Table of Contents
Introduction
Electrochemical processes are chemical reactions in which electrons are transferred across an interface and the reaction is directly coupled to an electric current or an electric potential difference. They form the bridge between chemical thermodynamics, kinetics, and electricity:
- In spontaneous electrochemical processes, chemical energy is converted into electrical energy (galvanic or voltaic cells).
- In non‑spontaneous electrochemical processes, electrical energy is used to drive chemical change (electrolysis).
This chapter gives the general framework; the later subsections discuss conduction and electrolytes, electrodes and potentials, electrochemical cells, and electrolytic processes in detail.
What Makes a Process Electrochemical?
A process is electrochemical if:
- Redox reactions are involved (some species is oxidized, another reduced).
- The electron transfer does not occur directly between reactants in the same place, but via an external path (e.g. a metal wire) or across an interface.
- There is a measurable electrical quantity associated:
- Electric current $I$ (flow of charge per unit time)
- Cell voltage $U$ (potential difference between two points)
- Charge $Q$ passed through the system
Contrast:
- A simple combustion in a flame is a redox reaction but not an electrochemical process, because electrons are not collected or driven through an external circuit.
- A battery discharging in a device is an electrochemical process: electron flow through the circuit is intrinsically tied to the chemical reaction inside.
Basic Components of an Electrochemical System
Most electrochemical systems share a few key components. These will be taken up again with more detail later.
Electrodes
An electrode is a phase (often a metal, sometimes a semiconductor or a conductive polymer) that allows electrons to enter or leave the chemical system. At an electrode:
- Oxidation occurs at the anode (electrons are released to the external circuit).
- Reduction occurs at the cathode (electrons are taken up from the external circuit).
The labels “anode” and “cathode” are tied to the process (oxidation/reduction), not to the sign:
- In a galvanic cell: anode is negative, cathode is positive.
- In an electrolytic cell: anode is positive, cathode is negative.
The electrode/electrolyte interface is where charge transfer and charge separation occur, leading to an electrical potential difference (electrode potential).
Electrolytes
The electrolyte is an ion‑conducting medium (solution, melt, or solid) that:
- Contains ions that can move under an electric field
- Completes the internal circuit between the two electrodes
- Participates in maintaining charge balance during operation
Electrolytes are essential because electrons cannot move freely through most liquids or ionic solids, whereas ions can.
External Circuit and Load
In galvanic cells, the electrodes are connected by:
- A metallic conductor (wire), possibly with a load (lamp, motor, resistor, or electronic device)
- The current in this external circuit is the useful electrical output of the cell
In electrolytic cells, the external circuit supplies power:
- A voltage source (power supply, battery) is connected to the electrodes
- The external source forces electrons in a direction opposite to that of a spontaneous reaction
Types of Electrochemical Processes
Electrochemical processes are commonly divided into two broad types.
Galvanic (Voltaic) Processes
In galvanic processes:
- A spontaneous redox reaction occurs.
- Chemical energy is transformed into electrical energy.
- The cell can deliver power to an external circuit.
Typical examples:
- Disposable batteries (primary cells)
- Rechargeable batteries (secondary cells) during discharge
- Corrosion processes (often undesirable galvanic reactions)
- Biological electrochemistry (e.g. nerve impulses involve ionic currents; certain organisms have natural “batteries”)
Characteristic features:
- The cell voltage at open circuit is related to the difference in electrode potentials of the two half‑reactions.
- The reaction direction is dictated by thermodynamic driving force (Gibbs free energy change).
- The current and rate of reaction depend on kinetic factors (electrode kinetics, mass transport).
Electrolytic Processes
In electrolytic processes:
- A non‑spontaneous redox reaction is driven by an external power source.
- Electrical energy is converted into chemical energy.
- The cell consumes power.
Typical examples:
- Electrolysis of water to produce hydrogen and oxygen
- Electrolytic refining and deposition of metals (electroplating)
- Production of chemicals (e.g. chlorine and sodium hydroxide)
- Recharge of secondary batteries (reverse of discharge)
Characteristic features:
- The applied voltage must exceed the sum of:
- The thermodynamic cell voltage (related to $Δ_\mathrm{r} G$)
- Additional overpotentials (kinetic barriers, ohmic losses)
- The direction of electron flow is set by the external power supply, overcoming the natural driving force.
Coupling of Chemical Change and Electric Quantities
A central idea in electrochemistry is that chemical reaction extent is directly measurable by electrical charge passed.
Faraday’s Laws (Qualitative View)
Faraday’s laws, in qualitative form:
- The amount of substance transformed at an electrode is proportional to the total charge $Q$ that has passed through the cell.
- For a given amount of charge, the amount of substance depends on the charge number of the ion or electrons exchanged.
Quantitatively, the relation involves the Faraday constant $F$ (charge per mole of electrons), introduced and used in detail in later subsections. Qualitatively:
- More charge $\Rightarrow$ more moles of electrons transferred $\Rightarrow$ more chemical change.
- A species requiring more electrons per molecule (higher oxidation state change) needs more charge to produce or consume a given amount.
This is why, for instance, you can calculate how long you need to run a current to deposit a certain mass of metal in electroplating.
Current, Charge, and Reaction Rate
The electrical current $I$ and time $t$ are linked to the charge:
$$
Q = I \cdot t
$$
Because the charge is tied to the number of electrons transferred, and the electrons are tied to a specific stoichiometry of the redox reaction, the current is directly related to:
- The rate of consumption of reactants at the electrodes
- The rate of formation of products
In bulk solution, overall reaction rates are also affected by:
- Mass transport (diffusion, convection, migration of ions)
- Electrode surface properties
These kinetic aspects connect electrochemistry to chemical kinetics.
Direction of Electron and Ion Flow
Electrochemical processes involve two parallel circuits:
- Electronic circuit (outer circuit):
- Through metal wires or external circuitry
- Electrons move from anode to cathode in galvanic operation
- The conventional current direction is opposite to electron flow
- Ionic circuit (inside electrolyte):
- Cations move towards the cathode; anions move towards the anode
- Ion migration maintains overall electroneutrality as electrons move externally
The separation of electron and ion paths allows us to:
- Harvest electrical energy separately from where chemical changes occur
- Control and measure the reaction via applied potential or current
Energetic Aspects
Electrochemical processes sit at the intersection of thermodynamics and electrical work:
- The cell voltage $U$ is related to the Gibbs free energy change of the net cell reaction.
- In galvanic cells, a positive cell voltage corresponds to a negative $Δ_\mathrm{r} G$ (spontaneous reaction) and the ability to do non‑expansion work (electrical work).
- In electrolytic cells, electrical work must be supplied to drive a positive $Δ_\mathrm{r} G$ reaction.
The detailed quantitative relationship between cell voltage and $Δ_\mathrm{r} G$ is treated in the chapter on chemical equilibrium and Gibbs free energy; here it is important only to recognize that:
- Electrochemical processes enable direct conversion between chemical and electrical forms of energy.
- The maximum useful electrical work obtainable is limited by the underlying thermodynamics.
Practical Importance of Electrochemical Processes
Electrochemical processes are central to many technologies and natural phenomena:
- Energy storage and conversion
- Batteries and accumulators
- Fuel cells
- Supercapacitors (involving electrostatic and some Faradaic processes)
- Materials processing
- Metal refining (e.g. electrolytic copper refining)
- Electroplating and surface finishing
- Chemical production
- Chlor‑alkali process
- Electrolytic production of aluminum
- Corrosion and corrosion protection
- Galvanic corrosion
- Cathodic protection systems
- Analysis
- Electroanalytical methods (potentiometry, voltammetry, amperometry)
- Biology and medicine
- Nerve conduction and membrane potentials
- Electrocardiography (ECG) and other bioelectric measurements
- Implantable batteries and sensors
Understanding the general features of electrochemical processes provides the basis for studying:
- How materials conduct electricity and what makes a substance an electrolyte
- How electrode potentials are defined, measured, and used to predict reaction direction
- How to construct useful cells and determine their voltage and performance
- How to drive and control non‑spontaneous reactions by applying external potentials
These topics are developed in the subsequent subsections on electrical conduction and electrolytes, electrodes and electrode potentials, electrochemical cells, and electrolytic processes.