Question

Identify the oxidation numbers for each element in the following equations. Then determine whether each equation describes a redox reaction. Justify your answer in terms of electron transfer. Describe the oxidation and reduction that occurs.

a. Ca + 2HCl → CaCl2 + H2 (4 points)

b. 2KOH + K2Cr2O7 → 2K2CrO4 + H2O (4 points)

c. AgNO3 + KI → AgI + KNO3 (4 points)

d. 2PbS + 3O2 → 2PbO + 2SO2 (5 points)

Answer 1

Like the problem suggests, assign oxidation states first. Based on that, see whether the reaction contains one oxidized *and* one reduced compound. If so, it’s a redox reaction. If not, it’s not a redox reaction.

I’ll start with problem a and let you try the rest on your own.

Ca is a pure element, so its oxidation state is 0.

HCl —> H is almost always +1, Cl must be -1 since the whole compound is uncharged, so the charges must sum to 0.

CaCl2 —> Cl is always -1 so Ca must be +2 to balance out the 2 chlorine atoms.

H2 —> pure element, so 0

Notice how Ca went from oxidation state 0 to 2. Because it increased we can say that Ca was oxidized.

Notice how the H in HCl went from +1 to 0. So we can say HCl was reduced.

Since the reaction contains both oxidation and reduction we can say this reaction was a redox reaction.

Using this same process can you try problems b-d? I’ll attach a link with oxidation state rules.

https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/22%3A_Oxidation-Reduction_Reactions/22.06%3A_Assigning_Oxidation_Numbers