# Chem. 31 – 2/23 Lecture. Announcements I Exam 1 –Next Monday (3/2) –Will Cover the parts we have covered in Ch. 1, 3 and 4 plus parts of Ch. 6 (through.

## Presentation on theme: "Chem. 31 – 2/23 Lecture. Announcements I Exam 1 –Next Monday (3/2) –Will Cover the parts we have covered in Ch. 1, 3 and 4 plus parts of Ch. 6 (through."— Presentation transcript:

Chem. 31 – 2/23 Lecture

Announcements I Exam 1 –Next Monday (3/2) –Will Cover the parts we have covered in Ch. 1, 3 and 4 plus parts of Ch. 6 (through thermodynamics, Le Châtelier’s Principle?) –Some of HW1.3 postponed (see posted solutions) –Review of Exam Topics on Wednesday –I can schedule a help session (11:45 to 12:45 on Friday?)

Announcements II First Lab Report –Resubmission deadline is 3/2 (2 weeks after first submission deadline) Today’s Lecture –Chapter 4 Material Least Square’s Analysis – quality and use of calibration –Chapter 6 Material Equilibrium Expressions from Reactions Thermodynamics

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) Absolute Uncertainty Relative Uncertainty Best Range: upper 2/3rds of standard range

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?

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 P A a replacing [A] a )

Equilibrium Equations Example problem: Write equation for reaction: AgCl(s) + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + (aq) + Cl - (aq) AgCl not included because it is a solid

Equilibrium Equations - manipulating reactions a)Flipping Directions - If for A ↔ B, K = K 1, then for B ↔ A, K = 1/K 1 b)Adding Reactions 1)NH 4 + ↔ NH 3 (aq) + H + 2)H + + OH - ↔ H 2 O(l) 3)NH 4 + + OH - ↔ NH 3 (aq) + H 2 O(l) Reaction 3) = rxn1) + rxn2) So K 3 = K 1 K 2

Equilibrium Equations - manipulating reactions c) Multiplication 2x[½ N 2 (g) + ½ O 2 (g) ↔ NO (g)] K = K 1 N 2 (g) + O 2 (g) ↔ 2NO (g) K = K 1 2

Equilibrium Equation Example Problem: If the following reactions have the given equilibrium constants: 1)Ag + + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + K = 1.70 x 10 7 2)NH 3 (aq) + H 2 O(l) ↔ NH 4 + + OH - K = 1.76 x 10 -5 3)H 2 O(l) ↔ H + + OH - K = 1.0 x 10 -14 Determine the equilibrium constant for the following reaction: Ag(NH 3 ) 2 + + 2H + → Ag + + 2NH 4 +

Thermodynamics 1.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 2.ΔS is related to disorder of system -If the final system is “more random” than initial system, ΔS > 0

Thermodynamics Entropy Examples: (Is ΔS > or < 0?) H 2 O(l) ↔ H 2 O(g) H 2 O(s) ↔ H 2 O(l) NaCl(s) ↔ Na + + Cl - 2H 2 (g) + O 2 (g) ↔ 2H 2 O(g) N 2 (g) + O 2 (g) ↔ 2NO(g) ΔS > 0 ΔS < 0 ΔS > 0

Thermodynamics ΔG = Change in Gibbs free energy This tells us if a process is spontaneous (expected to happen) or non-spontaneous ΔG < 0 process is spontaneous (favored) ΔG = ΔH - TΔS (T is absolute temperature) processes that are exothermic (Δ H 0) are favored at all T processes that have Δ H > 0 and Δ S > 0 are favored at high T

Example question The reaction N 2 (g) + O 2 (g) ↔ 2NO(g) has a positive  H. Under what conditions is this process spontaneous? - all temperatures - low temperatures - high temperatures - never

Thermodynamics ΔG and Equilibrium ΔG = ΔG° + RTlnQQ = Reaction Quotient (for A ↔ B, Q = [B]/[A]) At equilibrium, ΔG = 0 and Q = K ΔG° = -RTlnK

Thermodynamics Example Question: The ΔG° for the reaction Ca 2+ + 2OH - => Ca(OH) 2 (s) is -52 kJ/mol Determine K at T = 20.°C for Ca(OH) 2 (s) => Ca 2+ + 2OH -

Le Châtelier’s Principle Intuitive Method –Addition to one side results in switch to other side –Example: Mathematical Method AgCl(s) ↔ Ag + + Cl - Addition of Ag + When Q>K, ΔG>0 (toward reactants) When QK reaction shifts to reactants (more AgCl(s))

Le Châtelier’s Principle Stress Number 1 Reactant/Products: Addition of reactant: shifts toward product Removal of reactant: shifts toward reactant Addition of product: shifts toward reactant Removal of product: shifts toward product

Le Châtelier’s Principle Stress Number 1 Example: CaCO 3 (s) + 2HC 2 H 3 O 2 (aq) ↔ Ca(C 2 H 3 O 2 ) 2 (aq) + H 2 O(l) + CO 2 (g) 1. Add HC 2 H 3 O 2 (aq) 2. Remove CO 2 (g) 3. Add Ca(C 2 H 3 O 2 ) 2 (aq) 4. Add CaCO 3 (s) No effect because (s)

Download ppt "Chem. 31 – 2/23 Lecture. Announcements I Exam 1 –Next Monday (3/2) –Will Cover the parts we have covered in Ch. 1, 3 and 4 plus parts of Ch. 6 (through."

Similar presentations