Ch.4 - Chemical Quantities & Aqueous ReactionsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Oxidation and reduction deals with the transferring of electrons between reactants. The reactant that loses electrons is oxidized, while the reactant that gains electrons is reduced

Redox Reactions

Concept #1: Understanding Oxidation versus Reduction


We're going to say chemists usually use important terminology to describe the movement of electrons. Remember, electrons are the negative sub-atomic particles. We're going to say in redox reactions, we have the movement electrons from one reactant to another reactant. We’re going to use of funny type of sentence to help us remember what's going on. The sentence is LEO, the lion, goes GER. It might sound corny, but trust me helps a lot with understanding oxidation and reduction. We said LEO, the lion, goes GER. LEO – lose electrons oxidation. Goes GER – gain electrons reduction.
Remember, electrons are negatively charged. If you're losing negatively charged sub-atomic particles, you're becoming more positive. Lose electrons means you're becoming more positive. If you're gaining negative electrons, you’re gaining negative things, so you are going to become more negative.
Then here's the weird thing. If you've been oxidized, if you undergo oxidation, you’re called the reducing agent. You're the reducing agent. If you've been reduced, then you’re the oxidizing agent.

When in doubt about oxidation or reduction just remember the phrase: "LEO the lion goes GER."

Concept #2: Rules for Oxidation Numbers 


Now that we know the distinction between oxidation and reduction, it's important to set down some ground rules to help us figure out what’s the particular oxidation state on a particular element. I break rules for assigning oxidation numbers, which we’re going to abbreviate ON, so oxidation number is ON. I break it down into two sets of rules. We have our two general rules and then we have our more specific rules.
The two general rules. We're going to say for an atom in its elemental form, what does elemental form mean? Elemental form means two things. It means that the metal is by itself such as sodium. Sodium metal is by itself. Or the element is connected to itself. Oxygen here is connected to another oxygen. Sulfur here is connected to a bunch of sulfurs just like itself. If you're by yourself or you’re connected to copies of yourself and you have no charge, that is your elemental form. Let's write that down. Elemental form means you are by yourself or connected to copies of yourself and you have no charge. If you fit these types of categories, then your oxidation number is equal to 0.
The second general rule is for an ion, whatever the ion’s charge is, that's its oxidation number. Sodium here is by itself but it has a plus one charge, so its oxidation numbers is plus 1. Calcium is by itself but it has a plus 2 charge, so its oxidation number is plus two. NO3 minus, that’s our nitrate ion, our polyatomic ion. The oxidation number for the entire compound is negative 1. If we want to find the oxidation state of each of the individual elements, that's when we move on to our specific rules.
Our specific rules, we're going to say group 1A, plus 1. Group 2A, plus 2 based on the charge distributions that we learned earlier. What we’re going to say if a hydrogen, hydrogen is in group 1A but a hydrogen is not always plus 2. We're going to say it's plus 1 when it's connected to non-metals, so if it's connected to Cl or perchlorate or in water. In these three examples, water is connected to only non-metal so it’s plus 1. But if a hydrogen is connected to a metal or boron, it’s minus 1. If we had NaH or BH3, then in those cases hydrogen would be negative 1.
We’re going to say fluorine is always minus 1 no matter what. Oxygen on the other hand can be a pain because oxygen has various oxidation states. It's all based on what kind of compound it’s in. We're going to say for oxygen, when it's a peroxide, it's going to be negative 1. What exactly is a peroxide? A peroxide has the formula X2O2. X equals a group 1A element, so hydrogen, lithium, potassium – all of those are group 1A metals. A good example here, you could have hydrogen peroxide or sodium peroxide. Both are peroxides because both have the formula X2O2. In this case, oxygen would be minus 1.
Oxygen will be negative a half when it’s a superoxide. A superoxide has the formula XO2. X again is a group 1A element. Example, you could have sodium superoxide or lithium superoxide. We're going to basically say if oxygen is not a peroxide or superoxide, we can say that it's -2. This is the normal state of oxygen because we seldom see peroxide or superoxide. But if we do not see one of those two, remember oxygen would be minus 2.
Group 7A elements or halogens. They're going to be minus 1 except when they're connected to oxygen. When they're connected to oxygen, we're going to have to use some mathematical knowledge in order to do it. Simple algebra. We’ll get to see how this works when we do practice questions dealing with this concept. Remember, we have our two generals and then we have our more specific rules.

We use these oxidation number rules whenever we are asked to determine the oxidation number of an entire compound or the individual elements within a compound. 

Example #1: What is the oxidation number of each underlined element? 

Instead of being asked who is oxidized or reduced, sometimes you may be asked to determine the reducing and oxidizing agents

Example #2: In the following reaction identify the oxidizing agent and the reducing agent

Try your skills at determing the oxidation number of elements within molecules or polyatomic ions. 

Practice: What is the oxidation number of each underlined element?

a) P4                                  b) BO33–                                c) AsO42–                           d) HSO4

Just remember if you've been oxidized then you're the reducing agent and if you've been reduced then you're the oxidizing agent.

Practice: In the following reaction identify the oxidizing agent and the reducing agent:

Cr2O72- + 6 Fe2+ + 14 H→ 2 Cr3+ + 6 Fe3+ + 7 H2O

Basic Redox Concepts

Concept #3: OXIDATION-REDUCTION (REDOX) reactions deal with the transfer of electrons from one reaction to another.

Concept #4: From our understanding of redox reactions we can draw connections to voltage, current, charge and power.