1. Set up the apparatus as shown in the following diagram. Use two styrofoam cups, one inside the other, and a thermometer clamp to hold the thermometer.
Introduction
In the course of any chemical reaction there is a change in enthalpy, D H. If the reaction is exothermic, heat is given off and D H has a negative value. When heat must be added in order for a reaction to occur, the reaction is endothermic and D H has a positive value. The change in Enthalpy, D H, is a measure of the change in heat content and can be measured by subtracting the heat content of the reactants from the heat content of the products.
| D H REACTION | = | D H PRODUCTS -D H REACTANTS |
The heat content of a compound cannot be measured directly, but the change in enthalpy that occurs during a chemical reaction can be measured.
The chemical reaction for which you will determine H is a neutralization reaction. When an acid and a base react, the net result is the production of a salt and water. If the salt is water soluble, the net ionic equation for the reaction is
| H+ (aq) + OH- (aq) | ® | H2O |
A calorimeter is used to measure the heat change. This device was used in the experiment during which you found the specific heat of a metal. You will be using a variation of the calorimeter (double Styrofoam cups). The heat capacity of the calorimeter must be calculated first, then the heat change for the neutralization can be determined.
The heat released (D H) = heat absorbed by the water (or solution) + heat absorbed by calorimeter
The maximum temperature that is reached at
the time of mixing ("time zero") is determined by plotting Temperature
vs Time graph and extrapolating back to time zero.
Equipment
| 2 styrofoam cups | ring stand |
| test tube clamp | thermometer clamp |
| thermometer (0-100° C) | stirring rod |
| graduated cylinder | 150mL beaker |
Chemicals
student 2.0 M HCl(aq), 2.0 M NaOH
Spill/Disposal
Acid/Base Spill/Disposal: B1
Neutralization Product: Spill/Disposal A
Procedure
Part I. Heat Capacity of the Calorimeter
1. Set up the apparatus as shown in the following
diagram. Use two styrofoam cups, one inside the other, and a thermometer
clamp to hold the thermometer.
2. Measure out 50mL of distilled water as accurately as possible. Pour it into the calorimeter.
3. Measure out another 50mL of distilled water, as accurately as possible, into a 150mL beaker and heat to about 60° C on a hot plate. DON'T OVERHEAT!
4. Remove the beaker from the hotplate. Measure the temperature (to the nearest 0.1° C) of the cool water in the calorimeter. Stir the water as you do this. Then quickly measure the temperature of the warm water. Again, stir the water as you do this. These temperature readings must be taken just before the solutions are mixed. Then quickly add the warm water to the cool water in the calorimeter as your lab partner starts the stopwatch. Record the temperature on the Data Sheet every 30 seconds for 5 minutes. Keep gently stirring the mixture. Do not read the temperature at the time of mixing.
5. Repeat the above for Run 2.
6. Plot Temperature vs Time on graph paper as explained in the introduction and determine the temperature at the time of mixing. D T for the warm water will be equal to its initial temperature minus the maximum temperature reached upon mixing the warm water with the cool water. D T for the cool water will be equal to the maximum temperature upon mixing minus the initial temperature of the cool water. D T must always be positive.
7. Use the above D T values to calculate the heat capacity of the calorimeter for both runs. Use the average of these two values for your heat capacity.
Part II. Heat of Neutralization
1. Dry the inner Styrofoam cup used in Part 1. Measure out 50mL (accurately measured) 2.0 M NaOH and pour it into the calorimeter. Into a dry 150mL beaker, pour 50mL (accurately measured) of 2.0M HCl. Rinse out graduated cylinder in between measurements.
2. Determine the temperature of the acid to the nearest 0.1° C. Rinse the thermometer with distilled water, wipe it dry and measure the temperature of the base to the nearest 0.1° C. If the temperatures are not within 0.2° C of each other, either heat up or cool down the acid with a test tube containing either hot or cold water. Record both temperatures.
3. Pour the acid into the base quickly and carefully with gentle stirring. Continue to stir and record the temperature at one-minute intervals for 10 minutes. Again, do not record the temperature at the time of mixing.
4. Repeat the above for Run 2.
5. Plot the Temperature vs Time on graph paper and determine the temperature change for the reaction.
6. Calculate D H for the reaction.
Hand in Preliminary Data sheets (one per group and signed by all) before you leave the lab.
8. Each student is to hand in the Data Sheet and calculation sheet for both part I and part II. These reports are your own individual work. Your results may not be exactly the same as the other members of your group because you may plot or interpret the results in a slightly different manner. If your lab reports are exact copies it strongly suggests collusion and a zero grade for the lab report.
Disposal
All reactants and products may be disposed of into the sink.