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Chemical Kinetics

Chemical Kinetics. TURN ON SPECs NOW!. First things first…. Safety: Put bags away Goggles Lab Jacket Gloves Basic Reactants! LAB! Warm up Spec 20. Reaction Rate.

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Chemical Kinetics

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  1. Chemical Kinetics TURN ON SPECs NOW!

  2. First things first… • Safety: • Put bags away • Goggles • Lab Jacket • Gloves • Basic Reactants! • LAB! • Warm up Spec 20

  3. Reaction Rate • The rate of reaction may be defined (and measured) as the rate of disappearance of any reactant, ot the rate of appearance of any product. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

  4. Rate Expression • The mathematical equation showing how reaction rate depends on reactant concentrations is called the rate expression.

  5. Crystal Violet Lab • We are studying the reaction of crystal violet to sodium hydroxide and how they are related to rate law and reaction order crystV+(aq) + OH-(aq) --------> crystV-OH(aq) (purple) (colorless) • Complete Rate Law: Rate = k [crystV+]x [OH-]y • Because NaOH is 1000x more concentrated, doesn’t change much over time so we are actually looking at the Modified Rate law: • Rate = k' [crystV+]x where k' = k [OH-]y

  6. Beer-Lambert Law • We are measuring the reaction rate by using a UV-vis spectrometer to measure the color change over time • Obtaining the % Transmittance • Absorbance = -log(%T/100) • Absorbance is proportional to Concentration

  7. Procedure • Warm up Spec 20 for 15 min. • Set to 540nm • Two standard concentrations will be analyzed: • 0.025 M NaOH – two trials • 0.050 M NaOH – two trials • Blank: Blank Spec with clean test tube and NaOHsol’n (around ¾ full) • Mix crystal violet solution: Use one drop of dye in NaOH solution, invert quickly to mix, and record at time zero. • Record your %T: Record %T readings every 30 seconds for 15 minutes

  8. How to blank a spec – write down! • Blank a spec before every trial • Without test tube in spec 20, turn the dial until % transmittance is 0 • Fill a test tube ½-3/4 full with blank solution (the NaOHsoln) • Put test tube in spec 20, make sure white tag is not blocking the light, and close the door opening • Turn the dial until %T is 100

  9. Due by end of lab • Clean up the lab station and hood closest to your station • Turn in your lab notebook with following: purpose, equipment/chemicals, procedure, observations/new equipment, filled out data table.

  10. Lab Report • Due in TWO weeks (2/27/2012) • Typed, given to me in hard copy during class.

  11. Lab Report Contain • Contains following in order: Title, Data tables, Graphs, Calculations, Conclusion/Discussion, References • Title Page: Name, Date, Lab Partner, Sect. • Data Tables ( excel) • Raw data (%T vs T) • Calculated data (Abs vs. T)

  12. Graphs • 6 graphs total (excel) • 3 graphs of A vs. T (0 order), ln A vs. T(1st order), 1/A vs. T(2nd order) for Trial 1 0.025 M NaOH • Find the linear trendline y=mx+b and regression • Choose which graph has the most linear line • Linear graph will det. your overall reaction order • For Trial 2 0.025 M, Trial 1 0.05 M, Trial 2 0.05 M….1 graph per trial • Make linear graphs for each trial. • You determined which reaction order graph was most linear based upon the one for Trial 1 0.025 M • Find linear trendline y=mx+b and regression for each trial.

  13. Calculations – typed or blue/black ink • Show one example calc how convert %T to Absorbance • You are trying to find k and x and y and complete rate law: Rate: k[crystV+]x [OH-]y • X order (crystal V) • You will determine order from which graph was most linear • A vs. T (0 order) • ln A vs. T(1st order) • 1/A vs. T(2nd order)

  14. K’ • You get from slope of linear graph from the trendline. • If you have a linear graph from…. • 0 order: k’ = -m • 1st order: k’ = -m • 2nd order: k’ = +m • Get average k’ for 0.025 M and average k’ for 0.05 M

  15. Find y order (NaOH) • Comparing two equations • (k1' / k2' ) = ([OH-]1 / [OH-]2)y • k1’ = 0.025 M k’ , k2’ = 0.05 M k’ • [OH-]1 = 0.025 M, [OH-]2 = 0.05 • Y = ln((k1' / k2' )) /ln(([OH-]1 / [OH-]2)) • Find k • Plug in values k’, [OH-], and y: • Get Average k from ALL four trials

  16. Complete your rate law, plug in your k, x, and y value you found previously • Rate = k [crystV+]x [OH-]y

  17. Discussion/Analysis • 1 page, double spaced • What values did you find for your results aka what rate law did you get and how get it • Is it precise, accurate? • Error analysis – how did your errors affect your data (did it increase, decrease values?) • More questions and prompts you can find on the pbwiki

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