Electrochemistry

The One Thing

Electrochemistry examines the conversion between chemical and electrical energy through redox reactions. Galvanic cells generate electrical energy from spontaneous reactions, whereas electrolytic cells use electrical energy to drive non-spontaneous reactions.

Reactions, Processes and Experiments

Key Terms

• Redox reaction: A reaction involving oxidation, which is loss of electrons, and reduction, which is gain of electrons.
• Electrochemical cell: A device in which a redox reaction either produces electrical energy or is caused by electrical energy.
• Galvanic cell: A cell that converts the chemical energy of a spontaneous redox reaction into electrical energy.
• Electrolytic cell: A cell in which electrical energy is supplied to drive a non-spontaneous chemical reaction.
• Anode: The electrode at which oxidation occurs. It is negative in a galvanic cell and positive in an electrolytic cell.
• Cathode: The electrode at which reduction occurs. It is positive in a galvanic cell and negative in an electrolytic cell.
• Salt bridge: A device containing an inert electrolyte that completes the electrical circuit, maintains electrical neutrality, and reduces liquid junction potential.
• Cell notation: A symbolic representation of a cell; for example, Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s). A single line represents a phase boundary and a double line represents the salt bridge.
• Electrode potential: The potential difference developed between an electrode and the solution containing its ions.
• Standard electrode potential: The electrode potential measured under standard conditions, usually 1 M concentration, 1 bar gas pressure, and 298 K.
• Standard hydrogen electrode: The reference electrode assigned a standard reduction potential of 0.00 V under standard conditions.
• Cell potential or EMF: The potential difference between the cathode and anode when no current is drawn from the cell.
• Conductance: The ability of a solution or conductor to carry electric current; it is the reciprocal of resistance.
• Conductivity: The conductance of a solution of unit length and unit cross-sectional area; it is represented by kappa.
• Molar conductivity: The conductance of the volume of solution containing one mole of an electrolyte placed between electrodes of unit area and separated by unit distance.
• Strong electrolyte: An electrolyte that is almost completely ionised in solution, such as HCl, KNO3, or NaOH.
• Weak electrolyte: An electrolyte that is only partially ionised in solution, such as CH3COOH or NH4OH.
• Nernst equation: An equation that gives the electrode or cell potential under non-standard conditions in terms of temperature and the reaction quotient.
• Electrolysis: The chemical decomposition of an electrolyte by passing direct electric current through its molten or aqueous form.
• Faraday's laws of electrolysis: Laws stating that the amount of substance deposited depends on the charge passed and that equal charges deposit chemically equivalent amounts of different substances.
• Primary cell: A cell that cannot be conveniently recharged after its reactants are consumed, such as a dry cell or mercury cell.
• Secondary cell: A rechargeable cell in which the cell reaction can be reversed by supplying electrical energy, such as a lead storage battery.
• Fuel cell: A galvanic cell that continuously converts the chemical energy of externally supplied fuel and oxidant into electrical energy.
• Corrosion: The gradual deterioration of a metal due to electrochemical oxidation by the environment.

Easily Confused

• Anode and cathode: Oxidation always occurs at the anode and reduction always occurs at the cathode; their signs change between galvanic and electrolytic cells.
• Galvanic and electrolytic cells: A galvanic cell uses a spontaneous reaction to generate electrical energy, whereas an electrolytic cell uses electrical energy to drive a non-spontaneous reaction.
• Electron flow and conventional current: Electrons move through the external circuit from the anode to the cathode, while conventional current is taken in the opposite direction.
• Conductivity and molar conductivity: Conductivity generally decreases on dilution, whereas molar conductivity increases because they measure different aspects of ionic conduction.
• Strong and weak electrolytes: Strong electrolytes are almost completely ionised, whereas weak electrolytes are only partially ionised.
• Primary and secondary cells: Primary cells cannot be conveniently recharged, whereas secondary cells can be recharged by reversing the cell reaction.
• Cell potential and electrode potential: Cell potential is the potential difference between the cathode and anode; electrode potential is the potential difference between one electrode and its ion-containing solution.
• Thermodynamic feasibility and reaction rate: Cell potential and Gibbs energy determine whether a reaction is feasible, whereas kinetics determines how rapidly it occurs.

What Gets Asked

• Determine the electrode roles, signs, and electron direction in a galvanic or electrolytic cell. The key slip is assigning the anode or cathode by its sign rather than by oxidation or reduction.
• Calculate cell potential using E_cell = E_cathode - E_anode, using reduction potentials. A negative value means that the written reaction is non-spontaneous under the stated conditions.
• Apply the thermodynamic relations Delta G° = -nFE°cell and Delta G = -nFEcell. A spontaneous cell reaction under standard conditions has E°cell > 0.
• Use the Nernst equations E = E° - (0.0591/n) log Q and Ecell = E°cell - (0.0591/n) log Q at 298 K. Pure solids and pure liquids must be omitted from Q; aqueous ions and gases are included according to their stoichiometric powers.
• At equilibrium, use Ecell = 0 and Q = K, giving E°cell = (0.0591/n) log K at 298 K. The common error is failing to use the equilibrium conditions.
• Calculate conductance, conductivity, molar conductivity, or deposited mass using R = V/I, G = 1/R = I/V, kappa = G multiplied by the cell constant, Lambda_m = (kappa x 1000)/C, and m = ZQ = ZIt. The specific quantity and its units must not be confused.