Like all of chemistry, electrochemistry is concerned with what electrons do. Electrons in atoms, molecules or ions are bound with a particular energy. When oxidation-reduction (redox) reactions occur, electrons are transferred from the substance being oxidized to the substance being reduced. In the process, energy is either released or absorbed, depending on the electron-binding energy difference between the reacting substances. A battery (a voltaic cell) is a device that allows the chemical energy released by a spontaneous oxidation-reduction reaction to do electrical work (e.g., light a light bulb, power a radio.) An electrolytic cell can reverse this process by using external electrical energy to bring about a nonspontaneous redox reaction (e.g., electroplating an automobile bumper.) In both cases, the electron transfer reactions occur at electrodes. The oxidation reaction occurs at the anode, and the reduction reaction occurs at the cathode.
Some substances give up or accept electrons easier than others and can be organized by their relative ability to undergo reduction reactions. A table of standard reduction potentials orders substances by their ability to accept electrons from a common donor (hydrogen gas). Among common substances, fluorine has the highest value (+2.87 V) for its reduction potential. It might be considered the Tyrannosaurus Rex of the Periodic Table because it can gobble electrons from anything. Lithium does not undergo reduction readily and has a very low reduction potential (3.05 V). The difference between the reacting species in terms of reduction potential is an indication of the driving force for an electron-transfer reaction. In a voltaic cell, this difference defines the cellÕs potential and is the same as the electric potential for a standard single-cell battery. In an electrolytic cell, the cell potential is the electrical pressure (electric potential, expressed in volts) needed to drive the oxidation-reduction reaction.
Electrical charge is conducted through a solution by movement of cations and anions. In a voltaic cell, electrons are transferred from the cathode to the species being reduced. Cations must migrate to the cathode to offset the increase of negative charge near the electrode. Similarly, anions must migrate to the anode to offset the buildup of positive charge generated by the oxidation reaction at that electrode (see Transparencies 1 and 2 in the Appendix). The charge on the electrodes and the direction of movement of ions is just the opposite in electrolytic cells (seeTransparencies 3 and 4).
The quantity of work that can be produced by a battery or the quantity of work needed to run an electrolytic cell can be calculated from the electric potential, the current, and the time the cell operates. Thus, electrochemistry is the study of the interconversion of electrical and chemical energy.
The study of electrochemistry relies heavily on conceptual understanding of redox reactions. In essence, electrochemistry involves practical applications of redox chemistry. When students balance redox reactions by the half-reaction method, they are essentially separating the full reaction into two electrochemical half cells.
The structure and chemical properties of elements and molecular and ionic substances are governed by the chemical activity of their valence electrons. The energy that binds these electrons determines the value of a substance's standard reduction potential. Consequently, strong ties are apparent between electrochemistry and periodicity.
Whether an electrochemical cell operates in a voltaic or electrolytic mode depends on the relative free energy value for each half-cell. Observed phenomena in electrochemistry therefore directly relate to differences in thermodynamic functions of state (free energy, entropy, and enthalpy) and to temperature.
In electrochemistry, electrons are transferred in multiples of whole numbers. Therefore, there is a stoichiometric relationship between the quantity of charge transferred and the amounts of species oxidized and reduced.
1. Electrochemistry is a subtopic of oxidation/reduction.
2. Electrochemical cells contain several essential components:
a. Cathode, where reduction occurs
b. Anode, where oxidation occurs
c. Electrolyte (can be liquid or gel; is sometimes a salt bridge), which serves to conduct charge via moving ions in the cell
d. External circuit, where charge is conducted as a current of moving electrons
3. There are two kinds of electrochemical cells:
a. Voltaic (also called Galvanic)
Reactions spontaneous
Chemical energy transformed to electrical energy
b. Electrolytic (includes, but not limited to, electroplating)
Reactions not spontaneous
Electrical energy transformed to chemical energy
4. Electrochemical reactions occur at the surface of the electrodes.
5. Substances are categorized by half-reaction reduction potentials,which can be used to predict the spontaneity of oxidation-reduction reactions and the electric potential of batteries.
6. Technological applications of electrochemistry include batteries, fuel cells, electroplating, protection from corrosion, and chemical instrumentation.
7. Electrochemistry can be treated quantitatively at several levels of sophistication:
a. Calculation of cell potentials for batteries (essential)
b. Calculation of stoichiometric relationships (can be omitted if time is needed to treat other important chemistry topics)
c. Electrical work (should be treated only if time permits and if students have completed physics as a prerequisite)
d. Calculations using the Nernst equation or equations related to it or the Gibbs free energy equation (should not be treated in the general first-year high school course)
1. Electrical energy
2. Electric potential energy difference (volts)
3. Chemical energy
4. Spontaneous and nonspontaneous reactions
5. Conservation of charge
6. Anions, cations, and electrolytes
7. Electrostatic attraction/repulsion
8. Mole relationships in chemical reactions, balancing equations
9. Oxidation-reduction
10. Electronegativity, ionization energy, and electron affinity
Upon completion of their study of electrochemistry, students will be able to:
1. distinguish between anions and cations.
2. define anode and cathode in terms of oxidation and reduction.
3. describe how a voltaic cell produces an internal ionic flow and an external electron flow.
4. write half-cell equations and total equations for voltaic cells.
5. in relation to a voltaic cell, define or explain: anode, cathode, electric potential (volts), salt bridge, internal circuit, external circuit.
6. describe how a battery produces electrical energy.
7. identify the substance being oxidized and the substance being reduced in an electrochemical cell.
8. describe the operation of an electrolytic cell.
9. define cathode, anode, and explain the charge on the cathode and anode in an electrolytic cell.
10. explain similarities and differences between voltaic and electrolytic cells.
11. explain the operation of an apparatus for electroplating with metals.
12. describe the zero-potential hydrogen half-cell.
13. given a table of standard reduction potentials: determine whether a redox reaction will occur; predict the electric potential of a voltaic cell made from two different half-cells; predict the products of an electrolysis reaction.
14. given the reduction potential of one half-cell and the electric potential of a voltaic cell, calculate the reduction potential of the other half-cell.
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