.
g.
g.
Mass of the calorimeter
g.
g.
g.
Mass of the calorimeter + water
g.
g.
g.
Mass of water
g.
g.
g.
Total mass of the reactants
g.
g.
g.
Initial temperature of the water
°C
°C
°C
Final temperature of the water
°C
°C
°C
Change in the temperature
of the water, )t
°C
°C
°C
Number of moles of material
dissolved in the water
mol
mol
mol
Energy produced or released
by the dissolving salt, )q
J
J
J
Heat of solution, )H
Average Heat of Solution:
Theoretical Heat of Solution from Table I:
Percent error:
%
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