Kaplan MCAT General Chemistry Review

Reactions that involve the transfer of electrons from one chemical species to another can be classified as oxidation-reduction reactions. Now, that’s quite a mouthful to say, so these reactions are commonly called “redox” reactions as a shorthand way of noting that oxidation and reduction are always coupled.

OXIDATION AND REDUCTION

The law of conservation of charge states that an electrical charge can be neither created nor destroyed. Thus, an isolated loss or gain of electrons cannot occur; oxidation (loss of electrons) and reduction (gain of electrons) must occur simultaneously, resulting in an electron transfer called a redox reaction. An oxidizing agent causes another atom in a redox reaction to undergo oxidation and is itself reduced. A reducing agent causes the other atom to be reduced and is itself oxidized. There are various memory devices designed to help you remember these terms. One that is especially well known is OIL RIG, which stands for “Oxidation Is Loss; Reduction Is Gain.”

Mnemonic

OIL RIG stands for ^“Oxidation Is Loss, Reduction Is Gain,^” because, as we do often, we are talking about those all-important electrons. Alternatively, reduction is just what it sounds like: reduction of charge.

ASSIGNING OXIDATION NUMBERS

It is important, of course, to know which atom is oxidized and which is reduced. Oxidation numbers are assigned to atoms in order to keep track of the redistribution of electrons during chemical reactions. From the oxidation numbers of the reactants and products, it is possible to determine how many electrons are gained or lost by each atom. The oxidation number of an atom in a compound is assigned according to the following rules:

1. The oxidation number of free elements is zero. For example, the atoms in N₂, P₄, S₈, and He all have oxidation numbers of zero.

2. The oxidation number for a monatomic ion is equal to the charge of the ion. For example, the oxidation numbers for Na⁺, Cu²⁺, Fe³⁺, Cl^–, and N^3– are +1, +2, +3, -1, and -3, respectively.

MCAT Expertise

Don’t forget that you have the periodic table available to you on Test Day, so use it to organize your thoughts with regard to these rules rather than memorizing these on their own. Beware of transition metals but realize we can often figure their oxidation by default.

3. The oxidation number of each Group IA element in a compound is +1. The oxidation number of each Group IIA element in a compound is +2.

4. The oxidation number of each Group VIIA element in a compound is -1, except when combined with an element of higher electronegativity. For example, in HCl, the oxidation number of Cl is -1; in HOCl, however, the oxidation number of Cl is +1.

5. The oxidation number of hydrogen is -1 in compounds with less electronegative elements than hydrogen (Groups IA and IIA). Examples include NaH and CaH₂. The more common oxidation number of hydrogen is +1.

6. In most compounds, the oxidation number of oxygen is -2. This is not the case, however, in molecules such OF₂. Here, because F is more electronegative than O, the oxidation number of oxygen is +2. Also, in peroxides, such as BaO₂, the oxidation number of O is -1 instead of -2 because of the structure of the peroxide ion, [O-O]^2–. (Note that Ba, a Group IIA element, cannot be a +4 cation.)

7. The sum of the oxidation numbers of all the atoms present in a neutral compound is zero. The sum of the oxidation numbers of the atoms present in a polyatomic ion is equal to the charge of the ion. Thus, for SO₄^2-, the sum of the oxidation numbers must be -2.

Key Concept

The conventions of formula writing put cation first and anion second. Thus NaH implies H^– while HCl implies H⁺. So use the way the compound is written on the MCAT along with the periodic table to help you determine oxidation states.

Example: Assign oxidation numbers to the atoms in the following reaction in order to determine the oxidized and reduced species and the oxidizing and reducing agents.

SnCl₂ + PbCl₄ SnCl₄ + PbCl₂