electrochemistry

Redox reaction

A redox reaction is a chemical reaction involving electron transfer connecting chemical energy and electrical energy.

Oxidation

Oxidation is the loss of electrons and is associated with energy release.

Reduction

Reduction is the gain of electrons and is associated with energy storage.

Redox process

Redox means oxidation plus reduction occurring together through electron transfer.

Electron flow in circuit

Directed electron flow through an external circuit produces electrical current.

Non-spontaneous redox reaction

A non-spontaneous redox reaction can be driven by applying an external voltage.

Redox pair

A redox pair consists of an oxidized form and a reduced form that can reversibly exchange electrons.

Redox pair equation

Ox + ne− ⇌ Red.

Oxidized form (Ox)

The oxidized form is the species that has lost electrons.

Reduced form (Red)

The reduced form is the species that has gained electrons.

Oxidation number

The oxidation number is the hypothetical charge an atom would have if shared electrons were assigned to the more electronegative atom.

Purpose of oxidation number

Oxidation numbers are bookkeeping tools used to track electron loss or gain in redox reactions.

Evidence of redox reaction

A change in oxidation number indicates that a redox reaction has occurred.

Oxidation number of free elements

Elements in their natural uncombined state have oxidation number 0.

Fluorine oxidation number

Fluorine always has oxidation number −1 in its compounds.

Hydrogen oxidation number

Hydrogen is +1 when bonded to nonmetals and −1 when bonded to metals.

Oxidation number of monatomic ions

The oxidation number of a monatomic ion equals its ionic charge.

Oxygen oxidation number

Oxygen is −2 in oxides −1 in peroxides and −1/2 in superoxides.

Group I metals oxidation number

Alkali metals always have oxidation number +1 in compounds.

Group II metals oxidation number

Alkaline earth metals always have oxidation number +2 in compounds.

Sum of oxidation numbers neutral compound

In a neutral compound the sum of oxidation numbers equals zero.

Sum of oxidation numbers polyatomic ion

In a polyatomic ion the sum of oxidation numbers equals the ion charge.

Half-reaction method

A method for balancing redox reactions by separating oxidation and reduction half-reactions.

Order of balancing redox reactions

Atoms are balanced first then electrons then charges.

Balancing redox in acidic vs basic media

Redox reactions are balanced differently in acidic and basic solutions.

Galvanic cell

A galvanic cell uses a spontaneous redox reaction to generate electrical energy.

Energy conversion in galvanic cell

Chemical energy is converted into electrical energy in a galvanic cell.

Electrolytic cell

An electrolytic cell uses applied electrical energy to drive non-spontaneous reactions.

Anode

The anode is the electrode where oxidation occurs.

Cathode

The cathode is the electrode where reduction occurs.

Electron flow in galvanic cell

Electrons flow from anode to cathode through the external circuit.

Half-cell

A half-cell consists of an electrode in contact with a solution containing ions of the same element.

Electrode-solution interface

Oxidation or reduction occurs at the interface between electrode and solution.

Electrochemical cell

An electrochemical cell is formed when two half-cells are connected by a wire and salt bridge.

Salt bridge

A salt bridge maintains ionic balance while allowing electron flow in the external circuit.

Cell potential (Ecell)

Cell potential is the electrical potential difference between two half-cells.

Cell potential equation

Ecell = Ecathode − Eanode.

Positive cell potential

A positive cell potential indicates a spontaneous redox reaction.

Daniell cell

A Daniell cell consists of a Zn/Zn2+ anode and a Cu2+/Cu cathode.

Standard cell potential (E°cell)

The cell potential measured under standard conditions is the standard cell potential.

Standard conditions for E°

Standard conditions are 1 M solutions 1 atm pressure and 25 °C.

Standard electrode potential (E°)

Standard electrode potential measures the tendency of a half-reaction to be reduced.

Standard Hydrogen Electrode (SHE)

The SHE is the reference electrode with defined potential of 0.000 V.

SHE half-reaction

2H+ + 2e− ⇌ H2(g).

SHE conditions

The SHE operates at 1 M H+ 1 atm H2 and 25 °C.

Platinum electrode in SHE

Platinum provides a catalytic surface for rapid H2/H+ exchange.

Oxidizing agent strength

A species with more positive E° is a stronger oxidizing agent.

Reducing agent strength

A species with more negative E° is a stronger reducing agent.

Spontaneous redox prediction

A redox reaction is spontaneous if E°cell is positive.

Ranking redox agents

Oxidizing and reducing agents are ranked by comparing standard electrode potentials.

Gibbs free energy (ΔG)

Gibbs free energy is the maximum useful work obtainable at constant temperature and pressure.

Gibbs free energy equation

ΔG = ΔH − TΔS.

Exergonic reaction

An exergonic reaction has ΔG < 0 and releases energy spontaneously.

Endergonic reaction

An endergonic reaction has ΔG > 0 and requires energy input.

Equilibrium and ΔG

At equilibrium ΔG = 0.

Redox potential (E)

Redox potential measures the tendency of a species to gain or lose electrons.

Relationship between ΔG and E

ΔG = −nFE.

Meaning of negative sign in ΔG equation

A positive cell potential corresponds to a negative ΔG and a spontaneous reaction.

Faraday constant (F)

The Faraday constant is 96485 C mol−1.

Redox tower

The redox tower arranges redox couples by E° to predict reaction direction and energy yield.

ΔG° and equilibrium constant

ΔG° is related to the equilibrium constant K.

Spontaneous reaction and K

For spontaneous reactions K > 1.

Equilibrium condition and K

At equilibrium K = 1.

Non-spontaneous reaction and K

For non-spontaneous reactions K < 1.

Nernst equation

The Nernst equation relates cell potential to non-standard conditions.

General Nernst equation

Ecell = E°cell − RT/nF ln Q.

Nernst equation at 25 °C

Ecell = E°cell − 0.0592/n log Q.

Reaction quotient (Q)

Q is the ratio of product activities to reactant activities.

Effect of Q < 1 on Ecell

When Q < 1 the cell potential is greater than E°cell.

Effect of Q > 1 on Ecell

When Q > 1 the cell potential is less than E°cell.

Equilibrium and Nernst equation

When Q = 1 Ecell equals E°cell.

Use of Nernst equation

The Nernst equation describes electrochemical cells under non-standard conditions.