   Chapter 21, Problem 45PS

Chapter
Section
Textbook Problem

When boron hydrides burn in air, the reactions are very exothermic (a) Write a balanced equation for the combustion of B5H9(g) in air to give B2O3(s) and H2O(g). (b) Calculate the enthalpy of combustion for B5H9(g) (ΔfH° = 73.2 kJ/mol), and compare it with the enthalpy of combustion of B2H6 (−2038 kJ/mol). (The enthalpy of formation of B2O3(s) is −1271.9 kJ/mol.) (c) Compare the enthalpy of combustion of C2H6(g) with that of B2H6(g). Which transfers more energy as heat per gram?

(a)

Interpretation Introduction

Interpretation:

The balanced chemical equation for the combustion of B5H9(g) should be written.

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
Explanation

Boron hydride of the formula B5H9(g) reacts with the oxygen present in air to give oxide of boron. The equation is said to be balanced if it has an equal number of atoms on both product and reactant side. This is done by multiplying the compounds with the stoichiometric coefficient.

The balanced chemical equation for the combustion of B5H9(g) in air is,

(b)

Interpretation Introduction

Interpretation:

The enthalpy of combustion of B5H9(g) should be determined.

Concept introduction:

Enthalpy of reaction is the energy required or energy released when a chemical reaction occurs. If the process is endothermic then the energy is absorbed on the formation of the product. In such a case the sign of enthalpy change is positive. If the process is exothermic, then the energy is released on the formation of the product. In such a case the sign of enthalpy change is negative. In general, combustion reactions are exothermic in nature.

The standard enthalpy change is expressed as,

ΔrH°=nΔfH°(products)nΔfH°(reactants)

(c)

Interpretation Introduction

Interpretation:

The enthalpy of combustion of C2H6(g) should be given and compare the value with the enthapy of combustion of B5H9(g).

Concept introduction:

Enthalpy of reaction is the energy required or energy released when a chemical reaction occurs. If the process is endothermic then the energy is absorbed on the formation of the product. In such a case the sign of enthalpy change is positive. If the process is exothermic, then the energy is released on the formation of the product. In such a case the sign of enthalpy change is negative. In general, combustion reactions are exothermic in nature.

The standard enthalpy change is expressed as,

ΔrH°=nΔfH°(products)nΔfH°(reactants)

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