Heat of Dissolving Lab
Name ______________________________
Date ___________________ Period ______
Purpose: To measure the ________________________ as different _________________ dissolve in
_________________ and determine the _________________________________________.
Materials:
goggles and apron
distilled water
thermometer
stirring rod
CaCl2
graduated cylinder
foam cup as a calorimeter
balance
NH4Cl
Procedure:
1.
Put on goggles and lab apron. Measure 100.0 mL of distilled water at room temperature
and pour it into the plastic foam cup. Record the mass of the water by recalling that the
density of water is 1g/mL. Record the temperature of the water in the data table to the
nearest 0.1C. Do not remove thermometer from the cup, but be careful that it does not
tip over.
2.
Using your laboratory balance, measure out 8-10 grams of ammonium chloride on a
piece of paper. Record the mass to the nearest 0.01 g.
3.
Without removing the thermometer from the cup, shake the NH4Cl from the paper into
the water and stir gently with the stirring rod until the solid is dissolved. CAUTION:
Both of the solutions in the lab are irritating to the skin. Avoid contact with them.
4.
Make sure that the bulb of the thermometer is fully immersed in the liquid. If the
temperature rises, record the highest temperature reached. If the temperature falls,
record the lowest temperature reached.
5.
Dispose of the solution by pouring it down the drain, followed by plenty of water. Rinse
the cup, dry and return it and the thermometer to the lab bench.
6.
Repeat steps 2-5 using calcium chloride.
Observations:
Data Table
Solute
Solute
Mass (g)
Mass of
Water (g)
Mass of
Solution
(g)
Initial T
(C)
Final T
(C)
(+/-) T
(C)
Exothermic
or
Endothermic
NH4Cl
CaCl2
1
Calculations for _________________________
1. Calculate the change in temperature. T = Tf ─ Ti
2. Calculate the heat absorbed or released by the solution. The specific heat of water is 4.184J/gC.
qsur = (mass of the solution) x (T of the water) x (specific heat of the water)
3. How much heat was released/absorbed (circle one) from/into the reaction? (qrxn = -qsur)
qrxn =
4. Using the periodic table, calculate the molar mass of the solute.
5. How many moles of the solute were used in the reaction?
6. Calculate the molar heat of solution (H) in (+/-) kJ for the solute from the formula:
Heat of the reaction
q
= rxn
∆H = Molar heat of solution =
moles solute dissolved
mole
7. Calculate the percent error of your experimental value.
% error =
accepted value - experimental value
x 100% =
accepted value
The accepted value for H for NH4Cl is +14.8 kJ/mol.
The accepted value for H for CaCl2 is –81.3 kJ/mol.
2
Calculations for _________________________
1. Calculate the change in temperature. T = Tf ─ Ti
2. Calculate the heat absorbed or released by the solution. The specific heat of water is 4.184J/gC.
qsur = (mass of the solution) x (T of the water) x (specific heat of the water)
3. How much heat was released/absorbed (circle one) from/into the reaction? (qrxn = -qsur)
qrxn =
4. Using the periodic table, calculate the molar mass of the solute.
5. How many moles of the solute were used in the reaction?
6. Calculate the molar heat of solution (H) in (+/-) kJ for the solute from the formula:
Heat of the reaction
q
= rxn
∆H = Molar heat of solution =
moles solute dissolved
mole
7. Calculate the percent error of your experimental value.
% error =
accepted value - experimental value
x 100% =
accepted value
The accepted value for H for NH4Cl is +14.8 kJ/mol.
The accepted value for H for CaCl2 is –81.3 kJ/mol.
3
Critical Thinking: Analysis and Conclusions
1. When sodium chloride dissolves in water, the ions dissociate. The equation for this reaction is
NaCl(s)  Na+(aq) + Cl-(aq) Write similar ionic equations to show the dissociation in water of each
of the solutes used in the investigation.
_______________________________________________________________________________
_______________________________________________________________________________
2. Which reaction was exothermic? Which was endothermic?
_______________________________________________________________________________
3. Rewrite each of the ionic equations from Question 1 showing the molar heat of solution as a
reactant or a product.
_______________________________________________________________________________
_______________________________________________________________________________
4. When the reactants get colder in an endothermic reaction, what has happened to the heat energy?
_____________________________________________________________________________
5. Is the change in enthalpy positive or negative for an exothermic reaction? Explain. __________
_____________________________________________________________________________
6. Suggest two uses for these solution reactions in sports injuries or camping.
_______________________________________________________________________________
7. Which solids from this investigation could be used in each of your answers in the previous
question? Explain your reasoning.
_______________________________________________________________________________
_______________________________________________________________________________
8. Why is a plastic foam cup used instead of a beaker in this experiment? For what piece of equipment
is this cup a substitute?
______________________________________________________________________________
______________________________________________________________________________
9. How could you reduce experimental errors in this investigation? Explain your reasoning.
_______________________________________________________________________________
_______________________________________________________________________________
4