Cho phản ứng xảy ra trong pha khí sau: H2 + Cl2 -> 2HCl Biểu thức tốc độ trung bình của phản ứng là

\text{v}\text{=}\frac{\Delta {\text{C}}_{{\text{H}}_{\text{2}}}}{\Delta \text{t}}=\frac{\Delta {\text{C}}_{\text{C}{\text{l}}_{\text{2}}}}{\Delta \text{t}}=\frac{\Delta {\text{C}}_{\text{H}\text{C}\text{l}}}{\Delta \text{t}}.

\text{v}\text{=}\frac{\Delta {\text{C}}_{{\text{H}}_{\text{2}}}}{\Delta \text{t}}=\frac{\Delta {\text{C}}_{\text{C}{\text{l}}_{\text{2}}}}{\Delta \text{t}}=\frac{-\Delta {\text{C}}_{\text{H}\text{C}\text{l}}}{\Delta \text{t}}.

\text{v}\text{=}\frac{-\Delta {\text{C}}_{{\text{H}}_{\text{2}}}}{\Delta \text{t}}=\frac{-\Delta {\text{C}}_{\text{C}{\text{l}}_{\text{2}}}}{\Delta \text{t}}=\frac{\Delta {\text{C}}_{\text{H}\text{C}\text{l}}}{\Delta \text{t}}.

\text{v}\text{=}\frac{-\Delta {\text{C}}_{{\text{H}}_{\text{2}}}}{\Delta \text{t}}=\frac{-\Delta {\text{C}}_{\text{C}{\text{l}}_{\text{2}}}}{\Delta \text{t}}=\frac{\Delta {\text{C}}_{\text{H}\text{C}\text{l}}}{2\Delta \text{t}}.