Chem 30: Chp 13 - Redox

Electrochemistry

study of how electrons are transferred from one chemical species to another

Redox Reaction

a chemical reaction in which electrons are transferred between entities

• oxidation and reduction must happen together, cant happen alone

  • always one is reduced, and the other is oxidized

Electron Transfer in Redox Reactions

transfer of valence electrons from the strongest reducing agent (SRA) to the strongest oxidizing agent (SOA)

• SRA → SOA

Spontaneous Reaction

OA is higher than the RA on the redox table.

• always have evidence of reaction:

  • change in energy (exo/endothermic?)

  • colour change

  • change in odour

  • formation of precipitate/gas

Non-Spontaneous Reaction

RA is higher than the OA on the redox table.

• no evidences of a reaction

IMPORTANT RULE IN REDOX REACTIONS

• total # of electron gain = total # of electron lost

• must have both oxidation and reduction process occurring

Electrochemical reaction involves…

• loss of electrons by one reactant

• gain of electrons by another reactant

  • only one type of reaction.. not every reaction is a electrochemical reaction!

Electrochemical reactions may either be…

EXOTHERMIC:

• reactants → products + electrons

ENDOTHERMIC:

• reactants + electrons → products

Half-Reaction

a balanced chemical equation that represents either a loss or gain of electrons by a substance

• created from net-ionic equations (separated into ions)

Reduction

gain of electrons

• reactant electron = “gain”

• ion → atom

Oxidation

loss of electrons

• product electron = “loss”

• atom → ion

OIL RIG

• Oxidation Is Loss

• Reduction Is Gain

Non-Ionic Equations vs. Net-Ionic Equations (or Total Ionic Equations)

Atom

no charge

• # of electrons = # of protons → charge is 0

Ion

atom that has a charge

• cation = positive charge

• anion = negative charge

Species

ion or an atom

Spectator Species

chemical species that are present in a reaction but do not participate by reacting

• seen in the “net-ionic equations”

Half-Reaction Generalizations

OXIDATION:

• atom → ion (needs to lose electrons to become charged)

• losing electrons = electrons in product

REDUCTION:

• ion → atom (needs to gain electrons to equal out)

• gaining electrons = electrons in reactant

Reducing Agent

the species that is being oxidized

Oxidizing Agent

the species that is being reduced

Disproportionation

reaction in which a species is both oxidized and reduced

Oxidation State

an atom in an entity is defined as the apparent net electric change that it would have the more electronegative atom took all the shared electrons.

Oxidation Number

a positive or negative number corresponding to the oxidation state assigned to an atom, indicating its loss or gain of electrons

“ON”

abbreviation for Oxidation Number

IMPORTANT RULES FOR DETERMINING OXIDATION NUMBERS

• do not count coefficients

  • does not matter if its balanced or not

• no change in ON = no redox reaction

• ON ↑ = ON ↓

  • since during redox reactions, both oxidation and reduction occurs…

Oxidation Number Rules

1. Elements ON = ZERO

2. Simple Ions ON = CHARGE

3. Oxygen ON = -2

   • peroxides = -1

4. Hydrogen ON = +1

   • metallic hydrides = -1

5. Compounds:

   • Group 1 = +1

   • Group 2 = +2

   • Group 3 / Aluminum = +3

6. Total of Neutral Compound ON = ZERO

7. Total of Ionic Compound ON = ZERO

8. Total of Complex/Polyatomic Ion = CHARGE

ON (Oxidation #) ↑

an increase in oxidation number is an oxidation

• results in the reducing agent

ON (Oxidation #) ↓

a decrease in oxidation number is a reduction

• results in the oxidation agent