CHEMISTRY

Hess’ Law

Peter Jeschofnig, Ph.D. Version 42-0158-00-01

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.

Part 1: Reaction: HCl & NaOH → NaCl + H2O

Data Table 1: Sample data
Initial Temperature of HCl – oC 26.2 C°
Initial Temperature NaOH – oC 26.6 C°
Average Initial Temperature – oC 26.4 C°
Final Temperature of mixture

(extrapolated)

38.8 C°
Change in Temperature of mixture, ΔT 12.4 C°

( Ex p e ri m e n t ) ( Hes s ’ L a w )

( 90 ) ( © H a nds-On L a bs, I n c . ) ( ww w . L a b P a q . c o m )

Data Table 2: Sample data
Time after mixing- seconds Temperature – °C
20 38.5 C°
40 38.5 C°
60 38.4 C°
80 38.3 C°
100 38.3 C°
120 38.3 C°
140 38.1 C°
160 38.0 C°
180 37.9 C°
200 37.7 C°
220 37.5 C°
240 37.4 C°
260 37.3 C°
280 37.1 C°
300 37.0 C°

Part 2: Reaction 2: NH4Cl + NaOH → NH3 + NaCl + H2O

Data Table 3:
Initial Temperature of NaOH – oC 27.2 C°
Initial Temperature NH Cl – oC

4

27.4 C°
Average Initial Temperature – oC 27.3 C°
Final Temperature of mixture (extrapolated) 27.8 C°
Change in Temperature of mixture, ΔT 0.5 C°

( 94 ) ( © H a nds-On L a bs, I n c . ) ( ww w . L a b P a q . c o m )

Data Table 4:
Time after mixing- seconds Temperature – oC
20 27.8 C°
40 27.8 C°
60 27.8 C°
80 27.8 C°
100 27.8 C°
120 27.7 C°
140 27.7 C°
160 27.7 C°
180 27.7 C°
200 27.7 C°
220 27.7 C°
240 27.7 C°
260 27.7 C°
280 27.7 C°
300 27.7 C°

Part 3: Reaction : NH3 + HCl → NH4Cl

Data Table 5:
Initial Temperature of HCl – oC 27.8 C°
Initial Temperature NH – oC

3

28.2 C°
Average Initial Temperature – oC 28 C°
Final Temperature of mixture (extrapolated) 32.5 C°
Change in Temperature of mixture, ΔT 4.5 C°

( 97 ) ( © H a nds-On L a bs, I n c . ) ( ww w . L a b P a q . c o m )

Data Table 6:
Time after mixing- seconds Temperature – oC
20 32.5
40 32.4
60 32.3
80 32.2
100 32.1
120 32.1
140 32
160 31.9
180 31.9
200 31.8
220 31.8
240 31.7
260 31.7
280 31.6
300 31.6

Questions

For A. through E. See the calculations for the Data Tables above.

A. Using the data from your data tables calculate ΔT for all three reactions: 17.4 C° is total change in temperatures of all 3 reactions

B. Calculate the heat loss or gain of the three solution mixtures:

17.4 C° is total heat gain in temperatures of all 3 reactions

C. Use Hess’ Law and ΔH for the first two reactions: NaOH (aq) + HCl (aq) → H2O (l) + NaCl (aq)

NaOH (aq) + NH4Cl (aq) → NH3 + NaCl + H2O (l)

to determine ΔH for this reaction: NH3 + HCl → NH4Cl

D. Compare the results of step 3 above with the experimental results of the

NH3 + HCl → NH4Cl

( 101 ) ( © H a nds-On L a bs, I n c . ) ( ww w . L a b P a q . c o m )

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

80.29 + 167.2 -132.5-167.2 = -52.21 kJ/mol

F. What was the ΔH value obtained for NH3 + HCl à NH4Cl from Hess’ Law method? G. What was the ΔH value obtained for NH3 + HCl à NH4Cl experimentally?

H. What was the calculated ΔH value obtained for NH3 + HCl à NH4Cl using published thermodynamic data?

I. What was the % error of the various methods used? (i.e. comparing the results of the results of Hess’ Law method and the experimental results to the calculated value?

J. Name three examples of the practical application for the use of ΔH values.

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