Chemistry: An Atoms-Focused Approach
Chemistry: An Atoms-Focused Approach
14th Edition
ISBN: 9780393912340
Author: Thomas R. Gilbert, Rein V. Kirss, Natalie Foster
Publisher: W. W. Norton & Company
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Chapter 9, Problem 9.92QA
Interpretation Introduction

To calculate:

The enthalpy changes of the following reactions under standard conditions using average bond energies.

a) CO2g+H2gH2Og+CO(g)

b) N2g+O2g2 NO(g)

c) Cs+CO2g2CO(g)

The heat of sublimation of graphite Cs is 719 kJ/mol.

Expert Solution & Answer
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Answer to Problem 9.92QA

Solution:

a) CO2g+H2gH2Og+CO(g); Hrxno=36 kJ

b) N2g+O2g2 NO(g); Hrxno=87 kJ

c) Cs+CO2g2CO(g); Hrxno=173 kJ

Explanation of Solution

1) Concept:

Bond energy is the amount of energy required when one mole of bonds in the gas phase is broken. This can be used to calculate the heat of reaction (enthalpy change) by combining the H values (bond energies) for bonds that are broken and bonds that are formed.

H values are positive for bonds broken.

H values are negative for bonds formed.

Hrxno=Hf bond breakingo-Hf  bond formingo

2) Formula:

Hrxno=Hf bond breakingo-Hf  bond formingo

3) Given:

The heat of sublimation of graphite Cs is 719 kJ/mol.

Bond energies from Appendix 4: Table A4.1

Bonds Bond energy (kJ/mol)
C=O 743
H-H 436
O-H 463
CO 1072
NN 945
O=O 498
NO 678

4) Calculation:

a) CO2g+H2gH2Og+CO(g)

Bonds Number of bonds (mol) Bond energy (kJ/mol) ΔH
Bonds broken C=O 2 799 kJ/mol 2 mol ×799 kJ/mol
H-H 1 436 kJ/mol 1 mol ×436 kJ/mol
Bonds formed O-H 2 463 kJ/mol -2 mol ×463 kJ/mol
CO 1 1072 kJ/mol -1 mol ×1072 kJ/mol

Hrxno=2 mol ×799 kJ/mol+1 mol ×436 kJ/mol-2 mol ×463 kJ/mol+1 mol ×1072 kJ/mol

Hrxno=2034 kJ-1998 kJ=36 kJ

b) N2g+O2g2 NO(g)

Bonds Number of bonds (mol) Bond energy (kJ/mol) ΔH
Bonds broken NN 1 945 kJ/mol 1 mol ×945 kJ/mol
O=O 1 498 kJ/mol 1 mol ×498 kJ/mol
Bonds formed NO 2 678 kJ/mol -2 mol ×678 kJ/mol

Hrxno=1 mol ×945 kJ/mol+1 mol ×498 kJ/mol-2 mol ×678 kJ/mol

Hrxno= 661 kJ-1356 kJ=87 kJ

c) Cs+CO2g2CO(g)

CsC(g); Hsub=719 kJ/mol

Bonds Number of bonds (mol) Bond energy (kJ/mol) ΔH
Bonds broken C=O 2 799 kJ/mol 2 mol ×799 kJ/mol
Bonds formed CO 2 1072 kJ/mol -2 mol ×1072 kJ/mol

Here, solid carbon is converted to gaseous form. Therefore, the heat of sublimation also need to be taken into consideration.

Hrxno=1 mol ×719 kJ/mol+2 mol ×799 kJ/mol-2 mol ×1072 kJ/mol

Hrxno= 2317 kJ-2144 kJ=173 kJ

Conclusion:

The enthalpy change is the difference between the H values (bond energies) for bonds that are broken and bonds that are formed.

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Chapter 9 Solutions

Chemistry: An Atoms-Focused Approach

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