Chapter 8: Electrochemistry
Introduction
Electrochemistry is the study of chemical processes that involve the transfer of electrons, connecting the realms of chemistry and electricity. In this Chapter, we will explore how redox reactions can either generate electrical energy, as in batteries, or use electricity to drive chemical transformations, as in electrolysis. At the heart of electrochemistry are electrochemical cells, devices that harness these electron transfers. Galvanic cells, for example, convert the energy of spontaneous redox reactions into usable electrical power.
Electrochemistry is also fundamental to biological processes. In neurobiology, the movement of ions like sodium (Na + ) and potassium (K + ) across cell membranes generates electrical impulses that enable communication between neurons. This mechanism, essential for thought, movement, and sensation, mirrors principles observed in electrochemical cells.
Figure 8.1: The electrochemistry of a neuron’s synapse involves the movement of charged ions across cell membranes, a phenomenon governed by electrochemical gradients. The process begins when an action potential (i.e. an electrical signal) reaches the neuron, triggering the opening of voltage-gated calcium (Ca 2 + ) channels. The influx of Ca 2 + ions facilitates the release of neurotransmitters - like serotonin, dopamine, and oxytocin - which are shown above in yellow. These neurotransmitters diffuse and bind to receptor proteins (pink/purple above) on the postsynaptic neuron, opening new ion channels that allow sodium (Na + ) or potassium (K + ) ions to flow. The resulting change in ion distribution alters the membrane electrochemical potential of the postsynaptic neuron, either exciting or inhibiting the likelihood of a synapse. (CC-BY 4.0 2023; Chengpeng Jiang et. al., Nature , accessed via WikiMedia Commons ).
Beyond biology, electrochemistry drives innovations in energy storage, such as lithium-ion batteries, which power smartphones and electric cars. It also plays a role in corrosion prevention, where techniques like cathodic protection shield metals from oxidative damage, and in water treatment, where electrolysis is used to remove contaminants.
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- 8.1: Oxidation-Reduction Reactions
- Oxidation–reduction reactions are balanced by separating the overall chemical equation into an oxidation equation and a reduction equation. In oxidation–reduction reactions, electrons are transferred from one substance or atom to another. We can balance oxidation–reduction reactions in solution using the oxidation state method, in which the overall reaction is separated into an oxidation equation and a reduction equation.
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- 8.2: Voltaic Cells - Spontaneous Redox Reactions
- A galvanic (voltaic) cell uses the energy released during a spontaneous redox reaction to generate electricity, whereas an electrolytic cell consumes electrical energy from an external source to force a reaction to occur. Electrochemistry is the study of the relationship between electricity and chemical reactions. The oxidation–reduction reaction that occurs during an electrochemical process consists of two half-reactions, one representing the oxidation process and one the reduction process.
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- 8.4: Free Energy, Equilibrium, and Cell Potential
- A coulomb (C) relates electrical potential, expressed in volts, and energy, expressed in joules. The faraday (F) is Avogadro’s number multiplied by the charge on an electron and corresponds to the charge on 1 mol of electrons. Spontaneous redox reactions have a negative ΔG and therefore a positive Ecell. Because the equilibrium constant K is related to ΔG, E°cell and K are also related. Large equilibrium constants correspond to large positive values of E°.
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- 8.3: Standard Reduction Potentials
- Redox reactions can be balanced using the half-reaction method. The standard cell potential is a measure of the driving force for the reaction. \(E°_{cell} = E°_{cathode} − E°_{anode} \] The flow of electrons in an electrochemical cell depends on the identity of the reacting substances, the difference in the potential energy of their valence electrons, and their concentrations. The potential of the cell under standard conditions is called the standard cell potential (E°cell).
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- 8.5: Non-Spontaneous Cells - Electrolysis
- In electrolysis, an external voltage is applied to drive a nonspontaneous reaction. Electrolysis can also be used to produce hydrogen and oxygen gas from water. Electroplating is the process by which a second metal is deposited on a metal surface. The amount of material consumed or produced in a reaction can be calculated from the stoichiometry of an electrolysis reaction, the amount of current passed, and the duration of the electrolytic reaction.
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- 8.6: Batteries
- Commercial batteries are galvanic cells that use solids or pastes as reactants to maximize the electrical output per unit mass. A battery is a contained unit that produces electricity, whereas a fuel cell is a galvanic cell that requires a constant external supply of one or more reactants to generate electricity. One type of battery is the Leclanché dry cell, which contains an electrolyte in an acidic water-based paste.