## Hess's Law Hands-On Labs, Inc. Version 42-0158-00-02 Lab Report Assistant This document is not meant to be a substitute for a formal laboratory...

Use Hess’s Law to determine ΔH for the first 2 reactions and then add them together to determine ΔH for the third reaction: NH3 + HCl à NH4Cl.

Hess’s Law

Hands-On Labs, Inc.

Version 42-0158-00-02 Lab Report Assistant

This document is not meant to be a substitute for a formal laboratory report. The Lab Report

Assistant is simply a summary of the experiment’s questions, diagrams if needed, and data

tables that should be addressed in a formal lab report. The intent is to facilitate students’

writing of lab reports by providing this information in an editable file which can be sent to an

instructor. Exercise 1: Testing Hess’s Law

Data Table 1. Temperature (C) of the Solution (NaOH + HCl).

Initial Temperature of NaOH (C)

Initial Temperature of HCl (C)

Average Initial Temperature of Two Separate Solutions (C)

Highest Temperature of Mixture (May Be Extrapolated from Graph) (C)

Change in Temperature of Mixture (C), ΔT

Data Table 2. Temperature of Solution Every 20 Seconds.

Time after mixing (min:sec)

Initial

0:20

0:40

1:00

1:20

1:40

2:00

2:20

2:40

3:00

3:20

3:40

4:00

4:20

4:40

5:00 Temperature (oC)

26.7

35.4

35.9

35.8

35.9

35.9

35.8

35.7

35.6

35.5

35.3

35.1

35.0

34.9

34.7

34.6 24.8

24.9

24.85

35.9

9.2 Graph 1. Reaction 1 (NaOH + HCl).

Graph (NaOH + HCl)

40

35 35.4 35.9 35.8 35.9 35.9 35.8 35.7 35.6 35.5 35.3 35.1 35 34.9 34.7 34.6 20 40 1:00 1:20 1:40 2:00 2:20 2:40 3:00 3:20 3:40 4:00 4:20 4:40 5:00 30

26.7

25

20

15

10

5

0

Initial Data Table 3. Temperature (°C) of the Solution (NaOH + NH4Cl).

Initial Temperature of NaOH (C)

Initial Temperature NH4Cl (C)

Average Initial Temperature (C)

Highest Temperature of Mixture (May Be Extrapolated from Graph) (C)

Change in Temperature of Mixture (C), ΔT

Data Table 4. Temperature of Solution Every 20 Seconds.

Time after mixing (sec)

Initial

0:20

0:40

1:00

1:20

1:40

2:00

2:20

2:40 Temperature (oC)

23.7

25.3

25.5

25.8

26

25.8

25.7

25.8

25.7 24.9

25.6

25.25

26

2.3 Time after mixing (sec)

3:00

3:20

3:40

4:00

4:20

4:40

5:00 Temperature (oC)

25.7

25.8

25.8

25.7

25.7

25.7

25.7 Graph 2. Reaction 2 (NaOH + NH4Cl).

Graph (NaOH + NH4Cl)

26.5

26 26

25.8 25.5 25.8 25.7 25.8 25.7 25.7 2:40 3:00 25.8 25.8 3:20 3:40 25.7 25.7 25.7 25.7 4:00 4:20 4:40 5:00 25.5

25.3 25

24.5

24

23.7

23.5

23

22.5

Initial 20 40 1:00 1:20 1:40 2:00 2:20 Data Table 5. Temperature (°C) of the Solution (NH3 + HCl).

Initial Temperature of NH3 (C)

Initial Temperature HCl (C)

Average Initial Temperature (C)

Highest Temperature of Mixture (May Be Extrapolated from Graph) (C)

Change in Temperature of Mixture (C), ΔT

Data Table 6. Temperature of Solution Every 20 Seconds.

Time after mixing (sec)

Initial Temperature (oC)

25.2 25.7

25.3

25.5

32.8

7.6 0:20

0:40

1:00

1:20

1:40

2:00

2:20

2:40

3:00

3:20

3:40

4:00

4:20

4:40

5:00 32.5

32.7

32.8

32.7

32.6

32.4

32.3

32.1

32

31.9

31.8

31.7

31.6

31.5

31.4 Graph 3. Reaction 3 (NH3 + HCl).

Graph (NH3 + HCl)

35

32.5 32.7 32.8 32.7 32.6 32.4 32.3 32.1 32 31.9 31.8 31.7 31.6 31.5 31.4 20 40 1:00 1:20 1:40 2:00 2:20 2:40 3:00 3:20 3:40 4:00 4:20 4:40 5:00 30

25.2

25

20

15

10

5

0

Initial Questions

A. What were the starting temperatures for each of the reactions? Use the scatter plots in

Graph 1, Graph 2, and Graph 3 to support your answers. 26.7

23.7

25.2

B. Calculate the heat loss or heat gain of the 3 solution mixtures (qrxn). Then calculate the ΔH

for each reaction. Show your calculations. C. Use Hess’s Law to determine ΔH for the first 2 reactions and then add them together to

determine ΔH for the third reaction: NH3 + HCl NH4Cl. D. Compare the results of question “C” with the experimental results of the reaction:

NH3 + HCl NH4Cl (calculate the percent difference). E. Use the thermodynamic quantities given below to calculate the theoretical ΔH for this

reaction: NH3 + HCl NH4Cl ΔH°f for NH3 (aq) = - 80.29 kJ/mol ΔH°f for HCl (aq) = - 167.2 kJ/mol ΔH°f for NH4+ (aq) = - 132.5 kJ/mol ΔH°f for Cl- (aq) = - 167.2 kJ/mol F. What was the percent difference of the various methods used when comparing the results

of Hesss Law method and the experimental results to the theoretical value? G. Were the reactions performed in this laboratory exercise exothermic or endothermic? How

could you determine this? H. Define Hess’s Law. Did your experimental results support Hess’s Law? Use your data to

explain your answer. I. What are some possible sources of error in this experiment?

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