General Chemistry I

Question

Hello.

I am taking General Chemistry I. I have a homework question that I do not know how to solve.

It tells me that zinc is more reactive than chromium, and that chromium is more reactive than iron.

It then asks me, "Which of the following combinations will result in a reaction?"

The combinations they provide are as follows . . .

Combination A: Cr_(s) + Zn²⁺ _(aq)

Combination B: Cr²⁺ _(aq) + Zn_(s)

Combination C: Cr_(s) + Fe²⁺ _(aq)

Combination D: Cr²⁺_(aq) + Fe_(s)

I am looking to understand the question and how to solve the problem. Short and long answers are both fine with me. Thanks!!!

Answer 1

Let's look at the reactivity of the three metals:

Zn > Cr > Fe

This means that the more reactive metal is capable of replacing/displacing a less reactive metal in a chemical reaction. It follows from this that the less reactive a metal is, the harder it is to oxidize that metal and thus the more positive will be the standard reduction potential. Put another way, the more reactive the metal, the greater the standard oxidation potential.

Looking at the various combinations: These all have to do with oxidation/reduction. The ones that will result in a reaction are the ones where the solid metal can replace/displace the aqueous cation because the aqueous cation is more reactive and will be oxidized readily.

Combination A: Cr_(s) + Zn²⁺ _(aq) ==> no reaction since Cr can't replace Zn

Combination B: Cr²⁺ _(aq) + Zn_(s) ==> Zn²⁺(aq) + Cr(s) A reaction occurs

Combination C: Cr_(s) + Fe²⁺ _(aq) ==> Cr²⁺ + Fe(s) A reaction occurs

Combination D: Cr²⁺_(aq) + Fe_(s) ==> no reaction since Fe can't replace Cr