Electrochemical Cells & Standard Electrode Potentials (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about electrochemical cells and standard electrode potentials.

Electrode potential measures how easily a species is reduced, and the standard electrode potential gives every half-cell a common reference so their values can be compared and used to predict which way a redox reaction actually goes.

When a metal is dipped into a solution of its own ions, an equilibrium is set up between the metal and the solution, creating a potential difference called the electrode potential. Measuring that potential under standard conditions, against a standard hydrogen electrode, gives every half-cell a common reference point. Comparing two of these standard electrode potentials lets you calculate the cell potential and predict which way a reaction will go.

This video starts with how electrochemical cells are represented, then covers what a standard electrode potential is and how it's measured, and finishes with how those values are used to predict the direction a reaction takes.

Dipping a metal rod into a solution of its own ions sets up an equilibrium between the metal and its ions in solution, creating a potential difference called the electrode potential. Chemists represent electrochemical cells using a shorthand convention: a single vertical line shows a phase boundary between a solid and a solution, and a double vertical line represents a salt bridge. The substance with the highest oxidation state in each half-cell is written next to the salt bridge, and the cell potential is calculated as the potential of the right-hand half-cell minus the potential of the left-hand half-cell.

Because electrode potential depends on temperature, pressure and concentration, it can only be compared meaningfully under standard conditions: a temperature of 298 kelvin, a pressure of 100 kiloPascals, and an ion concentration of 1.00 mole per decimetre cubed. These standard electrode potentials are measured relative to the standard hydrogen electrode, which is given a value of exactly 0.00 Volts, so every other half-cell's potential is really a comparison against hydrogen. Once the standard electrode potential of two half-cells is known, the standard cell potential can be calculated as the potential of the reduction half-cell minus the potential of the oxidation half-cell.

Comparing the standard electrode potentials of two half-cells shows which one more readily accepts electrons: the half-cell with the more positive value is reduced, and the half-cell with the less positive value is oxidised, with electrons flowing from the negative pole to the positive pole through the external circuit. A more positive standard electrode potential means a species is more easily reduced. The reaction is feasible — likely to occur — when the standard cell potential works out positive.

Oxidation happens at the negative electrode, and reduction happens at the positive electrode — you're losing electrons at the negative. Never multiply a standard electrode potential by the number of electrons in the half-equation — always just subtract the two values directly.

A half-cell sets up an electrode potential from the equilibrium between a metal and its ions. The standard electrode potential puts every half-cell on the same scale, measured against the standard hydrogen electrode. The standard cell potential compares two half-cells directly, and a positive value predicts that a reaction is feasible.