
Concept explainers
Interpretation:
Enthalpy change of combustion per gram of methanol and isooctane is to be determined.
Concept Introduction:
The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one gram of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
ΔrH0=ΣnΔfH0(products) - ΣnΔfH0(reactants)
The change in enthalpy, ΔH in kJ per mole of a given reactant for the reaction can be calculated as:
ΔrH=enthalpy changenumber of moles

Answer to Problem 85GQ
The enthalpy change of combustion per gram of isooctane, is -47.82 J/gK and the enthalpy change of combustion per gram of methanol is -22.68J/gK
Explanation of Solution
For isooctane the balanced reaction is
C8H18+ 25/2 O2→ 8 CO2+ 9 H2O
ΔrH0=ΣnΔfH0(products) - ΣnΔfH0(reactants)
ΔrH0=[(8mol×-393.5kJ/mol) +(9mol×-285.83kJ/mol)] -[(1mol×-259.2kJ/mol +0)]
ΔrH0=-5461.3kJ
The change in enthalpy is -5461.3kJ
The change in enthalpy, ΔH in kJ per mole of a given reactant for the reaction can be calculated as:
ΔrH=enthalpy changenumber of moles
ΔrH0=-5461.3kJ/mol ×1mol114.2g=-47.82 J/gK
So, the enthalpy of isooctane is -47.82 J/gK
For methanol,
2 CH3OH + 3 O2 → 2CO2 + 4 H2O
ΔrH0=ΣnΔfH0(products) - ΣnΔfH0(reactants)
ΔrH0=[(2mol×-393.5kJ/mol) +(4mol×-285.83kJ/mol)]-[(2mol×-238.4kJ/mol +0)]
ΔrH0=-1453.5kJ
The change in enthalpy is -5461.3kJ
The change in enthalpy, ΔH in kJ per mole of a given reactant for the reaction can be calculated as:
ΔrH=enthalpy changenumber of moles
ΔrH0=-1453.5kJ2mol×1mol32.04g=-22.68J/gK
So, the enthalpy of methanol is -22.68J/gK
Enthalpy change of combustion per gram of methanol and isooctane was determined.
Want to see more full solutions like this?
Chapter 5 Solutions
Chemistry & Chemical Reactivity
- Chemistry & Chemical ReactivityChemistryISBN:9781133949640Author:John C. Kotz, Paul M. Treichel, John Townsend, David TreichelPublisher:Cengage LearningChemistry & Chemical ReactivityChemistryISBN:9781337399074Author:John C. Kotz, Paul M. Treichel, John Townsend, David TreichelPublisher:Cengage LearningChemistry: Principles and PracticeChemistryISBN:9780534420123Author:Daniel L. Reger, Scott R. Goode, David W. Ball, Edward MercerPublisher:Cengage Learning
- General Chemistry - Standalone book (MindTap Cour...ChemistryISBN:9781305580343Author:Steven D. Gammon, Ebbing, Darrell Ebbing, Steven D., Darrell; Gammon, Darrell Ebbing; Steven D. Gammon, Darrell D.; Gammon, Ebbing; Steven D. Gammon; DarrellPublisher:Cengage LearningChemistry by OpenStax (2015-05-04)ChemistryISBN:9781938168390Author:Klaus Theopold, Richard H Langley, Paul Flowers, William R. Robinson, Mark BlaserPublisher:OpenStaxChemistry: The Molecular ScienceChemistryISBN:9781285199047Author:John W. Moore, Conrad L. StanitskiPublisher:Cengage Learning





