Describe galvanic and electrolytic cells and calculate cell potentials from standard electrode potentials

What this dot point is asking

Electrochemistry connects redox reactions with electrical energy. There are two main cell types. A galvanic (or voltaic) cell converts the chemical energy of a spontaneous redox reaction into electrical energy. An electrolytic cell does the opposite, using an external electrical supply to force a non-spontaneous redox reaction to proceed. Both rely on separating oxidation and reduction so that electrons travel through an external circuit.

In any electrochemical cell, oxidation occurs at the anode and reduction occurs at the cathode. A useful memory aid is that both anode and oxidation begin with vowels, and cathode and reduction with consonants. The sign of each electrode differs between cell types, which is a common point of confusion. In a galvanic cell the anode is negative and the cathode is positive. In an electrolytic cell the anode is positive and the cathode is negative, because the power supply imposes the polarity.

A galvanic cell has two half-cells, each containing an electrode in an electrolyte. Electrons flow from the anode, through the external wire, to the cathode. A salt bridge connects the two half-cells and allows ions to move so that electrical neutrality is maintained. Without the salt bridge, charge would build up and the reaction would stop almost immediately. Anions in the salt bridge move toward the anode and cations move toward the cathode to balance the charge produced by electron flow.

The driving force of a cell is measured by the cell potential, or electromotive force (EMF), in volts. Each half-reaction has a standard electrode potential, $E^\circ$, measured relative to the standard hydrogen electrode, which is defined as 0 volts. Standard conditions are 25 degrees Celsius, 1 mol/L concentrations, and 100 kPa pressure for gases. These potentials are tabulated as reduction potentials, with the strongest oxidising agents at the top.

The standard cell potential is calculated as:

where both values are taken from the reduction potential table as written. A positive $E^\circ_{cell}$ indicates a spontaneous reaction, which is the case for any working galvanic cell. A negative value means the reaction as written is non-spontaneous and would require electrolysis to proceed.

To predict which reaction occurs, locate both half-reactions on the table. The half-reaction with the more positive reduction potential proceeds as a reduction at the cathode, and the other reverses to become an oxidation at the anode. This top-left to bottom-right rule helps you decide the spontaneous direction.

In electrolysis, an external voltage drives reactions at inert or active electrodes. When more than one species could react, the one easiest to reduce (highest reduction potential) is favoured at the cathode, and the one easiest to oxidise at the anode, although factors such as concentration and overpotential can change the actual product. Electrolysis is used industrially to extract reactive metals such as aluminium and to electroplate objects.

When answering, state the half-equations, identify anode and cathode with correct signs for the cell type, and use the standard potential table to justify the spontaneous direction.