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 3–5 sentences, describe the mistakes that the student made. Based on the oxidation numbers, is this a redox reaction? Explain your answer.

Answer 1

The student’s error is treating elemental diatomic molecules as already having their usual compound oxidation states: any element in its elemental form (H2, I2) has oxidation number 0, not +1 or −1. Also, hydrogen does not always have +1 (e.g., in metal hydrides it is −1) and group trends apply to compounds, not to an element by itself. In this reaction H goes 0 → +1 (oxidation) and I goes 0 → −1 (reduction), so electrons are transferred. Therefore the reaction is a redox reaction (half‑reactions: H2 → 2H+ + 2e− and I2 + 2e− → 2I−).