1 / 10

Chem. 31 – 10/22 Lecture

Chem. 31 – 10/22 Lecture. Announcements. HW 2.1 Additional Problem due today Water Hardness Lab Report due Monday Quiz Today Today’s Lecture Advanced Equilibrium: (Ch. 7) more on activity in equilibria real pH equation the systematic method – a way to deal with multiple equilibria.

quemby-joyce
Download Presentation

Chem. 31 – 10/22 Lecture

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. Chem. 31 – 10/22 Lecture

  2. Announcements • HW 2.1 Additional Problem due today • Water Hardness Lab Report due Monday • Quiz Today • Today’s Lecture • Advanced Equilibrium: (Ch. 7) • more on activity in equilibria • real pH equation • the systematic method – a way to deal with multiple equilibria

  3. Ionic Strength EffectsEffects on Equilibrium - Quantitative • Calculate expected [Mg2+] in equilibrium with solid MgCO3 for cases both with and without NaCl. • Go to Board

  4. Ionic Strength EffectsReal Equation for pH • pH = -logAH+ = -log(gH+[H+]) • Example Problem: Determine the pH of a solution containing 0.0050 M HCl and 0.020 M CaCl2. • Note: H2O  H+ + OH- also affected by ionic strength

  5. Second Part to Chapter 7The Systematic Method • Question: Why can’t we apply the ICE (initial, change, equilibrium) method to any type of equilibrium problem? • Answer: That method is best designed for cases where there is only one relevant equilibrium reaction. • Examples of failures: • Solubility of MgCO3 • pH of 5.0 x 10-8 M HCl solution (Show failure of Chem. 1B method) • Note: both problems can be solved using ICE method, but problem set up is more complicated

  6. The Systematic MethodSolubility of MgCO3 – Why did it fail? • MgCO3 Mg2+ + CO32- x x Equil. (in ICE) • So x = (Ksp)1/2 = 1.87 x 10-4 M (neglecting ionic strength effects) • Problem is both ions can react further: CO32- + H2O  HCO3- + OH- And HCO3- + H2O  H2CO3 + OH- Also, Mg2+ + OH-  MgOH+ And Mg2+ + CO32-  MgCO3 (aq) Finally, we also have H2O  H+ + OH- re-establishing equilibrium • Each additional reaction results in greater dissolution • To properly solve problem we must consider 6 reactions not just 1 • Measured “[CO32-]” from titration = [CO32-] + 0.5[OH-] + 0.5[HCO3-] + [MgCO3] + 0.5[MgOH+] • The “further” reactions makes [Mg2+] ≠ [CO32-], so ICE method fails (or needs modification by ICE tables for other reactions) • Actual solubility is greater than ICE method finds [Mg2+]total = solubility ~ 3.3 x 10-4 M (from systematic approach) Predicted HCl needed = 3.3 mL (close to that measured) These calculations didn’t include activity which would lead to a ~10% increase in solubility (~3.6 mL HCl needed). In 0.1 M NaCl, I get 6.1 mL HCl needed enhancements: (% over rxn 1 only) 90% 0% 9% 16%

  7. The Systematic MethodThe Six Steps • Write out all relevant reactions • Write a “Charge Balance Equation” • Write “Mass Balance Equations” • Write out all equilibrium equations • Check that the number of equations (in 2 to 4 above) = (or maybe >) the number of unknowns (undefined concentrations) • Solve for the desired unknown(s) by reducing the equations to one equation with one unknown. Then solve for remaining unknowns Note: the emphasis of teaching the systematic method is steps 1 to 5. Step 6 will be reserved for “easy” problems with 2 to max 3 unknowns

  8. The Systematic MethodpH of 5.0 x 10-8 M HCl • Demonstrate Method on Board

  9. The Systematic MethodConceptual Approach to Mass Balance Equations • With every source of related species, there should be one mass balance equation (or one set for ionic compounds) • Example: • Solubility of AgCl in water with 0.010 M 1,10-phenathroline (Ph) • Reactions: 1) AgCl(s)  Ag+ + Cl- 2) Ag+ + 2Ph  Ag(Ph)2+ • Mass Balance equations: • if only rxn 1) [Cl-] = [Ag+] • w/ rxn 2) [Cl-] = [Ag+] + [Ag(Ph)2+] 1,10-phenathroline Ag+ Ph Ph Ph Ph Ag+ Cl- Ag+ Cl- Ag+ Ag+ Cl- Cl- 2nd Mass Balance Equation: [Ph]o = 0.010 M = [Ph]Total = [Ph] + 2[Ag(Ph)2+] AgCl(s) Notes: with rxn 1) only, 2 Ag+s = 2 Cl-s; with rxn 2) also, 3 Cls = 2 Ags + 1 Ag(Ph)2 Initially 4 Phs, then 2 Phs + one complex containing 2 Phs (so total # of Phs remains constant)

  10. The Systematic Method2nd Example • An aqueous mixture of CdCl2 and NaSCN is made • Initial concentrations are [CdCl2] = 0.0080 M and [NaSCN] = 0.0040 M • Cd2+ reacts with SCN- to form CdSCN+ K = 95 • HSCN is a strong acid • Ignore any other reactions (e.g. formation of CdOH+) • Ignore activity considerations • Determine the concentrations of all species

More Related