Question

Many oxides of nitrogen have positive values for the standard free energy of formation. Using $\mathbf{\text{NO}}$as an example, explain why this is the case.

Short answer

Many oxides of nitrogen have positive values for the standard free energy of formation because $NO$and other nitrogen oxides have weaker bonds compared to the triple bond of${\text{N}}_{\text{2}}$ and the double bond of $O_{\text{2}}$.

Step-by-step solution

Concept Introduction

The free energy change associated with the production of a substance from the elements in their most stable state is known as the standard free energy of a substance.

Formation of Nitrogen Oxide

The reaction of nitrogen oxide formation is shown below.

$\frac{1}{2}{\text{N}}_{\text{2}}\text{(g) +}\frac{{\displaystyle 1}}{{\displaystyle 2}}{\text{O}}_{\text{2}}\text{(g)}\to \text{NO(g)}$

We can calculate the free energy change that accompanies the formation of of $1mol$that compound from its elements in their standard states.

$$\begin{gathered} {\text{∆G°=∆G}}_f^{°}\text{(NO) -}\frac{{\displaystyle 1}}{{\displaystyle 2}}∆G_f^{°}\left({\text{N}}_{\text{2}}\right)\text{-}\frac{1}{2}∆G_f^{°}\left({\text{O}}_{\text{2}}\right) \\ ∆G°\text{=1mol.87KJ/mol} \\ ∆G°=87kJ \end{gathered}$$

Explanation for positive standard free energy

We see that the value of the free energy of formation for$NO$is positive.$NO$, and some other nitrogen oxides have higher(positive) free energies of formation as compared to the relatively stable${\text{N}}_{\text{2}}$and$O_{\text{2}}$.

The reason for this is that nitric oxide and some other nitrogen oxides have weaker bonds compared to the triple bond of${\text{N}}_{\text{2}}$ and the double bond of $O_{\text{2}}$.