Oxidation & Reduction....I'm Already Lost!

"He who controls the past controls the future. He who controls the present controls the past." -George Orwell (1984)

For most students, it is either the start or the end of your much needed Spring Break. Don't let the idea of Spring Break damped your mood, at least you have the NCAA Basketball tournament looking forward to help you endure the brief moments of final exam week which is the sign of semester coming to a close or the preparation for AP exams for most high school students :(.

I have a good inclination that if you are reading this blog article that you are either in college or high school trying to perform well on your exam or almost on the brink of giving up (figuratively). I would like to give you a warm hug welcome if this is your first time viewing the website or a returning student. The idea of someone introducing or speaking about "oxidation or reduction" in the same sentence either conjure an emotion of confusion or that moment when you have a "blank" stare. You are free to cast that heavy burden of stress from your shoulder and call your favorite shipping courier to pick up the "desserts" package you patiently wrapped. (lol)

In this article, I will cover the concept of oxidation and reduction and how you can identify or recognize if a reaction involve is an oxidation and/or reduction.


Let's start with a simple example as our ice-breaker to illustrate the key parts of an oxidation-reduction reaction. Below is a reduction reaction between an iron ore and carbon monoxide to give iron metal and carbon dioxide.

 Fe₂O₃(s) + 3 CO(g) ———————> 2 Fe(s) + 3 CO₂((g)

As you can see from the reaction provided, the carbon monoxide gained an extra oxygen atom in the form of carbon dioxide. This process is called oxidation. In addition, carbon dioxide is also the agent (reactant) that brings about the reduction of iron ore to iron metal. CO is coined as reducing agent.

3 CO(g) ---> 3 CO₂((g), an oxygen atom is gained and is oxidized.

Reducing agent: the substance that donates electrons and is oxidized in the redox reaction.

Oxidation: refers to the loss of an electron by an atom, ion, or molecule.

Fe₂O₃(s) ---> 2 Fe(s), loss oxygen and is reduced

Another way to look at oxidation is any process in which oxygen is added to another substance. Let look at another chemical reaction example to visually see the process oxidation.

2 Mg(s) + O₂(g) ———————> 2 MgO(s)

The balanced chemical equation provided is between magnesium and oxygen. In this case, magnesium combined with oxygen and thus oxidized. Oxygen is the oxidizing agent because it is the substance that brings the oxidation.

2 Mg(s) ---> 2 MgO(s), magnesium gains an oxygen atom and oxidized

O₂(g) --->  2 MgO(s), oxygen loss an oxygen atom and is reduced.

Oxidizing agent: the substance that gain electrons and is reduced in the redox reaction.

Reduction: refers to the gain of an electron by an atom, ion, or molecule.

"Take-Away Message"

From the two examples presented, there are several insights that are observed:

1. If one substance undergoes oxidation (loss of e^-), another substance in the same reaction must be reduced (gain of e^-). Thus the name oxidation & reduction or aka redox reaction.
2. Reducing agent is itself oxidized, and the oxidizing agent is reduced.
3. Oxidation is the opposite of reduction. i.e. ( removal of oxygen is reduction and adding of oxygen is oxidation.

Mental Note: Point # 2 & # 3 are related. This is the hardest part understanding oxidation and reduction because if you mislabeled the incorrect substance it affects your identification if something is reduced or oxidized. Here is a neat trick on how I keep those terminology in my head.

Reducing agent - the word "reducing" implies opposite. "reducing" in the word "reducing agent" means oxidation. 

Oxidizing agent - the word "oxidizing" implies opposite. "oxidizing" in the word "oxidizing" agent means reduction.

Let's switched gears and talked about how the electron transfer from one substance to another substance in the redox reactions.

Word of Caution: Not all redox reaction involves oxygen, but all oxidation and reduction reactions INVOLVE transfer of electrons between substances.

With that being said and out of the way, let use an example to illustrate this point.

                                     2 Ag⁺(aq) + Cu(s) ———————> 2 Ag(s) + Cu²⁺(aq)

Let take the given chemical reaction and break into two part which is make it easier to see the election transfer.

2 Ag⁺(aq) + e^- ---> 2 Ag(s), accepts electrons from Cu and is reduced. Ag⁺ is the oxidizing agent by definition.

Cu(s) ---> Cu²⁺ + 2 e^-

Cu donates electron to Ag⁺ and is oxidized to Cu²⁺. Cu is the reducing agent.

"Take-Away Message"

In a typical redox (oxidation & reduction) reaction depicted as X + Y ———————>  + Y^n- can be divided into 2 parts:

• X ———> X^n- + : n e^-: X is oxidized, losing n electrons (to Y) to form Xⁿ⁺.  X is the reducing agent in the process.
• Y + n e^-———> Y^n-: Y is reducing, gaining n electrons (supplied by X) to form Y^n-.  X is the oxidizing agent in the process.

*Remember oxidizing and reducing agent are ALWAYS substances in the reactant (located on the left of the chemical equation)

So now, we had a better understanding of what a redox reaction involving electron transfer from one substance to another substance. At this point, we have not answer the question "How can you tell an oxidation-reduction reaction when you see one"?

When the reaction given is pretty obvious, then identify if it is a redox reaction is easy. What happened when the reaction is not obvious? We use the concept called oxidation number to answer that question.

Oxidation number: number assigned to each element in a compound to in order to keep track of the electrons during a reaction.

I know your next obvious question is how to determine the oxidation number in a redox reaction. Here are the guideline for assigning oxidation numbers.

Assigning Oxidation Guideline

1. Each atom in a pure element has an oxidation number of zero.

• i.e. (Cu(s), I₂ , S₈ , have value of "0" oxidation number.

     2. For monatomic ions, the oxidation number is equal to the charge of an ion.

• i.e.  Mg²⁺ has an oxidation value of 2.

     3. Fluorine always has an oxidation number of -1 in compounds.

     4. Cl, Br, and I always have oxidation number of -1 in compounds, EXCEPT when combined with    oxygen and fluorine.

• i.e. NaCl (Na has an oxidation value of +1 so Cl has an oxidation value of -1.
• i.e.  ClO^- (O has an oxidation value of -2 so Cl has an oxidation value of +1 in order to get a net charge of "-1"

      5. The oxidation number of H is +1 and of O is -2 in most compounds. Few exceptions apply.

• i.e. when H forms a binary compound with a metal, the metal forms a positive charge and H becomes a hydride ion (means H has a negative charge),  H^-.
• i.e.  CaH₂ (Ca has an oxidation value of +2 and H has an oxidation value of -1 in order to make the compound charge neutral.)
• i.e oxygen can have an oxidation number of -1 in a class of compounds called peroxides.
• i.e.  H₂O₂ (H is assigned its usual oxidation number of +1, and so O is -1).

       6. The algebraic sum of the oxidation number in a neutral compound must be zero; in a polyatomic ion, the sum must equal to the ion charge.

Let put our oxidation number guideline that we just learn to the test by going over an example to highlight the key points.

2 Na(s) + Cl₂(g) ———————> 2 NaCl (s)

2 Na(s): oxidation value of "0" by applying rule # 1

Cl₂(g) : oxidation value of "0" by applying rule # 1

2 NaCl (s): oxidation value for Cl is "-1" by applying rule # 4. oxidation value for Na is "+1" by applying rule # 6.

I am aware that the information we just covered can be overwhelming initially. So let summarized what we just discussed thus far.

Recognizing Oxidation-Reduction Reactions

                                                                 Oxidation                                              Reduction

1. In terms of oxidation #              increase in oxidation # of an atom               increase in oxidation # of an atom
2. In terms of electrons                    loss of electron by an atom                              gain of electrons by an atom
3. In terms of oxygen                       gain of one or more O atoms                           loss of one or more O atoms

Last but not least, here are the key vocabularies that were presented in this oxidation and reduction topic as it is very important to understand.

Key Terms

1. Reducing Agent
2. Oxidizing Agent
3. Reduction
4. Oxidation
5. Oxidation Number

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