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
1 · Read
A half cell pairs an element with its ions, such as zinc sitting in zinc ion solution. The standard hydrogen electrode, hydrogen gas over platinum in 1 M H+, is given 0.00 V so every other value has a baseline to stand on.
Standard means fixed conditions: 298 K, 100 kPa, and 1.00 mol per cubic decimetre ions. Keeping them fixed lets every value compare fairly with every other one.
Zinc is -0.76 V and copper is +0.34 V, so the cell potential is 0.34 minus -0.76, which is +1.10 V. Zinc is the negative end: it gives up electrons and its atoms enter the solution as ions, while copper ions plate out.
A positive cell potential means the reaction as written can go, while a negative one means it will not. Even a feasible one can stall on kinetics: hydrogen and oxygen sit mixed until a spark sets them off.
Half cells stand on the hydrogen zero, standard conditions keep values fair, differences give voltages, and kinetics can still say wait.
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.