Redox Reactions

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: Understanding Oxidation versus Reduction

Video Transcript

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: Rules for Oxidation Numbers 

Video Transcript

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: What is the oxidation number of each underlined element? 

I2                                                   Cs2O2                                                 HCO3


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

Example: 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. 

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


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.

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


Redox Reactions Additional Practice Problems

Consider the redox equation shown below and determine which of the statements(s) that follow is/are correct.

SeO32-(aq) + Cl2(aq) → SeO42-(s) + Cl-(aq)
i.  SeO42- is the reducing agent.
ii. Cl2 is the oxidizing agent.
iii. Cl- is the oxidizing agent.

A. i only

B. ii only

C. iii only

D. i and iii

E. ii and iii

Watch Solution

Identify the oxidation and reduction steps in the following reaction:

4 Fe + 3 O2 2 Fe2O3


Watch Solution

Identify the oxidation and reduction steps in the following reaction:

4Fe +3O2 2Fe2O3


Watch Solution

Which is the reducing agent in the (unbalanced) reaction?

S2O32-(aq) +  I2(aq) +  H+(aq)  →  I1-(aq)  + S4O62-(aq)


 A)  S2O32-    B)  H+    C) I 2      D)  S4O62-  E)  I1-


Watch Solution

What element is being oxidized in the following redox reaction?
H2O2(l) + ClO2(aq) → ClO21-(aq) + O2(g)

A) H
B) O
C) C
D) Cl
E) This is not a Redox Rxn

Watch Solution

Which of the following is the weakest oxidizing agent?

Sn2+ + 2e- → Sn°                                      -0.14
Cr3+ + 3e- → Cr°                                       -0.73
Sn4+ + 4e- → Sn2+                                    +0.15
Mn2+ + 2e- → Mn°                                     -1.18
Cr2+ + 2e- → Cr°                                       -0.23

A) Sn2+(aq)

B) Mn2+(aq)

C) Sn4+(aq)

D) Cr2+(aq)

E) Cr3+(aq)

Watch Solution

What element is being oxidized in the following redox reaction?

Cr(OH)4-(aq) + ClO-(aq) → CrO42-(aq) + Cl-(aq)

A) O

B) Cr

C) H

D) Cl


Watch Solution

Reduction involves which of the following?

A) decrease in oxidation number

B) loss of electrons

C) gain of electrons

D) increase in oxidation number

E) both A and C

Watch Solution

What element is being oxidized in the following redox reaction? 

Cr2O72-(aq) + Mn2O3(s) → Cr3+(aq) + MnO2(aq)

A) O

B) Cr

C) Mn

D) Cl

E) both Mn and Cl

Watch Solution

Which of the following is True?

A) reduction results in a decrease in the oxidation number

B) reduction is the loss of electrons

C) reduction occurs at the cathode

D) an increase in the oxidation number occurs only at the cathode

E) both A and C

Watch Solution

Consider the following standard reduction potentials in acid solution:

Al3+ + 3e- → Al(s)                                - 1.66
AgBr(s) + e- → Ag(s) + Br -                   + 0.07
Sn4+ + 2e- → Sn2+                              + 0.14
Fe3+ + e- → Fe2+                                 + 0.77

The strongest oxidizing agent among those shown above is 

A) Fe3+.

B) Fe2+.

C) Br -.

D) Al3+.

E) Al.

Watch Solution

What is the oxidizing agent in the (unbalanced) reaction?

Cu(s) + H+(aq) + NO3-(aq) → NO(g) + H2O(l) + Cu2+(aq)

A) Cu

B) H+

C) NO3-


E) Cu2+

Watch Solution

Of the substances below, which is the weakest reducing agent?

a) Na (s)

b) K+ (aq)

c) Cl2 (g)

d) Br (aq)

e) Fe2+ (aq)

Watch Solution

Which statement is most true of the reaction:

Bi2S3 (s) + 3 H+ (aq) → 2 Bi3+ (aq)+ 3 H2S (aq)

a) This is a redox reaction because Bi2S3 serves as the oxidizing agent and H+ as the reducing agent

b) This is a redox reaction because Bi2S3 serves as the reducing agent and H+ as the oxidizing agent

c) This is a redox reaction because Bi is oxidized and H is reduced

d) Both a and c are true

e) None of the above are true because the reaction is not a redox reaction



Watch Solution

Which substance below is the strongest reducing agent? See the attached table of standard reduction potentials.

a) Li+ (aq)

b) Li (s)

c) F (aq)

d) F2 (g)

e) H2 (g)

Watch Solution

Is the following reaction an example of a redox reaction?

Cr2O72– (aq) + H2O (l) ⇄ 2 CrO 42– (aq) + 2 H+ (aq)


a) Yes, Cr is oxidized and oxygen is reduced

b) Yes, Cr is oxidized and hydrogen is reduced

c) Yes, oxygen is oxidized and Cr is reduced

d) Yes, hydrogen is oxidized and Cr is reduced

e) No, neither hydrogen, oxygen, nor Cr are oxidized or reduced

Watch Solution

Which substance serves as the reducing agent in the following unbalanced reaction that occurs in acid:

Cr2O72– (aq) + Ni (s) → Ni 2+ (aq) + 2 Cr 3+ (aq) (acidic solution)


a) H+ (aq)

b) Cr2O72– (aq)

c) Ni (s)

d) Ni2+ (aq)

e) H2O (l)


Watch Solution

Using the table below, which substance can oxidize I (aq) to I2 (s)?

a) Br2 (l)

b) Ni2+ (aq)

c) Ag (s)

d) Cu2+ (aq)

e) More than one of the above

Watch Solution

Consider the following balanced redox reaction: 

3CuO(s) + 2NH3(aq) → N2(g) + 3H2O(l) + 3Cu(s)

Which of the following statements is true?

a) CuO(s) is the oxidizing agent and copper is oxidized.

b) CuO(s) is the oxidizing agent and copper is reduced.

c) CuO(s) is the reducing agent and copper is oxidized.

d) CuO(s) is the reducing agent and copper is reduced.

e) CuO(s) is the oxidizing agent and N2(g) is the reducing agent.

Watch Solution

Which one of the following is  not a redox reaction?

a) 2H2O2(aq) → 2H2O(l) + O2(g

b) C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

c) Na6FeCl8(s) + 2Na(l) → 8NaCl(s) + Fe(s)

d) Al(OH)4-(aq) + 4H+(aq) → Al3+(aq) + 4H2O(l)

e) CO2(g) + H2(g) → CO(g) + H2O(g)               

Watch Solution

You have an aqueous solution of FeCl 3  and CdCl2 . Using the table of standard reduction potentials, what metal ions or could you add to this solution in order to  selectively remove  the iron ions while not removing the cadmium ions? (circle all that apply)







Watch Solution

In which group can each substance act as an oxidizing agent?

A. Cl2, MnO2, Cu

B. Cl2, MnO4, Cu2+

C. Cl-, MnO4, Cu+

D. Cl2, Mn, Cu2+

Watch Solution

When designing a voltaic cell with the following cell notation:

                        Li(s) | Li+(aq) ‖ Zn2+(aq) | Zn(s)

That uses a salt bridge made of NaCl, what answer describes what is occurring at the anode?

A.  Lithium is being oxidized and Cl  joins the solution

B.  Lithium is being reduced and Cl  joins the solution

C.  Zinc is being oxidized and Cl joins the solution

D.  Zinc is being oxidized and Na+ joins the solution

E.  Lithium is being oxidized and Na+ joins the solution

Watch Solution

15. Which of the following reactions is/are redox reactions?

(1) K2CrO4 + BaCl2 → BaCrO4 + 2KCl

(2) Ag+ + Br → AgBr

(3) Cu + S → CuS      

A) only reaction (1)     

B) only reaction (2)     

C) only reaction (3)             

D) both reactions (1) and (3)            

E) both reactions (2) and (3)

Watch Solution

Given the overall redox reaction:  Fe2+(aq) + Ce4+(aq) → Fe3+(aq) + Ce3+(aq)

Identify the two half-reactions

Oxidation half-reaction                            Reduction half-reaction

A) Fe2+(aq) → Fe3+(aq) + e                     Ce4+(aq) + e → Ce3+(aq)

B) Ce4+(aq) + e → Ce3+(aq)                    Fe2+(aq) → Fe3+(aq) + e

C) Fe2+(aq) + e → Fe3+(aq)                     Ce4+(aq) → Ce3+(aq) + e

D) Ce4+(aq) → Ce3+(aq) + e                    Fe2+(aq) + e → Fe3+(aq)

E) Fe3+(aq) →Fe2+(aq) + e                      Ce4+(aq) → Ce3+(aq) + e

Watch Solution

Identify the oxidation and reduction steps in the following reaction:

4Fe +3O2 → 2Fe2O3

A.  Oxidize: O2 → 2 O2 + 2 e                  Reduced: Fe → Fe  3+ + 3 e

B.  Oxidize: Fe → Fe2+ + 2 e                    Reduced: O2 → 2 O2 + 4 e

C.  Oxidize: Fe → Fe3+ + 3 e                    Reduced: 4 e + O2 → 2 O2 

D.  Oxidize: Fe → Fe3+ + 3 e−                      Reduced: O2 → 2 O2+ 4 e

E.  Oxidize: O2 → 2 O2 + 4 e                  Reduced: Fe → Fe3+ + 3 e


Watch Solution

Of the substances below, which is the weakest reducing agent?

a. Na (s)

b. K+ (aq)

c. Cl2 (g)

d. Br (aq)

e. Fe2+ (aq)

Watch Solution

Which of the following statements is true of an oxidizing agent.

a) A strong oxidizing agent must be a weak reducing agent

b) An oxidizing agent facilitates the oxidation of another substance

c) An oxidizing agent is itself oxidized during a redox reaction

d) Both statements b and c are true

e) All of the above statements are true

Watch Solution

Use the standard reduction potentials below to determine which element or ion is the best reducing agent.

Pd2+ (aq) + 2 e        →      Pd (s) E° = + 0.90 V

2 H+ (aq) + 2 e –        →        H2 (g) E° = 0.00 V

Mn2+ (aq) + 2 e        →      Mn (s) E° = – 1.18 V


a) Pd (s)             b) H + (aq)              c) Mn2+ (aq)              d) H2 (g)

Watch Solution

In the chemical reaction, Zn (s) + CuSO4 (aq)   ⇌   ZnSO4 (aq) + Cu (s),


A. Metallic zinc is the reducing agent

B. Metallic zinc is reduced

C. Copper ion is oxidized

D. Sulfate ion is the oxidizing agent 

Watch Solution

Consider the reaction

2 Fe3+ (aq) + 2 I (aq)   ⇌   2 Fe2+ (aq) + I2 (aq)

Which statement for the reaction is true?


A. Fe3+ is oxidized

B. Fe3+ increases in oxidation number

C. Fe3+ is reduced

D. I is reduced 

Watch Solution