Redox reactions

Question 1

Define an oxidation half reaction.

Answer 1

An oxidation half reaction is one where electrons are lost in a redox reaction.

Question 2

Define a reduction half reaction.

Answer 2

A reduction half reaction is one where electrons are gained in a redox reaction.

Question 3

Define oxidizing agent.

Answer 3

An oxidizing agent is a chemical that gains electrons over the course of a redox reaction. An oxidizing agent causes a different chemical to be oxidized.

Question 4

Define reducing agent.

Answer 4

A reducing agent is a chemical that loses electrons over the course of a redox reaction. A reducing agent causes a different chemical to be reduced.

Question 5

Define oxidation number.

Answer 5

Oxidation number (O.N., which is the same as “oxidation state”) represents either the apparent or actual charge of an atomic particle. In the case of a monoatomic ion (e.g. Mg²⁺) the oxidation number is the actual charge. In the case of a molecular compound (e.g. SO₂), then the oxidation number represents the apparent charge. For SO₂, the oxygen has an oxidation number of -2 and the sulfur +4. This type of apparent charge is the result of unequal sharing of electrons in covalent bonds of a molecule, where it is as if oxygen was given all of the electrons.

Question 6

What is the oxidation number of an individual atom in a pure element?

Answer 6

The oxidation number of an individual atom of a pure element = 0. Therefore N₂, Cl₂, Fe, Hg, C all have oxidation numbers = 0.

Question 7

What is the oxidation number of a monoatomic ion?

Answer 7

The oxidation number of a monoatomic ion is equal to the charge of the ion. Therefore Na⁺ has an oxidation number = +1; S^2- has an oxidation number = -2.

Question 8

What is usually the oxidation number of hydrogen in a compound?

Answer 8

Hydrogen usually has an oxidation number = +1. An exception to this (which is uncommon in the CHEM 112 course) is if hydrogen is the anion in an ionic compound. Therefore in LiH, hydrogen has an oxidation number = -1.

Question 9

What is usually the oxidation number of oxygen in a compound?

Answer 9

Oxygen usually has an oxidation number = -2. An exception is this (which is uncommon in the CHEM 112 course) is when oxygen is in a peroxide compound. For instance in hydrogen peroxide (H₂O₂), hydrogen has priority over oxygen and has an oxidation number of +1. This means, in this unique case, that oxygen must have an oxidation number of -1.

Question 10

Why is it important to know if you have an ionic compound in determining oxidation numbers?

Answer 10

It is very important to know if you are working with an ionic compound! If so, you can separate the compound into the individual ions. Any monoatomic ions will have oxidation numbers equal to the charge of the ion. For example the ionic compound Na₂CO₃ consists of Na⁺ ions and CO₃^2- ions. The Na⁺ ions have oxidation number = +1. For the CO₃^2-, one can assign oxygen to have an oxidation number = -2 and therefore the carbon must be +4 (+4 - 3(-2) = -2).

Question 11

What is the oxidation number for Zn?

Answer 11

Zn consists of individual atoms in a free element therefore the oxidation number = 0.

Question 12

What is the oxidation number for Zn in ZnCl₂?

Answer 12

ZnCl₂ is an ionic compound. The monoatomic ion (Zn²⁺) has an oxidation number = +2.

Question 13

What is the oxidation number for Cl in ZnCl₂?

Answer 13

ZnCl₂ is an ionic compound. The monoatomic ion (Cl^-) has an oxidation number = -1.

Question 14

What is the oxidation number for Mn in MnSO₄?

Answer 14

MnSO₄ is an ionic compound. The monoatomic ion (Mn²⁺) has an oxidation number = +2. (This is in order to balance the 2- charge from the sulfate anion).

Question 15

What is the oxidation number for O in MnSO₄?

Answer 15

Oxygen (O) is part of the polyatomic anion SO₄^2-. As is typically the case, oxygen has the oxidation number = -2.

Question 16

What is the oxidation number for S in MnSO₄?

Answer 16

Sulfur (S) is part of the polyatomic anion SO₄^2-. Sulfur must have an oxidation number = +6 (+6 - 4(-2) = -2).

Question 17

In a chemical reaction zinc is changed from ZnCl₂ to Zn. Has zinc been oxidized or reduced?

Answer 17

Zinc is reduced as electrons are gained in converting Zn⁺² to Zn (zero oxidation number).

Question 18

In the chemical reaction: 2HCl + Zn → ZnCl₂ + H₂, which chemical is the reducing agent?

Answer 18

Zn is the reducing agent as it loses electrons (to become Zn²⁺) and reduce H in HCl to H₂.

Question 19

Is the following statement true or false: metal elements can only be reducing agents in redox reactions.

Answer 19

True statement. Metal elements in redox reactions can only lose electrons to become cations. A metal would never gain an electron to become an anion!

Question 20

If molecular oxygen (O₂) is involved in a redox reaction, is it likely to behave as an oxidizing agent or a reducing agent?

Answer 20

Molecular oxygen (O₂) will typically behave as an oxidizing agent. The oxidation states that we know for oxygen are -2 (and -1 in peroxides), therefore in changing from zero oxidation number to a negative oxidation number, electrons must be gained (which is the role of an oxidizing agent).