Question

Consider the following reaction: Mg(s) + ZnSO4(aq) → MgSO4(aq) + Zn(s) Which of the following statements regarding this reaction is correct?
The sulfate ion is reduced.
Zinc is the reducing agent.
Magnesium is neither oxidized nor reduced.
Zinc gains two electrons.
Magnesium is the oxidizing agent.

Answer 1

Answer:

Zinc gains two electrons.

Explanation:

First, you give each compound/element its oxidation number.

Zn(s) and Mg(s): 0 because they are both in its pure elemental form.

For the two compounds in the reaction, we find the oxidation number by finding the "charge" of each element in it.

ZnSO4: the Zn2+ has a +2 oxidation number and SO42- (which we can keep as a compound) has a -2 oxidation number.

MgSO4: same thing here... Mg2+ has a +2 oxidation number and SO42- still has -2.

Therefore, we can see that the Mg is going from a 0 to +2, the Zn is going from a +2 to 0, and the SO42- is unchanged at -2. So you can tell from this information that Mg is being oxidized and that Zn is being reduced. This makes Mg a reducing agent and Zn an oxidizing agent (remember, the agents are the ones causing the opposite reaction to happen).

Looking at the answer choices:

The sulfate ion is reduced.  False, because it remained unchanged.

Zinc is the reducing agent.  False, because it is the oxidizing agent.

Magnesium is neither oxidized nor reduced.  False, because it was actually oxidized.

Zinc gains two electrons.  True, because it went from a +2 oxidation number to 0, which meant it lost 2 oxidation numbers (i.e. gaining 2 negative charges).

Magnesium is the oxidizing agent. False, because it is the reducing agent.

Answer 2

Answer:

Zinc gains two electrons.

Explanation:

We can use oxidation numbers to figure out what is happening with the electrons.

Oxidation is the Loss Of Electrons. It is also an increase in oxidation number. The substance that is oxidized is the reducing agent.

Reduction is the Gain Of Electrons. It is also an decrease in oxidation number. The substance that is reduced is the oxidizing agent.

$\rm \stackrel{\hbox{0}}{\hbox{Mg}} + \stackrel{\hbox{+2}}{\hbox{Zn}}\stackrel{\hbox{+6}}{\hbox{S}}\stackrel{\hbox{-2}}{\hbox{O}}_{4} \,\longrightarrow \, \stackrel{\hbox{+2}}{\hbox{Mg}}\stackrel{\hbox{+6}}{\hbox{S}}\stackrel{\hbox{-2}}{\hbox{O}}_{4} + \stackrel{\hbox{0}}{\hbox{Zn}}$

The oxidation number of Zn decreases from +2 to 0. The zinc has lost two positive charges, so it has gained two electrons.

A is wrong. The oxidation numbers of the atoms in SO₄²⁻ have not changed, so the sulfate ion has not been oxidized or reduced.

B is wrong. Zn has gained electrons. It has been reduced, so it is the oxidizing agent,

C is wrong. The oxidation number of Mg has increased, so it has been oxidized.

E is wrong. Mg has been oxidized, so it is the reducing agent.