Use the chemical equation to complete the activity. 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.

Explanation:

H2(g)+I2(g)→2HI(g)H2(g)+I2(g)→2HI(g)

We can assign oxidation numbers to all the species present. The criterion is given below:

The sum of oxidation state for a neutral atom is always zero.

Free element or elements in the uncombined state has oxidation state zero.

So, oxidation state of both H2andI2H2andI2 are zero as they are present as a free elements.

In the case of HI, the oxidation state of H is +1 and I have an oxidation state of -1. Thus, the sum of oxidation number is equal to zero.

The oxidation numbers are included in the reaction as given below:

H20(g)+I20(g)→2H+1I−1(g)H20(g)+I20(g)→2H+1I−1(g)

We can see that the oxidation state of H changes from 0 to +1, this means that H loses one electron. So, it undergoes oxidation.

Similarly, we can see that the oxidation state of I changes from 0 to -1. This means that I gain one electron. So, it undergoes reduction.

Here, oxidation and reduction are taking place. Therefore, it is a redox reaction.

Now, we can analyze the observations made by the student:

1) Hydrogen always has the same oxidation number, so it has an oxidation number of +1 in both the reactant H2H2 and product HI.

This is false because the free element, H2H2 has an oxidation number of zero.

2) Iodine is in Group VIIA, so it has an oxidation number of 1 in both the reactant I2I2 and product HI.

This is false because I2I2 is present as the free element, and has an oxidation number of zero.

3) Neither hydrogen nor iodine changes the oxidation state, so the reaction is not a redox reaction.

This is false. The oxidation states of both hydrogen and iodine are changing. So, the reaction is a redox reaction.

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