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Chapter 14

Oxidation-Reduction Reactions

Shaun Williams, PhD

What is an Oxidation-Reduction Reaction?

Photographs of copper metal depositing out of  copper(II) sulfate solution onto a zinc strip.

Oxidation-Reduction Reaction

A graphic of copper(II) ions depositing on a zinc metal strip and the zinc in converted into an ion and dissolves into the solution.

Oxidation and Reduction

Oxidation and Reduction (cont.)

\[ \chem{\underset{\underset{0}\uparrow}{Zn}(s) + \underset{\underset{2+}\uparrow}{Cu}\underset{\underset{1-}\uparrow}{Cl_2}(aq) \rightarrow \underset{\underset{2+}\uparrow}{Zn}\underset{\underset{1-}\uparrow}{Cl_2}(aq) + \underset{\underset{0}\uparrow}{Cu}(s)} \]

Oxidizing and Reducing Agents

Oxidizing and Reducing Agents (cont.)

\[ \chem{\underset{\underset{0}\uparrow}{Zn}(s) + \underset{\underset{2+}\uparrow}{Cu}\underset{\underset{1-}\uparrow}{Cl_2}(aq) \rightarrow \underset{\underset{2+}\uparrow}{Zn}\underset{\underset{1-}\uparrow}{Cl_2}(aq) + \underset{\underset{0}\uparrow}{Cu}(s)} \]

Oxidation Numbers

Rules to Assign Oxidation Numbers

  1. The oxidation number of the atoms of an uncombined element is \(0\).
  2. The sum of the oxidation numbers of all atoms in a substance must equal the total charge: \(0\) for molecules, but the ionic charge for ions (including polyatomic ions).
  3. Fluorine has an oxidation number of \(1-\) in its compounds.
  4. Hydrogen has an oxidation number of \(1+\) unless it is combined with metals, where it has an oxidation number of \(1-\).

Rules to Assign Oxidation Numbers (cont.)

  1. The position of the element in the periodic table may be useful.
    1. Group 1 elements have oxidation numbers of \(1+\) in their compounds.
    2. Group 2 elements have oxidation numbers of \(2+\) in their compounds.
    3. Group 17 elements have oxidation numbers of \(1-\) unless combined with a more electronegative nonmetal.
    4. In binary compounds, Group 16 elements have oxidation numbers of \(2-\) unless combined with a more electronegative nonmetal.
    5. In binary compounds, Group 15 elements have oxidation numbers of \(3-\) unless combined with a more electronegative nonmetal.
  2. Oxygen has an oxidation number of \(2-\) except in peroxides (compounds containing the \(\chem{O_2^{2-}}\) ion) in which its oxidation number is \(1-\).

Batteries

Electricity and Batteries

A photograph of the open back of a wrist watch showing the battery.

Voltaic Cells

Two side by side beakers, one with a zinc solution and the right with a copper(II) solution. In the left solution is a zinc metal strip and in the right solution is a copper metal strip. These strips are connect to a light by wires.

Oxidation and Reduction

The Parts of a Voltaic Cell

A Grapical Voltaic Cells

Two side by side beakers: the left one contains a metal strip called the anode and the right one contains a metal strip called the cathode. In the left beaker oxidation occures while in the right beaker reduction occurs. Connecting the anode and cathode with a wire allows electricity to flow. A salt bridge also connects the two solutions.

A Grapical Voltaic Cells - Action of the Salt Bridge

Two side by side beakers: the left one contains a metal strip called the anode and the right one contains a metal strip called the cathode. In the left beaker oxidation occures while in the right beaker reduction occurs. Connecting the anode and cathode with a wire allows electricity to flow. A salt bridge also connects the two solutions. The salt bridge provided negative ions, such as the sulfate ion, to the oxidation side, and positive ions, such as sodium, to the reduction side.

Batteries

Balancing Simple Oxidation-Reduction Equations

Balancing Simple Reactions - Part 1

\[ \chem{Cu(s) + Ag^+(aq) \rightarrow Cu^{2+}(aq) + Ag(s)} \]

Balancing Simple Reactions - Part 2

\[ \chem{Cu(s) + Ag^+(aq) \rightarrow Cu^{2+}(aq) + Ag(s)} \]

Balancing Simple Reactions - Part 3

\[ \chem{Cu(s)+2Ag^+(aq) \rightarrow Cu^{2+}(aq)+2Ag(s)} \] Double-checking the numbers of atoms and the overall charge:

Reactants Products
\(\chem{Cu}\) 1 1
\(\chem{Ag}\) 2 2
Charge 2+ 2+

Helpful Hints in Balancing Simple Reactions

Helpful Hints in Balancing Simple Reactions (cont.)

  1. What factor can be used to multiply each coefficient in the balanced half-reactions to equalize the number of electrons gained or lost?
  2. When adding the two half-reactions, can any substances that are present in equal amounts on both sides of the equation be canceled out?

Balancing Complex Oxidation-Reduction Equations

  1. What reactant is oxidized and what reactant is reduced? Assign oxidation numbers to all the elements in the reaction to make the determination.
  2. What are the half reactions for the oxidation and reduction processes? For each half-reaction, determine each of the following:
    1. Is the element that changes the oxidation number balanced?
    2. Can any spectator ion be ignored until the final balancing step?

Balancing Complex Equations - Part 2

  1. Continued
    1. How many electrons must be added to the appropriate side of the equation to account for the change in oxidation number?
    2. Is the charge balanced after adding electrons? If not, what ions are present to balance the charge? If the reaction occurs in base, add \(\chem{OH^-}\) ions. If the reaction occurs in acid, add \(\chem{H^+}\) ions.
    3. Do the hydrogen and oxygen atoms balance? If not, add water molecules to the appropriate side of the equation.

Balancing Complex Equations - Part 3

  1. What factor can be used to multiply each coefficient to equalize the number of electrons gained and lost?
  2. When adding the two half-reactions, are any substances present on both sides of the equation? If so, cancel out an appropriate number. If substances are present on both sides of the equation in unequal amounts, cancel out the appropriate amount of substances on each side.

Electrochemistry

Activity Series

  • An activity series predicts the relative strengths of metals as reducing agents and metal compounds as oxidizing agents
  • As we go up the activity series, the metals become stronger reducing agents, which give up electrons more readily and are more reactive
  • Metals in metal compounds increase in oxidizing strength as we move down the activity series
The activity series, ranking the metals from most to least reactive.

Making a Voltaic Cell from Household Products

Adding a nail and a penny into a lemon and then connecting them with a wire produces electricity.

Electrolytic Cells

Electrolytic Cell - A Graphic

A bath of molten sodium chloride. Into this liquid, an anode and a cathode are placed and then connected to a voltage source. The electricity from the voltage source causes the chloride ions to be converted into molecular chlorine gas at the anode and sodium ions to be converted into liquid sodium at the cathode.

Corrosion Prevention

Corrosion

  • Slow deterioration of metals due to interaction with the environment
  • An oxidation-reduction reaction
  • Familiar signs are:
    • Red stains of rust on iron and steel
    • Green coating on copper and brass
    • Black coating on silver
A liquid water droplet on iron metal. Rust, iron(III) oxide, forms on the surface of the metal at the air-water droplet interface.

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