   Chapter 5, Problem 81GQ

Chapter
Section
Textbook Problem

The standard molar enthalpy of formation of diborane, B2H6(g), cannot be determined directly because the compound cannot be prepared by the reaction of boron and hydrogen. It can be calculated from other enthalpy changes, however. The following enthalpy changes can be measured.4 B(s) + 3 O2(g) → 2 B2O3(s)ΔrH° = –2543.8 kJ/mol-rxnH2(g) +½ O2(g) → H2O(g)ΔrH° = –241.8 kl/mol-rxnB2H6(g) + 3 O2(g) → B2O3(s) + 3 H2O(g)ΔrH° = –2032.9 kJ/mol-rxn(a) Show how these equations can be added together to give the equation for the formation of B2H6(g) from B(s) and H2(g) in their standard states. Assign enthalpy changes to each reaction.(b) Calculate ΔfH° for B2H6(g).(c) Draw an energy level diagram that shows how the various enthalpies in this problem are related.(d) Is the formation of B2H6(g) from its elements exo- or endothermic?

(a)

Interpretation Introduction

Interpretation:

The equation for the formation of B2H6 from B(s)andH2(g) has 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)

Explanation

First equation is:

4B+3O22B2O3ΔrH0=-2543.8kJ/mol

Divide the 1st equation by 2

2B+3/2O2B2O3ΔrH0=-1271

(b)

Interpretation Introduction

Interpretation:

The formation energy for B2H6(g) has 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)

(c)

Interpretation Introduction

Interpretation:

The energy level diagram for the various enthalpies in the problem has to be depicted.

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)

(d)

Interpretation Introduction

Interpretation:

The nature of the thermodynamic reaction has to be identified.

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)

When heat energy is transferred the directionality of heat transfer between the system and the surrounding is described either as endothermic or exothermic

Energy of the system decrease and energy of the surrounding increases, i.e qsys<0, Energy is transferred as heat from the surroundings to the system and work is done by the system -exothermic.

Endothermic process, the heat energy is transferred from surroundings to the system. Energy of system increases and that of surroundings decreases i.e, qsys>0

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