Chapter 19 | Electrochemistry

Oxidation-Reduction (Redox) Reactions

Chemical reactions that involve a transfer of electrons

Oxidation number

The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred

Known oxidation numbers

• Free elements have an oxidation number of 0

• Monatomic ions have an oxidation number equal to the charge of the ion

• Oxygen = -2

• Hydrogen = +1

• Group IA metals = +1

• Group IIA metals = +2

• Fluorine = -1

Oxidation (Anode)

The loss of electrons and an increase in oxidation number

Reduction (Cathode)

The gain of electrons or a decrease in oxidation number

Reducing agent

A substance that reduces something else

Oxidizing agent

A substance that oxidizes something else

Balancing redox equations

• Separate the equation into two half-reactions

• Balance the atoms other than O and H in each half reaction

• For reactions in acid, add H₂O to balance O atoms and H⁺ to balance H atoms

• Add electrons to one side of each half-reaction to balance the charges of the half-reaction

• If necessary, equalize the number of electrons in the two half-reactions by multiplying the half-reactions by appropriate coefficients

• Add the two half-reactions together and balance the final equation by inspection. The number of electrons on both sides must cancel

• Verify that the number of atoms and the charges are balanced

• For equations in basic solutions, add OH^- on both sides of the equation for every H⁺ that appears in the final equation

Salt Bridge

A pathway that lets ions flow between cell parts to keep balance and help electricity move

Free energy equation

• ΔG = free energy

• n = number of moles of electrons in reaction

• F = Faraday Constant = 96500 C/mol

E°_cell

E°_cell and equilibrium