Photochemical Reaction of Bromine with Hydrogen at Room Temperature

The rate of the photochemical production of hydrogen bromide was measured in a hydrogen–bromine system irradiated with 5780‐Å light. The rate was found to be proportional to the square of the concentration of bromine atoms. Absolute light intensities were determined and third‐order rate constants for the equation $d\mathrm{(HBr)}{\text{\hspace{0.17em}/\hspace{0.17em}d\hspace{0.17em}t\hspace{0.17em}=2k[Br]}}^2{\mathrm{[H}}_2]$ were determined at 0° and 25°C. The rate constant is given by ${\text{k\hspace{0.17em}=\hspace{0.17em}[\hspace{0.17em}(4.8\hspace{0.17em}±\hspace{0.17em}4)\hspace{0.17em}×\hspace{0.17em}10}}^5\exp \text{(0∓800/RT)}{\mathrm{liter}}^2{\mathrm{mole}}^{\text{−2}}·{\sec }^{\text{−1}}$ . The data of previous photochemical experiments at room temperature, usually but erroneously considered to be consistent with the chain mechanism, are shown to be consistent with the third‐order reaction.