H2(g) + I2(g) → 2HI(g) A student makes the following statements: Hydrogen always has the same oxidation number, so it has an oxidation number of +1 in both the reactant H2 and product HI. Iodine is in Group VIIA, so it has an oxidation number of –1 in both the reactant I2 and product HI. Neither hydrogen nor iodine changes oxidation states, so the reaction is not a redox reaction. In three to five sentences, describe the mistake that the student made, and determine whether or not the reaction is a redox reaction. Explain your answer.
The student made the mistake of assuming that hydrogen always has an oxidation number of +1 and iodine always has an oxidation number of -1 in all compounds. However, in the reactant H2, hydrogen has an oxidation number of 0 since it exists in its elemental form. In the product HI, hydrogen has an oxidation number of +1 and iodine has an oxidation number of -1. Therefore, the oxidation number of hydrogen and iodine change in this reaction, indicating that it is indeed a redox reaction.
