Unit 5: Chemical Kinetics Day 5: Begin Kinetics Lab

Warm Up If you are given 15 M stock solution and you need to make ONLY 20 mL of a 6 M solution, how much of the stock solution should you use? Answer: b

Agenda Demonstration Begin Lab Set Up Pre-Lab Questions Purpose Hypothesis Materials Pre-Lab Questions If Time: Begin Introductory Activity

Kinetics of Crystal Violet Fading Concentration (mol/L) Volume of 2.5 x10-5 CV (mL) Volume of distilled water 2.5 x10-5 = 25 μM 10.0 mL 0.0 mL 2.5 x10-6 = 2.5 μM 5.0 x10-6 = 5.0 μM 7.5 x10-6 = 7.5.0 μM 10.0 x10-6 = 10.0 μM 12.5 x10-6 = 12.5 μM 5.0 mL Pre-Lab 1.

Kinetics of Crystal Violet Fading Guided Inquiry Design and Procedure Part A CV+ + OH-  CVOH What percent of OH- ions will remain at the end of a reaction if the initial concentrations of each reactant were equal (1:1 ratio)?

Kinetics of Crystal Violet Fading Review: CV+ + OH-  CVOH purple colorless Rate = k[CV+]m [OH-]n

Kinetics of Crystal Violet Fading Rate Law: Rate = k’ [CV+]m where k’ = k [OH-]n The constant k’ is a new “pseudo” rate constant. The new pseudo rate law is valid when the concentration of OH- ions is much greater than the concentration of CV+ ions. Under these conditions, the [OH-] term in the original rate law will not change much over the course of the reaction and may be treated as a constant in the rate equation.

Kinetics of Crystal Violet Fading

Kinetics of Crystal Violet Fading BIG IDEA: The more color, the more concentrated, the more absorbance. Absorbance = Concentration

Part B: Procedure Outline NaOH conc. needs to be changed, not CV+, so that the effect on k’ can be calculated and the order of reaction with respect to OH- determined Colorimeter must be zeroed with a blank, equal volumes 0.01 M NaOH and water To make 10 mL of 0.01 M NaOH, measure 5.0 mL of 0.02 M NaOH in a serological pipet and… Clean the pipet with small portions of crystal violet solution. Measure 10 mL of 25 uM crystal violet solution… After the absorbance vs. time data are recorded, the absorbance values need to be converted into concentration, then separate data columns for ln[CV+] and 1/[CV+] need to be created and analyzed to determine k’ for the new concentration of NaOH, 0.01 M.