The discovery of electrical cells by Voltaire in the eighteenth century was one of the major scientific achievements of the millenium. Nowadays electrical cells are used in thousands of devices and applications. This section looks at the basic principles behind these electrochemical cells.
Understandings - SL
Essential idea: Voltaic cells convert chemical energy to electrical energy and electrolytic cells convert electrical energy to chemical energy.
Voltaic (Galvanic) cells: Voltaic cells convert energy from spontaneous, exothermic chemical processes to electrical energy.
Oxidation occurs at the anode (negative electrode) and reduction occurs at the cathode (positive electrode) in a voltaic cell.
Understandings - HL
Essential idea: Energy conversions between electrical and chemical energy lie at the core of electrochemical cells.
A voltaic cell generates an electromotive force (EMF) resulting in the movement of electrons from the anode (negative electrode) to the cathode (positive electrode) via the external circuit. The EMF is termed the cell potential (Eº).
The standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3 hydrogen ion and hydrogen gas at 100 kPa and 298 K. The standard electrode potential (Eº) is the potential (voltage) of the reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for gases. Eº of the SHE is 0 V.
ΔG° = -nFE°. When Eº is positive, ΔGº is negative indicative of a spontaneous process. When Eº is negative, ΔGº is positive indicative of a non-spontaneous process. When Eº is 0, then ΔGº is 0.
Applications and skills - SL
Construction and annotation of both types of electrochemical cells.
Explanation of how a redox reaction is used to produce electricity in a voltaic cell and how current is conducted in an electrolytic cell.
Distinction between electron and ion flow in both electrochemical cells.
Performance of laboratory experiments involving a typical voltaic cell using two metal/metal-ion half-cells.
Applications and skills - HL
Calculation of cell potentials using standard electrode potentials.
Prediction of whether a reaction is spontaneous or not using Eº values.
Determination of standard free-energy changes (ΔGº) using standard electrode potentials.
In Chapter 9.4