Investigating the Heat Associated with Dissolving Ionic Salts in Water Draft
2.03.02 tjg
Click here to go to Dissolving Salts Simulation
The computer will simulate a calorimetry experiment involving dissolving various ionic
salts in water. The following compounds should be available for testing: ammonium
nitrate, sodium nitrate, calcium nitrate, magnesium nitrate, sodium chloride, calcium
chloride, ammonium chloride, magnesium chloride. Work in pairs and choose two
compounds to test. One of your compounds should cause an increase in temperature of
the solution, the other compound should cause a decrease in the temperature of the
solution.
Write three specific questions that you can answer as a result of performing calorimetry
experiments.
Part IA. Design and carry out an experiment to determine the change in enthalpy, •H
(kJ), for the dissolving process for (write the formula of the compound)
___________________________.
In your experiment, what is the dependent variable?
______________________________
What is the independent variable(s)? __________________________________________.
Write an equation that describes the dissolving process for the salt. Classify this process
as endothermic, exothermic, or neither.
List the assumptions and or information that you need to make in order to do the
calculations.
In your experiment, identify the system ______________ and the surroundings
_________________.
In your experiment, what gains heat? ________________________________
What loses heat? _______________________________________________-
Calculations
The heat of reaction, qrxn for dissolving ____________ in water is ________________.
The change in enthalpy, •H, for dissolving ____________ in water is
________________.
Part IB. Design and carry out an experiment to determine the change in enthalpy, •H
(kJ), for the dissolving process for (write the formula of the compound)
___________________________.
Write an equation that describes the dissolving process for the salt. Classify this process
as endothermic, exothermic, or neither.
In your experiment, identify the system ______________ and the surroundings
_________________.
In your experiment, what gains heat? ________________________________
What loses heat? _______________________________________________-
Write an equation that relates the heat absorbed to the heat transferred.
.Calculations
The change in enthalpy, •H, for dissolving ____________ in water is
________________.
Part II. Design and carry out a series of experiments that investigates whether or not the
amount of compound dissolved in water has any effect on how much the temperature of
the solution will change. In your experiment, identify the dependent and independent
variables. Make a graph that illustrates a relationship between your variables.
Share your results with your classmates by placing your results on the board.
Questions. Dissolving Ionic Salts in Water (Computer Exercise)
1. Is dissolving an ionic salt in water a chemical or physical process? Explain.
2. Does doubling the amount of substance that dissolves in water correspond to a
doubling of a temperature change in the solution?
3. What is the difference between the heat of reaction, qrxn, and the enthalpy of reaction,
•Hrxn?
4. Can you make a prediction about which ionic compounds will cause an increase or
decrease in the temperature of the solution when they dissolve in water?
5. What causes the changes in temperature that you observed to occur (how does it
happen)? Draw a diagram at the molecular level that illustrates what is happening. In
each case, identify the system and surrounding.
Part IIIA. Dissolving Magnesium Sulfate in Water (laboratory activity)
Work in pairs. Obtain a sample of magnesium sulfate of known mass (between 3 to 8
gram s) using the analytical balance. Record the exact mass of your sample (use an
analytical balance) and formula for magnesium sulfate in your lab notebook. You will
share your data with your class. It is best if the class arranges to have the entire range of
mass investigated (i.e. 3, 4, 5, 6, 7, 8 grams). The class will have to come to some
agreement as to how to divide the work and what variable(s) will remain constant.
Using a graduated cylinder, obtain 100.0 mL of distilled water and place it in a
calorimeter. Set up a ring stand and clamp to suspend a thermometer into the water so
that it does not touch the bottom or sides of the calorimeter. Record the temperature of
the water every minute for four minutes or until the temperature has stabilized. Add your
sample of magnesium sulfate with vigorous mixing. Do not use the thermometer as a
stirrer. Record the temperature of the solution every 30 seconds for about 5 to 8 minutes,
until the temperature stabilizes or changes slowly. It will be helpful to have a watch that
can measure seconds. Determine the change in temperature that occurs. You should
record in a data table the exact mass of your magnesium sulfate sample, number of moles
of magnesium sulfate, the change in temperature of the solution, the total mass of
solution, the number of moles of magnesium sulfate used.
Calculate the heat of solution (qsolution = mc•T). Assume that the resultant solution has
a specific heat of 4.18 J/g°C. In this calculation, the mass is the total mass of solution.
Calculate the heat of the reaction, qrxn. Is there a relationship between the mass of
magnesium sulfate used, the number of moles of magnesium sulfate, the change in
temperature of the solution, and qrxn? Calculate the change in enthalpy for this reaction,
•Hrxn. Is this process endothermic or exothermic? On the board, place the value for your
mass of magnesium sulfate, •T, qrxn, and •Hrxn on the board. Compare your value of
•Hrxn for the dissolving of magnesium sulfate in water to the literature value. Calculate
a percent error.
Part IIIB. Dissolving Magnesium Sulfate in Water. (Details and understanding what
occurs)
Write an equation that describes the process of dissolving magnesium sulfate in water.
Determine the •T, the temperature change for the process. Show clearly how you
determined •T. It will be helpful to graph your data by plotting temperature versus time.
Draw the best curved line through the points and explain what is occurring in each
section of the curve. What part of your curve represents •T for the dissolving process?
Assume that your calorimeter is a perfect insulator - no heat escapes or enters the interior
of the calorimeter. Identify the system and the surrounding. Draw a diagram that
illustrates what is transferring heat to what. What gives up heat? What takes in heat?
How does the Law of Conservation of Energy apply to your experiment? Write an
algebraic equation that expresses this relationship.
Calculate the heat that was gained or lost by the solution, qsoln. The total mass of the
solution = the mass of the water plus the mass of your magnesium sulfate sample. Pay
particular attention to the sign (positive or negative) of your value of qsoln
Calculate qrxn . In order to determine qrxn, you need to make an inference or you need to
invoke the Law of Conservation of Energy. Pay particular attention to the sign (positive
or negative) of your value of qrxn
Your classmates have data for other sample sizes of magnesium sulfate. Record at least
five other data sets in you lab notebook so that you have data for 3, 4, 5, 6, 7, 8 grams of
magnesium sulfate.? Your graph should show a relationship between two variables in
your experiment. In your experiment, what is the dependent variable what are the
independent variables? What is graphed on the x-axis and what is graphed on the y-axis?
Is there a limiting reagent present? Do you need to do an additional experiment?
Calculate the change in enthalpy, •Hrxn, for dissolving magnesium sulfate in water. You
need to specify whether your experiment was carried out at constant pressure or constant
volume. If it was carried out at constant pressure than you can calculate •Hrxn, by
dividing qrxn by the number of moles of magnesium sulfate used. Use your textbook or a
handbook of physical constants (e.g. Handbook of Chemistry and Physics) to obtain the
change in enthalpy, •Hrxn, for dissolving magnesium sulfate in water and compare your
experimental value to the literature value.
Use your textbook to investigate the intermediate steps involved in dissolving salts in
water. Read pages 469- 472 of your textbook (BLB). Draw an energy level diagram for
dissolving magnesium sulfate similar to Figure 13.4, the one found on page 472 of your
textbook. What is the difference between an exothermic and a endothermic process for
dissolving ionic salts in water?