Glossary
Anode
The electrode in an electrochemical cell where oxidation occurs and electrons are released into the external circuit.
Example:
In a zinc-carbon battery, the zinc casing acts as the anode and slowly corrodes as it oxidizes.
Cathode
The electrode in an electrochemical cell where reduction occurs and electrons are consumed from the external circuit.
Example:
In a copper-silver cell, the silver electrode serves as the cathode, where silver ions are reduced and deposited.
Cell Potential (E°)
The overall potential difference of an electrochemical cell under standard conditions, calculated by combining the standard reduction potentials of the half-reactions.
Example:
A positive cell potential indicates that the redox reaction is spontaneous and can generate electricity.
Electrochemistry
The branch of chemistry that studies the relationship between chemical reactions and electrical energy, specifically how redox reactions produce or consume electricity.
Example:
A car battery operates on principles of electrochemistry, converting chemical energy into electrical energy to start the engine.
Electrolytic cells
Electrochemical cells that use an external electrical energy source to drive nonspontaneous redox reactions.
Example:
The industrial production of aluminum metal from its ore requires an electrolytic cell because the reduction of aluminum ions is nonspontaneous.
Electromotive force (EMF)
The potential difference that drives the flow of electrons in an electrochemical cell, measured in volts.
Example:
A standard AA battery typically provides an electromotive force of 1.5 V to power devices.
Galvanic (voltaic) cells
Electrochemical cells that produce electrical energy from spontaneous redox reactions.
Example:
The common alkaline battery used in remote controls is a type of galvanic cell.
Half-reactions
Equations that represent either the oxidation or reduction process separately, explicitly showing the electrons transferred.
Example:
For the combustion of methane, the oxidation half-reaction for carbon is CH₄ → CO₂ + 8e⁻ + 8H⁺.
Nonspontaneous
Describes a process or reaction that requires a continuous input of energy to occur, typically characterized by a negative cell potential (E° < 0).
Example:
The decomposition of water into hydrogen and oxygen gas is a nonspontaneous reaction that needs electrical energy to proceed.
Oxidized
The process by which a species loses electrons, resulting in an increase in its oxidation state.
Example:
When copper metal is exposed to air, it can be oxidized to form copper(II) oxide, turning green over time.
Oxidizing agent
A species that gains electrons and is itself reduced in a redox reaction, causing another species to be oxidized.
Example:
In the reaction 2Na + Cl₂ → 2NaCl, chlorine (Cl₂) is the oxidizing agent because it accepts electrons from sodium.
Redox reactions
Chemical reactions that involve the transfer of electrons between reactants, resulting in changes in oxidation states.
Example:
The rusting of iron is a classic redox reaction where iron loses electrons to oxygen.
Reduced
The process by which a species gains electrons, resulting in a decrease in its oxidation state.
Example:
In a photographic film, silver ions (Ag⁺) are reduced to solid silver (Ag) when exposed to light.
Reducing agent
A species that loses electrons and is itself oxidized in a redox reaction, causing another species to be reduced.
Example:
In the reaction 2Na + Cl₂ → 2NaCl, sodium (Na) is the reducing agent because it donates electrons to chlorine.
Salt bridge
A component of a galvanic cell that connects the two half-cells, allowing the flow of ions to maintain charge neutrality and complete the circuit.
Example:
Without a salt bridge, the buildup of charge in the half-cells would quickly stop the flow of electrons in a galvanic cell.
Standard reduction potentials
The voltage associated with a reduction half-reaction when all species are at standard conditions (1 M concentration for solutions, 1 atm pressure for gases, 25°C).
Example:
Chemists use tables of standard reduction potentials to predict the spontaneity of redox reactions and calculate cell potentials.
Volts (V)
The SI unit of electrical potential difference or electromotive force, representing the energy per unit charge.
Example:
A car battery provides 12 volts of electrical potential to power the vehicle's electrical system.