High School
SAT
ACT
University
MCAT
LSAT
DAT
Wize High School Grade 12 Chemistry Textbook > Electrochemistry

Redox Reactions

Oxidation StatesPrevious Section
Balancing Redox Reactions: Half-Reaction MethodNext Section
Popular Courses
Find My Course
0:00 / 0:00

Redox Reactions

OIL RIG! Oxidation Is Loss of electrons, Reduction Is Gain of electrons
OR
LEO GER! Loss of Electrons is Oxidation, Gain of Electrons is Reduction

OXIDATION (lose e-) REDUCTION (gain e−)

_____________

Reducing agent reduces others and is itself Oxidizing agent oxidizes others and is itself
oxidized. reduced.

Oxidation number increases. Oxidation number is reduced.


PAGE BREAK


Keep in mind that in organic chemistry the definitions of oxidation and reduction are different:
  • For reduction, we would look for more bonds to H and less bonds to O
  • For oxidation, we would look for less bonds to H and more bonds to O!


PAGE BREAK

How can you recognize if a redox reaction is occuring?

1. Can check oxidation #s of each element to see if they change from reactants to products (most important for now!)

2. Familiarize yourself with common oxidizing and reducing agents (you'll learn more about this in orgo!)
  • Oxidizing agents: O2, halogens, HNO3, Cr2O72-, MnO4-
  • Reducing agents: H2, metals, C



0:00 / 0:00
Example 1:
In the following reaction, what is being oxidized and what is being reduced? What is acting as the reducing agent and what is acting as the oxidizing agent?

2Mg + O2 --> 2MgO

Mg: 0 -> 2+ lost electrons, is oxidized, is the reducing agent
O: 0 -> -2 gained electrons, is reduced, is the oxidizing agent

Write the oxidation half reaction:
Mg --> Mg2+ + 2e-

Write the reduction half reaction:
O2 + 4e- --> 2O2-


PAGE BREAK

Example 2:
In the following reaction, what is being oxidized and what is being reduced? What is acting as the reducing agent and what is acting as the oxidizing agent?

Chemical Equation: Zn + CuSO4 --> ZnSO4 + Cu
Net ionic equation: Zn + Cu2+ --> Zn2+ + Cu

Zn: 0 -> 2+ lost electrons, is oxidized, is the reducing agent
Cu: 2+ -> 0 gains electrons, is reduced, is the oxidizing agent

Write the oxidation half reaction:
Zn -> Zn2+ + 2e-

Write the reduction half reaction:
Cu2+ + 2e- -> Cu
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is being oxidized?
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is the reducing agent?
0:00 / 0:00

Disproportionation Reaction


  • This is a reaction where an element is being oxidized and reduced!

For example:


2H2O2 --> 2H2O + O2





Write out the oxidation states for each of the atoms above.

What is being oxidized?

What is being reduced?



checklist
Mark Yourself Question
  1. Grab a piece of paper and try this problem yourself.
  2. When you're done, check the "I have answered this question" box below.
  3. View the solution and report whether you got it right or wrong.
Decide if the following reactions are redox reactions or not. Hint: To do this you'll need to assign oxidation numbers to each element. If the reaction is a redox reaction, identify the oxidizing agent and reducing agent.


1. Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g)





2. H2SO4(aq) + 2NaOH(aq) -> Na2SO4(aq) + 2H2O(l)



Practice: Oxidizing vs Reducing Agents

What is the reducing agent in the following chemical reaction?

NO + Au+ → NO3- + Au
Extra Practice
Electrochemistry: Reduction potential application
Reduction Potentials Application
Below we see a diagram showing mitochondrial cellular respiration.
If we just consider the electron carrier NADH, NADH molecules gives up their electrons to the first complex, then the electron goes to the 2nd complex, then the 3rd, then the 4th and finally oxygen (O2) is the final electron acceptor.
Electrochemistry: Reduction potential application
Reduction Potentials Application
Below we see a diagram showing mitochondrial cellular respiration.
If we just consider the electron carrier NADH, NADH molecules gives up their electrons to the first complex, then the electron goes to the 2nd complex, then the 3rd, then the 4th and finally oxygen (O2) is the final electron acceptor.
Electrochemistry: Redox
Write the half-reactions for the following redox reaction. Which is the oxidizing agent and which is the reducing agent?
Au2+(aq) + 2I-(aq) <----> Au(s) + I2(s)
Electrochemistry: Reduction potential application
Reduction Potentials Application
Below we see a diagram showing mitochondrial cellular respiration.
If we just consider the electron carrier NADH, NADH molecules gives up their electrons to the first complex, then the electron goes to the 2nd complex, then the 3rd, then the 4th and finally oxygen (O2) is the final electron acceptor.
Practice: Fill in the Blanks Review Activity (Oxidation vs Reduction)
Review of Oxidation/Reduction:
Redox reaction: involve changes in oxidation states or numbers.
Oxidation AND reduction must occur in order for it to be called a redox reaction!
A voltaic cell is constructed with an Sn/Sn2+ half-cell and a Zn/Zn2+ half-cell. The zinc electrode is negative.
Write a balanced overall reaction for this cell
Draw a diagram of this cell, labeling electrodes with their charges and showing the direction of electron flow in the wire and anion/cation flow in the salt bridge
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is being oxidized?
Electrochemistry: Standard Reduction Potentials
A galvanic cell is constructed from chromium and copper electrodes immersed in aqueous solutions of Cr3+and Cu2+ respectively (half-reactions shown below).
Cr3+(aq)+3e−⇌Cr(s)Cr^{3+}(aq)+3e^-⇌Cr(s) E°red=−0.74VE°_{red}=-0.74V
Cu2+(aq)+2e−⇌Cu(s)Cu^{2+}(aq)+2e^-⇌Cu(s) E°red=+0.34VE°_{red}=+0.34V
Electrochemistry: Redox
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is being oxidized?
Electrochemistry: Redox
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is the reducing agent?
Consider the following balanced redox equation:
16H(aq)++2MnO4(aq)−+10Cl(aq)−→2Mn(aq)2++5Cl2(g)+8H2O(l)16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}
Which species is being reduced?