Xét phản ứng trong quá trình tạo ra NO_x nhiệt:

${\text{N}}_2\left(\text{g}\right)+{\text{O}}_2\left(\text{g}\right)\to 2\text{NO}\left(\text{g}\right){\text{ \hspace{0.17em} \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}Δ}}_{\text{r}}{\text{H}}_{298}^{\text{o}}=180,6\text{ kJ}$

Nhiệt tạo thành chuẩn của $\text{NO}\left(\text{g}\right)$ là
A. 180,6 kJ / mol.
B. -180,6 kJ / mol.
C. -90,3 kJ / mol.
D. 90,3 kJ / mol.

Đáp án đúng là: D

$$\begin{gathered} \frac{1}{2}{\text{N}}_2\left(\text{ g}\right)+\frac{1}{2}{\text{O}}_2\left(\text{ g}\right)\stackrel{t°}{\to }\text{NO}\left(\text{g}\right) \\ {\Delta }_f{H}_{298}^0\left(NO\right)=\frac{180,6}{2}=90,3kJ \end{gathered}$$