Question

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 3–4 sentences, describe the mistakes that the student made. Based on the oxidation numbers, is this a redox reaction? Explain your answer. please make your answer simple, coherent, and intended for 9th grade.

Answer 1

The student forgot that elements in their natural, elemental form (H2 and I2) have oxidation number 0, not +1 or −1. In the product HI hydrogen is +1 and iodine is −1, so both elements do change oxidation numbers. Hydrogen goes 0 → +1 (it is oxidized, loses electrons) and iodine goes 0 → −1 (it is reduced, gains electrons). Because one species is oxidized and another is reduced, the reaction is a redox reaction.