Name ___________________________________________________________Class ________________Date ________________
Molar Heat of Solution
Introduction:
The molar heat of solution is the heat involved when a substance dissolves in water. The heat that is absorbed by the
substance as it dissolves is transferred from the water. When heat is absorbed by the substance dissolving the water solution gets
colder and the temperature goes down (endothermic). The heat that is released by the substance as it dissolves is absorbed by the
water. When heat is released by the substance dissolving the water solution gets warmer and the temperature goes up (exothermic).
As always this quantity of heat can be related back to the mole concept. The Molar Heat of Solution is the amount of heat released or
absorbed for every mole of solid that is dissolved.
In this experiment you will be able to calculate the molar heat of solution for several solids. You will dissolve a known mass
of solid in a known mass of water and measure the temperature change. The heat lost or gained by the substance as it dissolves is
equal to the heat gained or lost by the water. From this data you will be able to determine the Molar Heat of Solution.
Pre-Lab: On a separate sheet of paper answer the following questions
1) What is the difference between Heat and Temperature?
2) The sign of the change in Enthalpy (H) for a solid can tell you if a reaction is exothermic or endothermic. How can you tell?
3) Why will we be using an insulated foam cup during this experiment?
4) Why should you not stir solutions with a thermometer?
Procedure:
1) Read through the procedure and make a data table for the data to be collected for each solid.
2) Obtain one of the samples to be tested today. (NaOH, and NH 4NO3)
3) Measure out about 8.00 grams of the solid that you chose and record the exact mass of the solid.
4) Obtain a foam cup to be used as your calorimeter.
5) Measure out 100.0-ml of distilled water and put it in the cup. Measure the initial temperature of the water.
6) Add the solid that you have already massed out. Stir the solid while you monitor the temperature with a thermometer.
7) After the solid is dissolved record the highest or lowest final temperature of the solution.
8) Pour the solution down the sink and repeat 1-7 steps with the other solid.
In the space below construct a data table for the data to be collected.
DATA TABLE
NaOH
Mass of solid used
Mass of water used
Initial Temperature of the water
Final Temperature of the water
NH4NO3
Name ___________________________________________________________Class ________________Date ________________
Calculations:
1) Calculate the change in temperature (T ) for the water for each of the solids. (Tfinal-Tinitial)
NaOH:
NH4NO3:
2) Calculate the energy (heat) that was transferred when each solid was dissolved using the following equation.
The Heat that the water absorbed or lost (q) = (Mass of water) X (4.18 J/gC) X ( T
NaOH:
NH4NO3:
3) Using the molar masses of the solids convert the mass to moles for each solid.
NaOH:
NH4NO3:
4) Calculate the molar heat of solution (H) by dividing the heat transferred by the quantity of moles. (#2/#3)
NaOH:
NH4NO3:
5) Assign the proper sign to the H and write it here. (exo/endo)
NaOH:
NH4NO3:
Questions:
1) Which solid dissolved to give an exothermic and which dissolved to give an endothermic reaction?
2) Write the balanced equation for each of the reactions, remember ionic solids dissolve to form aqueous ions. Included the
experimental ΔH for each reaction.
3) State one source of error in this experiment and explain if it would make your value for ΔH to high or too low?
4) How could this experiment be improved so that heat was not lost to the surroundings?
5) A 12.8 g sample of KCl is dissolved in 75.0 g. of water. The temperature of the water goes from 30ºC to 20ºC. What is the H
for this reaction? (you know 4.18 J/gºC)