--- title: "Standard Electrode Potentials and Predicting Redox Reactions" description: "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 reacti" canonical: https://lightmysky.com/learn/science/standard-electrode-potentials-and-predicting-redox-reactions-mt_LDI10CjoAI source: https://lightmysky.com/learn/science/standard-electrode-potentials-and-predicting-redox-reactions-mt_LDI10CjoAI.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Chemistry · Ages 17 to 18 Page: https://lightmysky.com/learn/science/standard-electrode-potentials-and-predicting-redox-reactions-mt_LDI10CjoAI ## Ready when they can - 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 ## Lesson: Electrode potentials predict the winner 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. *(drawing: The potential predicts direction. Kinetics predicts how quickly the change appears.)* **Example.** 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Oxidation Numbers and Balancing Redox Equations](https://lightmysky.com/learn/science/oxidation-numbers-and-balancing-redox-equations-mt_4xuYKYgUex) ## Optional connections - [Electrolysis: Half Equations at the Electrodes](https://lightmysky.com/learn/science/electrolysis-half-equations-at-the-electrodes-mt_6FvxaD8wTY) - [Free Energy and Whether a Reaction Is Feasible](https://lightmysky.com/learn/science/free-energy-and-whether-a-reaction-is-feasible-mt_o1kZgBYWoM) ## Opens up - [Metabolic Diversity: Choosing an Electron Donor and Acceptor](https://lightmysky.com/learn/science/metabolic-diversity-choosing-an-electron-donor-and-acceptor-mt__2yCr9OSIn) - [Redox Potentials and the Proton-Motive Force](https://lightmysky.com/learn/science/redox-potentials-and-the-proton-motive-force-mt_lU8kgepFRN) - [Cells That Do Work: Batteries, Fuel Cells and Corrosion](https://lightmysky.com/learn/science/cells-that-do-work-batteries-fuel-cells-and-corrosion-mt_NQQzlecX4v) - [Redox Titrations with Manganate(VII) and Thiosulfate](https://lightmysky.com/learn/science/redox-titrations-with-manganate-vii-and-thiosulfate-mt_tKZTSfo0Ra) - [The Nernst Equation and Cells Away from Standard Conditions](https://lightmysky.com/learn/science/the-nernst-equation-and-cells-away-from-standard-conditions-mt_yqmRqDaadR)