Advanced Topic 594 of 1102 Acids, Bases & Electrochemistry
Ionic Mobility
Speed of an ion in an electric field, high for H+ because of proton hopping.
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In-depth explanation of Ionic Mobility
Ionic mobility is the speed an ion reaches per unit electric field strength. Hydrogen and hydroxide ions in water are exceptionally mobile because they move by the Grotthuss mechanism, in which protons hop along chains of hydrogen bonds rather than the whole ion diffusing. The mobility of other ions decreases as the size of their hydration shell grows, since larger hydrated ions drag more water and move more slowly. Mobility determines the molar conductivity contributed by each ion and the transport number. It is therefore central to understanding conduction in electrolytes.
Going deeper: Ionic Mobility
Key ideas: Mobility u is the drift speed per unit field: u = v/E in m2 V-1 s-1, typically 5e-8 m2 V-1 s-1 for most ions; H+ is 3.6e-7 and OH- 2.0e-7. Link to conductivity: lambda = z F u. Smaller hydrated radius means higher mobility, so Li+ moves slower than Cs+ in water. Temperature raises mobility by about 2 percent per kelvin.
Common mistakes and connections: Mistakes: assuming smaller crystal radius means faster ion (the hydration shell makes Li+ effectively larger), and forgetting H+ and OH- move by Grotthuss proton hopping, not by physical diffusion. Linked to viscosity (Walden rule) and transport numbers.
Visualization of Ionic Mobility
Limiting ionic conductivities in water: small Li+ is slowest because of its large hydration shell, while H+ and OH- hop by the Grotthuss mechanism. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Li+ has a lower mobility than K+ in water because its small size gives it a larger hydration shell.
Frequently asked questions about Ionic Mobility
What is Ionic Mobility?
Speed of an ion in an electric field, high for H+ because of proton hopping.
Is Ionic Mobility a beginner, intermediate or advanced topic?
Ionic Mobility is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Ionic Mobility?
Study these earlier topics in Acids, Bases & Electrochemistry first: Molar Conductivity, Kohlrausch's Law, Strong and Weak Electrolytes.
What comes after Ionic Mobility?
The next topics in Acids, Bases & Electrochemistry are Transport Number and Debye-Huckel Theory.
Can I practise Ionic Mobility with quizzes?
Yes. The interactive lesson includes a self-check question on Ionic Mobility, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Ionic Mobility fits in Acids, Bases & Electrochemistry
Ionic Mobility is topic 84 of 105 in Acids, Bases & Electrochemistry.