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Chemistry 2 Unit 7: Electrochemistry

Electrochemistry uses redox reactions to produce electricity in galvanic cells or uses electricity to drive reactions in electrolysis.

Covers: Oxidation–reduction reactions, galvanic cells, standard reduction potentials, cell potential and free energy, the Nernst equation, electrolysis and Faraday's law

Study guide

In a redox reaction, oxidation is the loss of electrons and reduction is the gain of electrons (OIL RIG). The oxidizing agent is reduced, and the reducing agent is oxidized. Oxidation numbers track electron transfer, and redox equations can be balanced using half-reactions, balancing atoms, charge and, in acidic or basic solution, H₂O and H⁺ or OH⁻.

A galvanic (voltaic) cell turns a spontaneous redox reaction into electrical energy. Oxidation happens at the anode and reduction at the cathode (an ox, red cat). Electrons flow through the wire from the anode to the cathode, and a salt bridge allows ions to flow to keep each half-cell neutral: anions move toward the anode and cations toward the cathode. Standard reduction potentials (E°) rank how strongly species gain electrons; the standard cell potential is E°cell = E°(cathode) − E°(anode). A positive E°cell means the reaction is favored. Free energy relates to cell potential: ΔG° = −nFE°, where n is moles of electrons and F is Faraday's constant (96,485 C/mol e⁻). E°cell also relates to K: a positive E° means K > 1.

Under nonstandard conditions, the Nernst equation gives E = E° − (RT/nF) ln Q, or at 25 °C, E = E° − (0.0592/n) log Q. As the cell runs, Q increases and E falls until it reaches zero at equilibrium, when the battery is dead. Concentration cells use the same half-reaction at different concentrations. An electrolytic cell uses an external power source to drive a nonspontaneous reaction, as in electroplating, recharging batteries and producing aluminum or chlorine. Faraday's law relates charge to the amount of substance: charge = current × time (q = It), and moles of electrons = q ÷ F.

In a cell with metal electrodes, the anode loses mass as its metal is oxidized into ions, and the cathode gains mass as metal ions are reduced and plate onto it. The size of each mass change comes from the moles of electrons transferred and each metal's molar mass and ion charge. Standard reduction potentials are intensive properties: multiplying a half-reaction by a coefficient to balance electrons does not change its E°. The standard hydrogen electrode is the reference, defined as 0 V.

Raising the reactant concentrations or lowering the product concentrations makes Q smaller, which raises the cell potential above E°. The reverse changes lower it.

Key ideas

• Oxidation is loss of electrons; reduction is gain (OIL RIG).
• Oxidation occurs at the anode, reduction at the cathode.
• E°cell = E°cathode − E°anode; positive means favored.
• ΔG° = −nFE° links cell potential to free energy.
• Electrolysis uses electricity to drive nonspontaneous reactions.

Formulas and rules

Common mistakes

• Multiplying E° values when balancing half-reactions; E° doesn't change with coefficients.
• Mixing up anode and cathode; oxidation is always at the anode.

Words to know

Oxidation

Loss of electrons.

Reduction

Gain of electrons.

Oxidizing agent

The species that is reduced.

Reducing agent

The species that is oxidized.

Oxidation number

A number tracking electrons assigned to an atom.

Half-reaction

One part of a redox reaction, showing oxidation or reduction.

Galvanic cell

A cell producing electricity from a spontaneous redox reaction.

Anode

The electrode where oxidation occurs.

Cathode

The electrode where reduction occurs.

Salt bridge

A connection allowing ions to flow between half-cells.

Standard reduction potential

A measure of a species' tendency to gain electrons.

Cell potential

The voltage produced by an electrochemical cell.

Faraday's constant

Charge of one mole of electrons: 96,485 C.

Nernst equation

Gives cell potential under nonstandard conditions.

Concentration cell

A cell with the same half-reaction at different concentrations.

Electrolytic cell

A cell using electricity to drive a nonspontaneous reaction.

Electroplating

Coating an object with metal by electrolysis.

Sample practice questions

13 more questions like these, from level 1 to 5, are in the practice set.