Electrochemical Cells, Standard Electrode Potentials, & Electrochemical Series

This is a continuation of the previous topics Introduction to Redox Reactions and Electrolysis. By definition, an electrochemical(voltaic) cell is a device that uses a spontaneous chemical reaction in an aqueous solution to generate electric current i.e converts chemical energy into electrical energy.

When a strip of metal, such as zinc is dipped into a solution of its salt, an electron potential difference is set up between the metal, M_(s), and its ions Mⁿ⁺_(aq) at equilibrium:

M_(s)  ⇌ Mⁿ⁺_(aq) + ne^–

This is represented as M_(s)/Mⁿ⁺_(aq). The slanting line /, represents the boundary or interface between the solid and the liquid. Under this condition, the setup constitutes a half cell which we defined in Introduction to Redox Reactions. When two different half-cells are separated by a porous partition that allows the diffusion of ions, electron transfer occurs externally through the connecting wire(a conductor) between the two electrodes, as indicated by the glowing of a bulb, or a deflection in the ammeter. Such a setup is called galvanic, voltaic, or electrochemical cell, and the flow of electrons through the cell produces an electric current.

Standard Electrode Potentials

When a voltmeter is connected between two electrodes in a voltaic cell, the force with which the electrons are being pushed around the cell is called electromotive force(emf) and it is measured in volts V. The emf value of a cell, E_cell, is a quantitative measure of the tendency of a redox reaction to occur in the cell. It is equal to the difference between the electrode potentials, E₁ and E₂ of the two electrodes that constitute the cell.

The electrode potential of an electrode is a quantitative measure of the ability of an atom or ion to gain electrons at the electrode, Its value depends on the nature of the metal and its ion(the electrode), the concentration of the ions in the solution, and the temperature of the cell.

For a half cell, the electrode potential cannot be measured; except the emf of a cell for a complete circuit with two electrodes. The potential is determined by coupling it with that of the hydrogen electrode; the reference standard is assigned zero potential(0.000V).

The standard hydrogen electrode consists of a platinized platinum electrode passing through a glass tube, dipping into a one mole per dm³ solution of hydrochloric acid(1.0moldm^-3 H₃O⁺) with pure hydrogen gas at one atmospheric pressure and 25^oC(298K) bubbling around the platinised platinum.

The half-cell reaction for the standard hydrogen electrode, written in the reduction form is:

2H⁺_(aq) + 2e^– → H_2(g) E^o = 0.00v

How To Measure Standard Electrode Potential

In order to measure the standard electrode potential of a half cell, its electrode is coupled to the standard hydrogen electrode via a salt bridge. A salt bridge is an inverted U-tube containing a saturated solution of an electrolyte, such as Na₂SO₄, KCl and KNO₃. It allows the migration of ions between two electrodes.

In order to compare the electrode potential of various half cells, electrode potentials are measured under the following standard conditions:

• The concentration of the electrolyte is 1.0moldm^-3
• Gaseous reactants is at one atmospheric pressure
• The temperature of the cell is 25^oC(298K)
• Platinium electrode is used when a metal is not involved in the half-cell

Under these conditions, the electrode potential is called the standard electrode potential(E^o). The superscript (^o) means standard state.

Electrochemical Series

Electrochemical series is an arrangement of common elements and redox agents in order of increasing values of their standard electrode potentials. The potential for each electrode is obtained relative to the standard hydrogen electrode. Some of the common applications of an electrochemical series include cathodic protection and zinc-plating & tin-plating.

The value of the standard(reduction) electrode potential is a measure of the tendency of a species to accept electrons and act as an oxidizing agent. A negative E^o value indicates a weak attraction to electrons hence, the species is a weaker oxidizing agent than the Hydrogen ion. A positive E^o value indicates a strong attraction to electrons; hence, a stronger oxidizing agent than a hydrogen ion.

The standard emf of a cell is used to predict whether a reaction will occur spontaneously or not. A redox reaction would be spontaneous if the standard emf of the cell, E^o_cell, is positive.