   Chapter 8, Problem 95SCQ

Chapter
Section
Textbook Problem

White phosphorus exists as P4 molecules with phosphorus atoms at the corners of a tetrahedron. (a) Elemental phosphorus reacts with Cl2 to form PCl3. Write a balanced chemical equation for this reaction. (b) The standard enthalpy of formation ( Δ f H ∘ ) of P4(g) is +58.9 kJ/mol; for PCl3(g) it is −287.0 kJ/mol. Use these numbers to calculate the enthalpy change for the reaction of P4(g) and Cl2(g) to give PCl3(g). (c) Based on the equation in (a), determine what bonds are broken and what bonds are formed. Then, use this information along with the value of Δ r H ∘ calculated in (b) and the bond dissociation enthalpies for the P—P and Cl—Cl bonds from Table 8.8 to estimate the bond dissociation enthalpy for the P—Cl bonds. How does your estimate compare with the value in Table 8.8?

(a)

Interpretation Introduction

Interpretation:

The balanced chemical equation for the reaction of phosphorus with chlorine to form PCl3 has to be determined.

Concept Introduction:

In a balanced equation the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product.

Coefficient is a number placed before a formula in a chemical equation.

A balanced equation should be obeying the law of conservation of mass. Law of conservation of mass states that, the number of atoms remains constant throughout the reaction, simply it can be stated as follows, “during a chemical reaction atoms are neither be created nor be destroyed”.

Explanation

White phosphorous exists as P4 molecule with phosphorous atoms at the corners of a tetrahedron.

Element phosphorous react with Cl2 to form PCl3

(b)

Interpretation Introduction

Interpretation:

The enthalpy change of the reaction of P4 and Cl2 to PCl3 has to be determined.

Concept Introduction:

Bond energy or more correctly the bond dissociation enthalpy is the enthalpy change when breaking a bond in a molecule with the reactant and products in the gas phase.

ΔrH=ΔH(bondsbroken)ΔH(bondsformed)

Enthalpy of formation:

ΔrH=ΔfH0(products)ΔfH0(reactants)

(c)

Interpretation Introduction

Interpretation:

The bond dissociation enthalpy for PCl bond has to be determined.

Concept Introduction:

Bond energy or more correctly the bond dissociation enthalpy is the enthalpy change when breaking a bond in a molecule with the reactant and products in the gas phase.

ΔrH=ΔH(bondsbroken)ΔH(bondsformed)

Enthalpy of formation:

ΔrH=ΔfH0(products)ΔfH0(reactants)

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