Heat of Solution
Name____________________________ Per____
The molar heat of solution (ΔHsoln) of a substance is the change in the chemical energy when one mole of
the substance is dissolved in a solvent. In this experiment you will measure 50 mL of room temperature water. Pour
the water into a Styrofoam cup (calorimeter). Place the calorimeter in a 250 mL beaker to stabilize it. Weigh out
10.0 g Sodium thiosulfate pentahydrate (Na2S2O3 · 5H2O). Record the actual mass used to three decimal places.
Record the mass and the temperature of the water in the cup to the nearest 0.1 ºC. Add the solid to the water and stir
until it is completely dissolved. (Use a data table to display your data.)
Measure the temperature again, and record it to the nearest 0.1 ºC. Calculate the thermal energy change
(ΔH) and the chemical energy change (-ΔH).
Calculate the amount (moles) of sodium thiosulfate pentahydrate which dissolved. Then calculate the heat
of solution (J/g) and ΔHsoln (kJ/mol) for thiosulfate pentahydrate.
Now, repeat the experiment by dissolving approximately 4.0 g of calcium chloride (CaCl2) in 50 mL of
water. Record the exact amounts used. Repeat the calculations to obtain the heat of solution for calcium chloride.
Heat of Solution – Write your report on a separate sheet or sheets of paper.
A. Purpose:
B. Safety:
C. Data: (Place in a table)
Na2S2O3·5H2O
CaCl2
Mass of solid
_____g
_____g
Moles of solid
_____mol
_____mol
Volume of water
_____mL
_____ mL
Mass of water (assume DH2O=1.00 g/mL) _____ g
_____ g
ti (initial temp.)
_____ºC
_____ºC
tf (final temp.)
_____ºC
_____ºC
tf-ti (change in temp.)
_____ºC
_____ºC
thermal energy change ΔH (qsurr)
_____kJ
_____kJ
chemical energy change –ΔH (qsys)
_____kJ
_____kJ
Gram heat of solution
_____kJ/g
_____kJ/g
Molar heat of solution (ΔHsoln)
_____kJ/mol
_____kJ/mol
Additional Observations:
D. Calculations: Show ALL calculations for both substances. Be sure to label all calculations.
1. qsurroundings: ΔHsurroundings = (msoln)(cH2O)(ΔTH2O)
2. qsystem
3. Gram heat of solution = -ΔH/g of substance dissolved
4. Molar heat of solution ΔHsoln= -ΔH/mol of substance dissolved
E. Conclusions:
F. Questions:
1. Which reaction is endothermic? What is the energy being used for? Where is the energy coming from?
2. Draw the potential energy diagram showing the heat flow for the endothermic process. Label all parts
including the axis, heat of dissociation, heat of hydration, heat of solution, reactants or products for
each section. For a hint, see pages 415 and 553 in your textbook.
3. Which reaction is exothermic? Where is the energy coming from? Where is the energy going?
4. Draw the potential energy diagram showing the heat flow for the exothermic process. Label all parts
including the axis, heat of dissociation, heat of hydration, heat of solution, reactants or products for
each section.
5. Calculate the thermal and chemical energy changes when 5.85 g of sodium chloride are dissolved
in 400.0 mL of water in an insulated calorimeter. The temperature drops by 0.32 ºC. Draw
sequential pictures of the process described at the molecular/atomic level.
6. Calculate the heat of solution (kJ/mol) of NaCl from the information in question 5.
7. If ΔHsoln CaCl2 is -82.8 kJ/mol, analyze your experimental results (include % error) and determine at
least two possible sources of error. Describe how each of those sources of error contributed to the final
answer (did the error result in an experimentally calculated value that was too large or too small? Why?