Oxidation Number Mcat

6. The elements in Group IIA form compounds (such as Mg₃N₂ and CaCO₃) in which the metal atom has a +2 oxidation number.

The oxidation number is the effective charge on an atom in a compound; the oxidation number is useful in determining the nomenclature, or name, of chemical compounds.

The oxidation number describes explicitly the degree to which an element can be oxidized (lose electrons) or reduced(gain electrons). There is a fixed set of rules that can be followed to determine the oxidation number. The oxidation number can typically be found above groups in the periodic table.

1. The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O₂, O₃, P₄, S₈, and aluminium metal all have an oxidation number of 0.

2. The oxidation number of simple ions is equal to the charge on the ion. The oxidation number of sodium in the Na + ion is +1, for example, and the oxidation number of chlorine in the Cl – ion is -1.

3. The oxidation number of hydrogen is +1 when it is combined with a nonmetal as in CH₄, NH₃, H₂O, and HCl.

4. The oxidation number of hydrogen is -1 when it is combined with metal as in LiH, NaH, CaH₂, and LiAlH₄.

5. The metals in Group IA form compounds (such as Li₃N and Na₂S) in which the metal atom has an oxidation number of +1.

6. The elements in Group IIA form compounds (such as Mg₃N₂ and CaCO₃) in which the metal atom has a +2 oxidation number.

7. Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O₂, O₃, H₂O₂, and the O₂ 2- ion.

8. The elements in Group VIIA often form compounds (such as AlF₃, HCl, and ZnBr₂) in which the nonmetal has a -1 oxidation number.

9. The sum of the oxidation numbers in a neutral compound is zero.

Redox reactions occur when one element is oxidized and another reduced in the same reaction. For example, the reaction between zinc and sulfur, which produces zinc sulfide:

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The half-reactions can be written:

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In the reaction above, the zinc is being oxidized by losing electrons. However, there must be another substance present that gains those electrons, and in this case, that is the sulfur. In other words, the sulfur is causing the zinc to be oxidized. Sulfur is called the oxidizing agent. The zinc causes the sulfur to gain electrons and become reduced and so the zinc is called the reducing agent. The oxidizing agent is a substance that causes oxidation by accepting electrons. The reducing agent is a substance that causes reduction by losing electrons. The simplest way to think of this is that the oxidizing agent is the substance that is reduced, while the reducing agent is the substance that is oxidized. The sample problem below shows how to analyze a redox reaction.

A different type of reaction where redox can be seen is a disproportionation reaction, where a compound with an intermediate oxidation number converts into two compounds, one with a higher and one with a lower oxidation number. In the example of hydrogen peroxide decomposition oxygen is present in the reactant and both products. In H ₂ O ₂ , oxygen has an oxidation state of -1. In H ₂ O, its oxidation state is -2, and it has been reduced. In O ₂ however, its oxidation state is 0, and it has been oxidized. Oxygen has been both oxidized and reduced in the reaction.

Practice Questions

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MCAT Official Prep (AAMC)

Chemistry Question Pack Question 55

Chemistry Question Pack Passage 16 Question 89

Chemistry Question Pack Passage 19 Question 107

Chemistry Question Pack Passage 20 Question 115

Sample Test C/P Section Passage 5 Question 22

Practice Exam 2 C/P Section Passage 5 Question 24

Practice Exam 4 C/P Section Passage 1 Question 3

Key Points

• The oxidation number tells how many electrons the element can give or be given

• The oxidation state of a pure element is always zero.

• The oxidation state for a pure ion is equivalent to its ionic charge.

• In general, hydrogen has an oxidation state of +1, while oxygen has an oxidation state of -2.

• The sum of the oxidation states for all atoms of a neutral molecule must add up to zero.

• Reduction is the gain of electrons, which causes a decrease in oxidation state

• Oxidation is the loss of electrons, which causes an increase in oxidation state

• In disproportionate reactions, a chemical species is both reduced and oxidized at the same time

Key Terms

Oxidation: the loss of electrons in a substance (Cu to Cu +2 )

Reduction: the gain of electrons in a substance (Cu +2 to Cu)

Disproportionation reaction: reaction where a compound with an intermediate oxidation number converts into two compounds, one with a higher and one with a lower oxidation number than the intermediate oxidation number

Oxidizing agent: a substance that causes oxidation by accepting electrons

Reducing agent: a substance that causes reduction by losing electrons

Oxidation number: the effective charge on an atom in a compound

Oxidation and Reduction Reactions for the MCAT: Everything You Need to Know

Learn key MCAT concepts about oxidation and reduction reactions, plus practice questions and answers

(Note: This guide is part of our MCAT General Chemistry series.)

Table of Contents

Part 1: Introduction to oxidation and reduction reactions

Part 2: Overview of atomic structure

Part 3: Oxidation and reduction

a) Definitions

b) Assigning oxidation states

c) Hydrides

Part 4: Applications of redox reactions

a) Writing and balancing redox reactions

b) Electrochemical cells

c) Electron transport chain

Part 5: High-yield terms

Part 6: Passage-based questions

Part 7: Standalone practice questions

Part 1: Introduction to oxidation and reduction reactions

Redox reactions constitute fundamental processes in our daily lives. Consider combustion, which creates fire and heat from oxygen gas and hydrocarbon fuel. Oxidation and reduction reactions are also key in creating energy from the food we eat. Additionally, these processes are key to the operations behind batteries, in which reduction and oxidation reactions generate the power we need to drive our cars.

Redox reactions refer to chemical reactions in which the exchange of electrons results in the oxidation of some atoms and the reduction of others. Success on this topic requires a detailed understanding of both oxidation and reduction and how oxidation states change over the course of a given reaction.

In this guide, we’ll start to break down the essentials of oxidation and reduction reactions needed to know for the MCAT. Throughout this guide, you will see several important words defined in bold. At the end of this guide, there are also several AAMC-style practice questions for you to test your knowledge against.

Part 2: Overview of atomic structure

How are electrons spatially distributed in an atom? The electron cloud model provides us with an answer and proves to be useful in visualizing how electrons are lost (oxidation) and gained (reduction) in redox reactions.

The electron cloud model, developed by Erwin Schrödinger in the 1920s, describes electrons as being distributed within certain regions of probability around the central nucleus. The electron cloud model—which is the preferred model of modern scientists—is distinct from Bohr’s atomic model in which electrons were imagined to move in discrete concentric orbits around the nucleus—much like satellites around the Earth or the cabins of a Ferris wheel.

The electron cloud model emphasizes the indistinct nature of electron distribution. The electron cloud model theorized that electrons do not move in static orbits around the central nucleus, such that electrons are always a specific and discrete distance away from the center of the atom. Instead, we can only guess where an electron might be—that guess is mathematically computed and described as a region of probability called the electron cloud. These regions of probability can be visualized and illustrated as certain shapes, which gives rise to the atomic theory of subshells and electron orbitals.

For more information on atoms and how they behave, be sure to refer to our guide on atomic and nuclear physics.