Chapter 20, Problem 20.38P

Interpretation Introduction

Interpretation:

For combustion of ammonia to nitrogen dioxide and water vapour ${\text{ΔS}}_{\text{rxn}}^{\text{o}}$ has to be calculated and the sign of ${\text{ΔS}}_{\text{rxn}}^{\text{o}}$ has to be identified whether it matches with the expected one.

Concept Introduction:

Entropy change: The sign of ${\text{ΔS}}^{\text{o}}$ for a reaction is determined by using the following rules:

• When a molecule is broken down and gives two or more smaller molecules.
• When the moles of gas is increases (by the breaking of molecules)
• Solid changes to liquid or gas state or liquid state changes to gas state.

Entropy$\left(S\right)$: it is used to describe the disorder. It is the amount of arrangements possible in a system at a particular state.

If the disorder increases in a system, then $\Delta S>0$ positive

If the disorder decreases in a system, then $\Delta S<0$ negative

If the disorder equal in a system, then $\Delta S=0$

Entropy is the measure of randomness in the system.  Standard entropy change in a reaction is the difference in entropy of the products and reactants.  ${\text{(ΔS}}^{\text{°}}{}_{\text{rxn}}\text{)}$ can be calculated by the following equation.

  ${\text{ΔS}}^{\text{°}}{}_{\text{rxn}}{\text{ = S}}^{\text{°}}{}_{\text{Products}}-{\text{ S}}^{\text{°}}{}_{\text{reactants}}$

Where,

  ${\text{S}}^{\text{°}}{}_{\text{reactants}}$ is the standard entropy of the reactants

  ${\text{S}}^{\text{°}}{}_{\text{Products}}$ is the standard entropy of the products