Chemistry 101 Laboratory

Experiment 10: Reaction Enthalpy

Pre-Laboratory

Look over the INSTRUCTIONS for this lab.

Complete the pre-lab questions for this lab.

Look at the Post-laboratory calculations and reports for this lab

Laboratory

Calorimetry measures the transferr of heat during a chemical reaction or a change in the physical state of a compound. It is typically carried out under conditions of constant temperature or constant pressure. The latter is of particular interest because the heat change measured equals the change in enthalpy for the process being studied. In this experiment, you will use a simple calorimeter to measure the enthalpy of neutralization of HCl by NaOH and the enthalpy of formation of the Cu(II) tetra-ammonium salt. To to this, you will also have to
determine the heat capacity of your calorimeter.

The fundamental equation for calorimetry is

Q_rxn = Q_cal + Q_soln

The first heat is that of the reaction being studied, the second that of the physical device (the calorimeter) and the third is the heat of the solution in which the reaction is studied. In each instance

Q = (heat capacity) x DT
= CDT
= (mass) x (specific heat) x DT
= (density) x V x (specific heat) x DT

In each of these equations DT is the difference between the initial and the final heat. The different forms of the heat equation are used in different calculations.

For Q_cal you calibrate the calorimeter by measuring the change in temperature when equal volumes of hot and cold water are mixed in the calorimeter. If no heat was transferred from the water to the calorimeter or to the surroundings, the final temperature would be the average between that of the temperatures of the cold and hot water. In fact, it is always lower. What you will do is to measure the temperature of a mixture of the hot and cold water as a function of time. You will carry out a linear curve fit in order to extrapolate to zero time and the temperature at that time T_final. This is a reference temperature taking into account the effects of calorimeter and the surroundings upon heat transfer. The heats lost by the hot and cold water to the surroundings and the calorimeter is

Q_h = V x d x (T_final - T_h) x q_s
Q_c = V x d x (T_final - T_c) x q_s

Q_cal = abs(Q_h - Q_c) = V x d x (T_h - T_c) x q_s

Where "abs" stands for the absoute value. This heat, divided by T_final - T_c, equals the heat capacity of the calorimeter.

For the second part of the experiment, you will measure the heat of neutralization of HCl by NaOH. The heat of the reaction calculation requires that you know that the density of the solution is 1.06 g/mL and that the heat capacity is 3.895 J/deg g. All of the other calculations are the same as above. For the third part of the experiment, the density of the solution is also 1.06 g/mL, but the heat capacity is 4.184 J/deg g.

Instructions

A.  Determination of Calorimeter Heat Capacity

1. Heat about 75 mL of water to 60-70°C. Monitor the temperature with the temperature probe.

2. Enter Science Workshop, set up the temperature probe as in previous experiments and set up sampling options to measure temperature every 30 seconds.

3. Measure 50 mL of room temperature water with a graduated cylinder. Pour this into the calorimeter and measure its temperature, T_c, with the temperature probe.

4. Measure 50 mL of the heated water with a graduated cylinder allowing the water to stand for about 30 seconds.

5. Stir the water gently and briefly, measure its temperature using the probe,T_h.

6. Insert the temperature probe into the calorimeter lid.

7. Pour the hot water into the calorimer as quickly and completely as possible, recording this time as t = 0.

8. Quickly put the lid on the calorimeter, making sure the probe and stirring rod are in place, see fig 7.1. Also, immediately hit the record button to initiate temperature readings.

(Click to Enlarge)

9. Stir the water gently for a few seconds.

10. Continue to record the temperature at 5 second intervals for three minutes.

11. Dry the calorimeter with a paper towel and repeat the determination. (Dry calorimeter again after the second trial)

B. Heat of Neutralization

1. Measure 50.0 mL of 2.0 M HCl and pour into a calorimeter. Using the temp probe, measure the temperature of the HCl.

2. Measure 50.0 mL of 2.0 M NaOH. Measure the temperature of NaOH (make sure to rinse the probe before measuring the temperature.

3. If the temperatures of the two solutions are different, allow them to stand until they are the same, or average the two and record this temperature as the initial temperature.

4. Pour the NaOH into the calorimeter with the HCl. IMMEDIATELY press record.

5. QUICKLY place the lid on the calorimeter with the probe and stirrer in place.

6. Stir the mixture gently for a few seconds.

7. Continue recording data for three minutes.

8. Discard the solution by dumping it down the drain

9. Rinse the calorimeter - first with tap water, then with distilled.

10. Dry thoroughly

11. Repeat the experiment

12. When finished, rinse and dry the calorimeter again.

C. Heat of formation of Cu(NH₃)₄²⁺

1. Measure 50.0 mL of 0.50 M CuSO₄ and (in another container) 50.0 mL of 6 M NH₃

2. Pour one of these solutions into a clean dry calorimeter.

3. Record the initial temperature of each and average the two if they are different as you did in part B.

4. Add the second solution to the first in the calorimeter and IMMEDIATELY press Record.

5. Quickly place the lid on the calorimeter with the probe and stirrer in place.

6. Stir the mixture gently for a few seconds.

7. Continue recording data for three minutes.

8. Discard the solution by dumping it down the drain

9. Rinse the calorimeter - first with tap water, then with distilled.

10. Dry thoroughly

11. Repeat the experiment

12. When finished, rinse and dry the calorimeter again.

Pre-lab Questions Post-lab Calculations

Department of Chemistry

Capital University

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