1 / 16

Concentrations vs. Activities

Concentrations vs. Activities. Remember: activities are “effective” concentrations E.g. the concentrations that control difference in chemical potential from standard state Activities ≠ concentrations Result from interactions of charged species

kemal
Download Presentation

Concentrations vs. Activities

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Concentrations vs. Activities • Remember: activities are “effective” concentrations • E.g. the concentrations that control difference in chemical potential from standard state • Activities ≠ concentrations • Result from interactions of charged species • Cause “non-ideal” behavior of the ions in solution

  2. Interactions • Electrostatic: between charged ions • Hydration shells: water as a polar molecule • Interactions adds structure to solution: • Increase entropy • reduces free energy e.g., DG • lowers activity of ions • Not complexes – these are specific species, deal with separately

  3. Examples • Seawater: • Salinity 3.5% • Activity of water in seawater: aH2O, seawater ≈ 0.98 concentration of water in seawater: mH2O, seawater aH2O,seawater= 0.98mH2O,seawater • Fresh water • awater ≈ mwater • Recall • Activity coefficient, g = a/m • gseawater = 0.98

  4. How to determine g? • Must consider two types of solutes: • Uncharged species • Charged species

  5. Uncharged Species • Uncharged solutes in dilute water (i.e. close to ideal) • Here: a ~ m; g ~ 1 • There is no electrical interaction • Only change from hydration of ions • Since no charge though, little hydration

  6. For uncharged species in concentration solutions: • g > 1 • E.g., a > m • Why? • Results from hydration of charged species • “Removes” water from solution

  7. Activity coefficient of uncharged species: g = 100.1I Where I is ionic strength: I = ½ Smizi2 Essentially sum of charges in solution

  8. Ionic Strengths • Complicated to determine in natural waters: • Need a total analyses of all dissolved solids • Commonly approximated with major elements • Possible to estimate with TDS or SpC using empirical relationships

  9. Values depend on type of solution: • I ≈ 2 x 10-5(TDS); NaCl waters • I ≈ 2.5 x 10-5(TDS); “average” waters • I ≈ 2.8 x 10-5(TDS); CaHCO3 waters

  10. Or: • I ≈ 0.8 x 10-5(SpC); NaCl waters • I ≈ 1.7 x 10-5(SpC); “average” waters • I ≈ 1.9 x 10-5(SpC); CaHCO3 waters • SpC typically closer to ionic strength because it measures charge of solution

  11. Examples of Ionic strength • On board

  12. Charged Species • Problem with calculating g • To determine gi of single dissolved species, need to know how much G varies as concentrations of single species changes • Impossible to change just one ion - violates electrical neutrality • Generally calculated in terms of uncharged components, e.g., NaClo

  13. For dilute solution, assume change in single ion concentration • With assumptions, can estimate single ion activity coefficient • Calculated using various models • Debye-Hückel • Extended Debye-Hückel • Güntelberg equation • Davies equation • Pitzer equations - Complicated, but thermodynamically more rigorous

  14. Assumptions of all these models: • Charged species are point charges – i.e. 1 D, not true, all ions have mass so are 3D • All interaction are electrostatic • Boltzmann distribution around ions

  15. Expression for g • On board

  16. Seawater Davies Debye- Huckel Extended Debye- Huckel Debye- Huckel Davies Extended Debye- Huckel

More Related