Chem. 31 – 2/19 Lecture

1. Chem. 31 – 2/19 Lecture

2. Announcements • Additional Problem 1.2 and Pipet/Buret Calibration Lab Report due today • Quiz 2 • Covering today: • Chapter 4 Material • Linear Regression and Calibration • Chapter 6 (Equilibria) • Reaction Equilibria • Manipulation of Reactions • Thermodynamics

3. Use of Calibration Curve- Quality of Results Quality of Results Depends on: Calibration Results R2 value (measure of variability of response due to conc.) Reasonable fit Range of Unknown Concentrations Extrapolation outside of range of standards should be avoided Best concentration range (see next slide) Better fit by curve

4. Use of Calibration Curve- Quality of Results Quality of Results Depends on: Calibration Results on last slide Range of Unknown Concentrations Extrapolation outside of range of standards should be avoided Best concentration range Range of Standards (0.02 to 0.4 ppm) Relative Uncertainty Absolute Uncertainty Best Range: upper 2/3rds of standard range

5. Calibration Question A student is measuring the concentrations of caffeine in drinks using an instrument. She calibrates the instruments using standards ranging from 25 to 500 mg/L. The calibration line is: Response = 7.21*(Conc.) – 47 The response for caffeine in tea and in espresso are 1288 and 9841, respectively. What are the caffeine concentrations? Are these values reliable? If not reliable, how could the measurement be improved?

6. Equilibrium Equations Equilibrium Equations from Chemical Equations (Reactions) Generic Example: aA + bB ↔ cC + dD (Reaction) Equilibrium Equation Compounds are in equation if in solution (not present as solid, or solvent). Concentrations are in M but K is unitless Similar equation for gases (except with PAa replacing [A]a)

7. Equilibrium Equations Example problem: Write equation for reaction: AgCl(s) + 2NH3(aq) ↔ Ag(NH3)2+(aq) + Cl-(aq) AgCl not included because it is a solid

8. Equilibrium Equations- manipulating reactions Flipping Directions - If for A ↔ B, K = K1, then for B ↔ A, K = 1/K1 b) Adding Reactions NH4+↔ NH3(aq) + H + H+ + OH-↔ H2O(l) NH4+ + OH-↔ NH3(aq) + H2O(l) Reaction 3) = rxn1) + rxn2) So K3 = K1K2

9. Equilibrium Equations- manipulating reactions c) Multiplication 2x[½ N2 (g) + ½ O2 (g) ↔ NO (g)] K = K1 N2 (g) + O2 (g) ↔ 2NO (g) K = K12

10. Equilibrium Equation Example Problem: If the following reactions have the given equilibrium constants: Ag+ + 2NH3(aq) ↔ Ag(NH3)2+ K = 1.70 x 107 NH3(aq) + H2O(l) ↔ NH4+ + OH-K = 1.76 x 10-5 H2O(l) ↔ H+ + OH-K = 1.0 x 10-14 Determine the equilibrium constant for the following reaction: Ag(NH3)2++ 2H+→ Ag+ + 2NH4+

11. Thermodynamics Some Definitions: H = Enthalpy S = Entropy G = Gibbs Free Energy T = Absolute temperature

12. Thermodynamics ΔH is related to heat of reaction - if a reaction produces heat, ΔH < 0 and reaction is “exothermic” - a reaction that requires heat has ΔH > 0 and is endothermic ΔS is related to disorder of system - If the final system is “more random” than initial system, ΔS > 0

13. Thermodynamics Entropy Examples: (Is ΔS > or < 0?) H2O(l) ↔ H2O(g) H2O(s) ↔ H2O(l) NaCl(s) ↔ Na+ + Cl- 2H2(g) + O2(g) ↔ 2H2O(g) N2(g) + O2(g) ↔ 2NO(g) ΔS > 0 ΔS > 0 ΔS > 0 ΔS < 0 ΔS > 0