   Chapter 23, Problem 119SCQ

Chapter
Section
Textbook Problem

Write balanced equations for the combustion of ethane gas and liquid ethanol (to give gaseous products).(a) Calculate the enthalpy of combustion of each compound. Which has the more negative enthalpy change for combustion per gram?(b) If ethanol is assumed to be partially oxidized ethane, what effect does this have on the enthalpy of combustion?

(a)

Interpretation Introduction

Interpretation:

The balanced equations for the combustion of ethane gas and liquid ethanol have to be written and calculate the enthalpy of combustion of each component.

Concept introduction:

• Balanced chemical equation of a reaction is written according to law of conservation of mass.
• Stoichiometry of a chemical reaction is the relation between reactants and products of the reaction and it is represented by the coefficients used for the reactants and products involved in the chemical equation.

Enthalpy of combustion:

• The standard enthalpy of combustion is the enthalpy change that occurs when one mole of substance burns completely under the standard conditions of 250C and 1 atm.
• The standard enthalpy of combustion is always negative.
Explanation

Let’s write the balanced equation of combustion of ethane:

2C2H6(g) + 7O2(g)  4CO2(g) + 3H2O(l)

Let’s calculate the enthalpy of combustion:

ΔHrxn=nΔHfo (products) - nΔHfo (reactants)= 4ΔHfo (CO2(g)) + 6ΔHfo (H2O(l)) - 7ΔHfo (O2(g))- 2ΔHfo(C2H6(g))= 4 × (-393.5 kj/mol) + 6×(-285.83) - 7 (0 kj/mol) - 2×(-83.85 kj/mol)= - 3121.3 kj

The combustion of one mole of ethane released - 3121.3 kj energy.

Let’s calculate the energy released by 1 gm of ethane:

Relaesed Energy for1gm ethane = -1560.6 kj/mol30 g/mol= -52 kJ/g

Therefore, Heat of combustion of ethane is - 52 kJ/g

(b)

Interpretation Introduction

Interpretation:

If ethanol is assumed to be partially oxidized ethane, then the effect does this have on the enthalpy of combustion should be determined.

Concept introduction:

Enthalpy of combustion:

• The standard enthalpy of combustion is the enthalpy change that occurs when one mole of substance burns completely under the standard conditions of 250C and 1 atm.
• The standard enthalpy of combustion is always negative.

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