Wize High School Grade 12 Chemistry Textbook > Electrochemistry

Redox Reactions

Redox Reactions

OIL RIG! Oxidation Is Loss of electrons, Reduction Is Gain of electrons

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.

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!

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

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-

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:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

Which species is being oxidized?

I don't know

Consider the following balanced redox equation:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

Which species is the reducing agent?

I don't know

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?

Mark Yourself Question

Grab a piece of paper and try this problem yourself.
When you're done, check the "I have answered this question" box below.
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)

I have answered this question

Practice: Oxidizing vs Reducing Agents

What is the reducing agent in the following chemical reaction?

NO + Au+ → NO3- + Au

Au+

NO3-

I don't know

Extra Practice

Consider the following balanced redox equation:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

Which species is being reduced?

Electrochemistry: Redox

Consider the following balanced redox equation:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

Which species is the reducing agent?

Electrochemistry: Redox

Consider the following balanced redox equation:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

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).

$Cr^{3+}(aq)+3e^-⇌Cr(s)$ $E°_{red}=-0.74V$

$Cu^{2+}(aq)+2e^-⇌Cu(s)$ $E°_{red}=+0.34V$

Consider the following balanced redox equation:

$16H^+_{(aq)}+2MnO^-_{4(aq)}+10Cl^-_{(aq)} \to 2Mn^{2+}_{(aq)}+5Cl_{2(g)}+8H_2O_{(l)}$

Which species is being oxidized?

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

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!

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.

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.

Redox Reactions

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Redox Reactions

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

_____________

How can you recognize if a redox reaction is occuring?

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Disproportionation Reaction

For example:

2H2O2 --> 2H2O + O2

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

What is being oxidized?

What is being reduced?

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Practice: Oxidizing vs Reducing Agents

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Extra Practice

Review of Oxidation/Reduction:

Reduction Potentials Application

Reduction Potentials Application

Reduction Potentials Application