Additivity of heats of reaction

LabQuest

18

Chemistry with Vernier 18 - 1

Additivity of Heats of Reaction: Hess’s Law

In this experiment, you will use a Styrofoam-cup calorimeter to measure the heat released by three reactions. One of the reactions is the same as the combination of the other two reactions. Therefore, according to Hess’s law, the heat of reaction of the one reaction should be equal to the sum of the heats of reaction for the other two. This concept is sometimes referred to as the additivity of heats of reaction. The primary objective of this experiment is to confirm this law. The reactions we will use in this experiment are: (1) Solid sodium hydroxide dissolves in water to form an aqueous solution of ions.

NaOH(s) Na+(aq) + OH–(aq) H1 = ? (2) Solid sodium hydroxide reacts with aqueous hydrochloric acid to form water and an aqueous solution of sodium chloride.

NaOH(s) + H+(aq) ) + Cl–(aq) H2O(l) + Na+(aq) + Cl–(aq) H2 = ? (3) Solutions of aqueous sodium hydroxide and hydrochloric acid react to form water and aqueous sodium chloride.

Na+(aq) + OH–(aq) + H+(aq) ) + Cl–(aq) H2O(l) + Na+(aq) + Cl–(aq) H3 = ? In the space below, combine two of the above equations algebraically to obtain the third equation. Indicate the number of each reaction on the shorter lines. ______ __________________________________________________________________ ______ __________________________________________________________________ ______ __________________________________________________________________

You will use a Styrofoam cup in a beaker as a calorimeter, as shown in Figure 1. For purposes of this experiment, you may assume that the heat loss to the calorimeter and the surrounding air is negligible. Even if heat is lost to either of these, it is a fairly constant factor in each part of the experiment, and has little effect on the final results. You will make three grand assumptions in this experiment:

1. The specific heat of solution is equivalent to the specific heat of water. 2. The density of solution is equivalent to the density of water. 3. All solutions are identical in temperature before mixing.

OBJECTIVES In this experiment, you will

Combine equations for two reactions to obtain the equation for a third reaction. Use a calorimeter to measure the temperature change in each of three reactions. Calculate the heat of reaction, H, for the three reactions. Use the results to confirm Hess’s law.

LabQuest 18

18 - 2 Chemistry with Vernier

MATERIALS

LabQuest 4.00 g of solid NaOH LabQuest App ring stand Temperature Probe utility clamp 50 mL of 1.0 M NaOH stirring rod 50 mL of 1.0 M HCl Styrofoam cup 100 mL of 0.50 M HCl 250 mL beaker 100 mL of water

PROCEDURE Reaction 1 1. Obtain and wear goggles.

2. Connect the Temperature Probe to LabQuest and choose New from the File menu. If you have an older sensor that does not auto-ID, manually set up the sensor.

3. Use a utility clamp to suspend a Temperature Probe from a ring stand as shown in Figure 1.

4. Place a Styrofoam cup into a 250 mL beaker as shown in Figure 1. Measure out 100.0 mL of water into the Styrofoam cup. Lower the Temperature Probe into the solution.

5. CAUTION: Handle the NaOH and resulting solution with care. Weigh out about 2 g of solid sodium hydroxide, NaOH, and record the mass to the nearest 0.01 g. Note: NaOH(s) is highly hygroscopic; it readily absorbs moisture from the surrounding environment. This additional water weight is not negligible and can directly impact your results. Make sure to weigh your solid NaOH quickly and proceed to the next step without delay. Please take care to keep the solid NaOH jar sealed when not in use. NaOH(s) that has been exposed to too much moisture will appear wet and shiny. Ask your TA to inspect the NaOH(s) is you are unsure.

6. Start data collection and obtain the initial temperature, t1. Monitor temperature (in °C) on the screen. It may take several seconds for the Temperature Probe to equilibrate at the temperature of the solution. After three or four readings at the same temperature have been obtained, add the solid NaOH to the Styrofoam cup. Using the stirring rod, stir continuously until the temperature has maximized and then begun to drop. At this point, all reactant has been consumed and your open-calorimeter system is losing heat to the surrounding environment. Record the maximum temperature, t2, before the drop in temperature occurred.

7. Data collection will stop after 3 minutes. You can manually stop data collection if your temperature peak and drop occurs before 3 minutes has elapsed. Conversely, if your peak needs longer than 3 minutes, you can adjust the LabQuest to record data for longer than the default 180 seconds.

8. To confirm the initial (t1) and final (t2) values you recorded earlier, examine the data points along the curve on the displayed graph. As you tap each point, the temperature and time values are displayed to the right of the graph.

Figure 1

Additivity of Heats of Reaction: Hess’s Law

Chemistry with Vernier 18 - 3

9. Rinse and dry the Temperature Probe, Styrofoam cup, and stirring rod. Dispose of the solution in the aqueous waste bin. Ask your TA for its location.

Reaction 2 10. CAUTION: Handle the HCl solution and NaOH solid with care. Repeat Steps 4–9, using

100.0 mL of 0.50 M hydrochloric acid, HCl, instead of water. Use the same amount of solid NaOH as before.

Reaction 3 11. CAUTION: Handle the HCl and NaOH solutions with care. Repeat Steps 4–9, initially

measuring out 50.0 mL of 1.0 M HCl (instead of water) into the Styrofoam calorimeter. In Step 5, instead of solid NaOH, measure 50.0 mL of 1.0 M NaOH solution into a graduated cylinder. After 3–4 temperature readings have been taken to determine the initial temperature of the 1.0 M HCl, add the 1.0 M NaOH solution to the Styrofoam cup.

PROCESSING THE DATA 1. Determine the mass of 100 mL of solution for each reaction (assume the density of each

solution is 1.00 g/mL).

2. Determine the temperature change, t, for each reaction.

3. Calculate the heat released by each reaction, q, by using the formula:

q = Cp•m• t (Cp = 4.18 J/g°C) Convert joules to kJ in your final answer.

4. Find H ( H = –q ). 5. Calculate moles of NaOH used in each reaction. In Reactions 1 and 2, this can be found from

the mass of the NaOH. In Reaction 3, it can be found using the molarity, M, of the NaOH and its volume, in L.

6. Use the results of the Step 4 and Step 5 calculations to determine H/mol NaOH in each of the three reactions.

7. To verify the results of the experiment, combine the heat of reaction ( H/mol) for Reaction 1 and Reaction 3. This sum should be similar to the heat of reaction ( H/mol) for Reaction 2. Using the value in Reaction 2 as the accepted value and the sum of Reactions 1 and 3 as the experimental value, find the percent error for the experiment.

LabQuest 18

18 - 4 Chemistry with Vernier

DATA AND CALCULATIONS Reaction 1 Reaction 2 Reaction 3

1. Mass of solid NaOH

g

g (no solid NaOH mass)

2. Mass (total) of solution

g

g

g

3. Final temperature, t2

°C

°C

°C

4. Initial temperature, t1

°C

°C

°C

5. Change in temperature, t

°C

°C

°C

6. Heat, q

kJ

kJ

kJ

7. H

kJ

kJ

kJ

8. Moles of NaOH

mol

mol

mol

9. H/mol

kJ/mol

kJ/mol

kJ/mol

10. Experimental value kJ/mol

11. Accepted value kJ/mol

12. Percent error % , lllll

ALL

102

ALL

102

ALL

ALL

100

ALL

27.7

ALL

33.6

ALL

28.8

ALL

23.0

ALL

23.1

ALL

22.8

ALL

4.7

ALL

10.5

ALL

6.3

ALL

4.18*100*3.2 =1337.6

ALL

102*4.185*6.1 =2603.9

ALL

100*4.18*3 =1254

ALL

0.05

ALL

0.49

ALL

-90483.8

ALL

-1387.76

ALL

-35817.6

ALL

2.5

ALL

1790.88

ALL

-4433.71

ALL

3469.4

ALL

-5260.8

ALL

-4433.71

ALL

7.23

Additivity of Heats of Reaction: Hess’s Law

Chemistry with Vernier 18 - 5

KEY LEARNING POINTS balancing reactions calorimeter/calorimetry Hess’s law heat (enthalpy) of reaction specific heat q = mC∆T

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