   Chapter 5, Problem 52PS

Chapter
Section
Textbook Problem

You wish to know the enthalpy change for the formation of liquid PCl3 from the elements.P4(s) + 6 Cl2(g) → 4 PCl3(ℓ) ΔrH° = ?The enthalpy change for the formation of PC15 from the elements can be determined experimentally, as can the enthalpy change for the reaction of PCl3 (ℓ) with more chlorine to give PCl5(s):P4(s) + 10 Cl2(g) → 4 PCl5(s)ΔrH° = –1774.0 kJ/mol-rxnPCl3(ℓ) + Cl2(g) → PC15(s)ΔrH° = –123.8 kJ/mol-rxnUse these data to calculate the enthalpy change for the formation of 1.00 mol of PCl3(ℓ) from phosphorus and chlorine.

Interpretation Introduction

Interpretation:

The enthalpy change for the formation of 1mole of PCl3 has to be determined.

Concept Introduction:

If a reaction proceeds in two or more other reaction,ΔrH0 for the overall process is the sum of ΔrH0 values of those reactions-Hess’s law.

Explanation

The required equation for the formation of PCl3 is found out by multiplying the 2nd equation by 4

4PCl3+4Cl24PCl5

And then by subtracting the 2nd equation from the 1st equation, so we get,

P4+6Cl24PCl3

ΔrH20=4×-123.8kJ/mol =-495

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