Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074



Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

Using thermodynamic data, estimate the normal boiling point of ethanol. (Recall that liquid and vapor are in equilibrium at 1.0 atm pressure at the normal boiling point.) The actual normal boiling point is 78 °C. How well does your calculated result agree with the actual value?

Interpretation Introduction


The normal boiling point of ethanol should be estimated using thermodynamic data which then has to be identified that how well it agreed with the actual value.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔrG. It is related to entropy and entropy by the following expression,


Here, ΔrH is the change in enthalpy and ΔrS is the change in entropy.

The value of ΔrG is zero at the boiling point of the liquid. Thus the entropy change can be written as,


Thus, the temperature at which vaporization takes place can be calculated if ΔrH and ΔrS values are known.


The normal boiling point of ethanol is calculated below.

The Appendix L referred for the values of standard entropies.

The standard entropy of C2H5OH(l) is 160.7 J/Kmol.

The standard entropy of C2H5OH(g) is 282.7 J/Kmol.

The standard enthalpy of C2H5OH(l) is 277.7 kJ/mol.

The standard enthalpy of C2H5OH(g) is 235.3 kJ/mol.

The given reaction is,


The expression for the standard entropy change is,

ΔrS°=nS°(products)nS°(reactants)=[(1 mol C2H5OH(g)/mol-rxn)S°[C2H5OH(g)] (1 mol C2H5OH(l)/mol-rxn)S°[C2H5OH(l)]]

Substituting the entropy values,

ΔrS°=[(1 mol C2H5OH(g)/mol- rxn)(282.7 J/Kmol)(1 mol C2H5OH(l)/mol- rxn)(160

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