Teacher’s Copy: Heat of Solution of Solid
This lab was adapted from:
www.saskschools.ca/~chem30_dev/1_energy/labs/heat_solution.rtf
staff.fcps.net/jswango/unit6/thermochem/hess_law%20lab.pdf
Objectives:
Content introduced:
Endothermic and exothermic reactions
Total heat of solution & calculations
Molar heat of solution & calculations
Content reinforced:
Ions
Balancing equations
Percent error
Skills introduced:
Skills reinforced:
How to measure temperature
Weighing materials
Using graduated cylinder
Curriculum Connections: Outcome and Indicators
Examine the relationships between heat energy and reactions.
! Recognize that energy changes are associated with chemical reactions.
! Differentiate between endothermic and exothermic reactions.
! Compare the energy changes in phases changes and chemical
reactions.
! Explain the difference between heat and temperature.
Understand the quantitative description of enthalpy change.
! Measure some energy changes in chemical reactions.
! Investigate how tables of standard heats (enthalpies) of formation are
created and used.
! Express the enthalpy change of a chemical reaction as a term in the
equation for the reaction, or as a heat of reaction ΔH).
Connections to Past concepts:
Thermochemistry:
Calorimetry
Energy
Law of Conservation of Heat
Temperature vs. Heat
Specific Heat
Connections to Future concepts:
Hess’s Law
Materials:
 2 Styrofoam cups
 thermometer
 ~15g of anhydrous sodium acetate (CH3CO2Na)
 ~15g of ammonium nitrate (NH4NO3)
 water
 100mL graduated cylinder
 balance
Procedure:
1. Accurately find the mass of about 150 mL of tap water. Record this value and other
measurements in the data table. Add the water to the Styrofoam cup calorimeter.
2. Accurately find the mass of about 15 g of solid ammonium nitrate.
3. Find and record the initial temperature of the water. Record to the nearest 0.2C.
4. Dissolve the solid ammonium nitrate in the water, stirring with the thermometer.
Record the maximum temperature difference from the initial reading.
5. Rinse out the cup, dry it thoroughly, and repeat the experiment using a sample of
about 15 g of sodium acetate in place of the ammonium nitrate.
Safety Concerns:
Ammonium nitrate and sodium acetate are skin and eye irritants therefore protective
clothing, eye goggles, and rubber gloves and MUST be worn.
Sample Data Table with Data:
Ammonium nitrate Sodium acetate
NH4NO3
CH3CO2Na
Mass of cup + water (g)
Mass of empty cup (g)
Mass of water (g)
Mass of solid + container (g)
Mass of empty container (g)
Mass of solid used (g)
Initial water temperature (°C)
Final water temperature (°C)
Change in water temperature (°C)
100.6
1.6
99.0
39.9
23.3
16.6
20.2
8.0
-12.2
103.4
1.6
101.8
38.4
23.2
15.2
19.8
27.6
+7.8
Calculations:
1. The total heat of solution can be calculated by determining the amount of heat absorbed or lost by
the water during the experiment. This is calculated using the formula:
Q
=
m
c
T
heat absorbed/
lost (joules)
=
mass of
water (g)
specific heat of
water (J/gC)
temperature
change (C)
The specific heat of water is more-or-less a constant, with a value of 4.18 J/ gC.
Important Note: The amount of heat absorbed (or lost) by the water is equal to the amount of
heat lost (or absorbed) by the solute. Therefore, to determine the value for heat gained/lost by
the solute, just reverse the sign for the value of heat gained/lost by the water.
The value for the amount of heat absorbed or lost by the solute will be converted into kJ.
Ammonium nitrate:
Q= mcT
Q= (99.0g)(4.18J/gC)(+12.2C)
Q= +5.04kJ
Sodium acetate:
Q= mcT
Q= (101.8g)(4.18J/gC)(-7.8C)
Q= -3.32kJ
2. The molar heat of solution, expressed as kJ/mol, is determined by converting the mass of the
solute used to moles:
molar heat solution=total heat solution
moles solute used
Total heat of solution tells us how much heat was gained or lost in our actual experiment with the
mass of the substances actually used, while
Molar heat of solution tells us how much energy would be gained or lost for one mole of the
substance.
Ammonium nitrate: molar heat solution = total heat solution
Moles solute used
= +5.04kJ
(80.052 g*mol-1/16.6g)
= +1.05 kJ/mol
Sodium acetate: molar heat solution = total heat solution
Moles soute used
= -3.32kJ
(82.03379 g*mol-1/15.2g)
= -0.62kJ/mol
Questions:
1. NH4+ and NO3CH3CO2- and Na+
2. ammonium nitrate: exothermic
sodium acetate: endothermic
3.
NH4NO3 (aq) + heat  NH4+(aq) + NO3-(aq)
NaCH3CO2(aq)  Na+(aq) + CH3CO2-(aq) + heat
4.
Observed value
NH4NO3
Expected value Percent error
+25.8 kJ/mol
CH3CO2Na
-17.4 kJ/mol
5.
6.
7.
We use a Styrofoam cup as a calorimeter, because it is a
container with good insulated walls to prevent heat exchange
with the environment.
Student’s Copy: Heat of Solution of Solid
Measure experimentally the amount of heat involved in the dissolving of
~15g ammonium nitrate (NH4NO3) and ~15g sodium acetate
(CH3CO2Na) in ~150mL of water.
Record your data in a representative format.
Using the equation discussed in class, calculate the total and molar heat
of solution for each of the two solids. (specific heat of water = 4.18J/g°C)
Questions:
1. Both of the substances used in this experiment consist of ions.
Give the chemical formulas for the two ions that make up each of
the compounds used in this experiment.
Ammonium nitrate:
Sodium acetate:
2. For each of the dissolving processes, state whether the overall
process was endothermic or exothermic.
3. Write balanced equations for the dissociation of each ionic
compound. Be sure to include the value for the energy term that
you obtained during your experiment.
4. Complete the table.
Observed value
NH4NO3
CH3CO2Na
Expected value Percent error
+25.8 kJ/mol
-17.4 kJ/mol
5. If there was discrepancy between your observed value and the
expected value resulting in a percent error, why did this occur?
6. What conclusions can you draw from your experimental data.
7. Why use a Styrofoam cup
Assessment Rubric
Adapted from: http://web.rbe.sk.ca/assessment/rubrics/#science
Criteria
Data &
Observations
1
 No data
present
 Graphs/table
s/drawings
absent
2
Questions
Conclusion
 No relations
to data
 Conclusion
absent or
unrelated to
purpose/hyp
othesis
 All data present
 All data present
 Graphs/tables
poorly
presented/missing
parts
 Some mislabelling
or missing labels
 Graphs/tables
complete
 Some data
presentation not
clear
 Data presented
appropriately and
clearly
 Questions
answered
inaccurately
 Most questions
accurately
answered
 Analysis shows
little thought, little
relation to data
 Analysis somewhat
thoughtful;
partially related to
data
 Questions
answered
thoroughly and in
complete
sentences
 Little relation to
purpose/hypothes
is & data
 Somewhat related
to
purpose/hypothes
is
 Student fully
relates to
purpose/hypothes
is
 Student somewhat
uses data, refers to
data
 Student uses data
to support
conclusion
 Proper safety
precautions are
used
 Proper safety
precautions are
always used
 does not use
data at all
Safety
 Not following
proper safety
precautions
 Needs to be
reminded
many times
during lab
Clean-up
 Clean up
done
incorrectly
and not
4
 Some data missing
 Data missing
 No thought
3
 Proper safety
precautions are
generally used
most of the time
 Thinks ahead to
ensure safety
 May need
reminding
 Proper clean up
procedures
sometimes used
 Station generally
 Analysis
thoughtful, directly
related to data
 Station is clean
 Station is clean
 Uses proper clean
up procedures
 Proper clean-up
procedures used
completely
clean
most of the time