Systems that can lose, or gain electrons - redox systems (pairs), can be connected together via an external circuit to cause a potential difference to arise, and a flow of electrical charge, if there is no external resistance. This section is about the measurement of the relative potentials developed by redox couples in comparison to one another.
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
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 9.6