Standard Electrode Potentials and Predicting Redox Reactions
Each half reaction gets a potential measured against a standard hydrogen electrode. Put two half cells together and the difference says which way the electrons will flow, and whether a proposed reaction goes at all.
What a learner can do afterwards
- Describes a half cell and explains what the standard hydrogen electrode is there for
- States the conditions a standard electrode potential is measured under and why they are fixed
- Calculates a cell potential from two standard values and identifies which species is oxidised
- Predicts whether a named redox reaction happens, and gives one reason a feasible one may still not be seen
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A half-cell contains an electrode and a redox couple, such as zinc metal with Zn2+ ions. A standard hydrogen electrode uses platinum, hydrogen gas at 100 kPa and H+ at 1.00 mol/dm3. At 298 K its reduction potential is defined as 0.00 V, giving other half-cells a comparison point.
Standard electrode potentials use 298 K, 100 kPa for gases and 1.00 mol/dm3 for ions. Fixed conditions matter because concentration, pressure and temperature can change a potential.
Oxidation happens at the anode and reduction at the cathode. Reduction happens in the half-cell with the more positive reduction potential, so its electrode is the cathode. In a cell the cathode is the positive electrode, the reverse of electrolysis, where it was negative. Ecell = E(cathode) - E(anode). With nickel at -0.26 V and silver at +0.80 V, Ecell = +0.80 - (-0.26) = +1.06 V. Nickel is oxidised at the negative anode and dissolves as Ni2+. Silver ions are reduced to silver metal.
A positive Ecell means the reaction is thermodynamically feasible as written under the stated conditions. A negative value favours the reverse direction. Feasibility does not give a reaction speed. A large activation barrier can leave a feasible mixture unchanged until a spark starts it.
The hydrogen electrode sets the zero, standard conditions make values comparable, and subtracting reduction potentials predicts direction but not reaction speed.
Original chemistry lesson and assessment text authored for os-taxonomy.
The rewritten readings on galvanic cells and standard reduction potentials were written for this dataset. The reading on feasibility and rate is original writing for os-taxonomy. The three videos are linked Khan Academy videos, not adaptations.
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