Ch13_RateNotes115b07

Ch13_RateNotes115b07 - Ch. 13. Chemical Kinetics - How fast...

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Ch. 13. Chemical Kinetics - How fast do reactions go? - What is the sequence of elementary steps that lead to the final result? Read Ch. 13.1-13.3 for Wednesday Concepts Rate of reaction: how fast products appear or how fast reactants disappear Rate laws: how does rate depend on concentration and conc. on time Rxn mechanism : elementary steps that lead to a final result - Rate limiting step - Steady state approximation - Rate of enzyme reactions and atmospheric reactions Factors that affect rate: - concentration - temperature - catalyst Special topics - femtochemistry - atmospheric chemistry
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Demonstration: hydrogen peroxide - iodate ion oscillating reaction (Bray rxn) - changes in concentration <-> changes in rate; catalysis (use colors to measure concentration) Ingredients: - hydrogen peroxide -H IO 3 (from potassium iodate + sulfuric acid) H 2 O 2 (aq) -> H 2 O (aq) + O 2 (g) RedOx rxns: Rxn 1. 5H 2 O 2 (aq) + 2HIO 3 (aq) - > 5O 2 (g) + I 2 (aq) + 6H 2 O (aq) Rxn 2. 5H 2 O 2 (aq) + I 2 (aq) -> 2HIO 3 (aq) + 4H 2 O (aq) Net effect: 2H 2 O 2 (aq) ->O 2 (g) + H 2 O (aq) Note: products of Rxn 1 are reactants of Rxn 2 and vice versa -> Rxn 1 speeds up Rxn 2 and Rxn 2 speeds up Rxn1 -> oscillations Colors: amber <-> I 2 dark blue I 2 + I - -> I 3 - /starch ( [I 2 ] must be large, and I - should be present)
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Insert: Video of oscillating reactions http://chemed.chem.purdue.edu/demos/main_pages/22.8.html L2: Rates, Instantaneous and initial
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Rate of rxn ~ speed of a car time 7:30 am S. Pasadena USC 0 14 miles Distance Instantaneous speed on 110 without traffic: Average speed to USC: mph h m t D S 19 75 . 0 14 = = Δ Δ = 8:15 am mph dt dD S 80 = =
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Definition of Rate ( R ): How fast products appear or how fast reactants disappear = change in concentration per unit time E.g., A -> B Average rate: Instantaneous rate: t A t t A A R i f i f Δ Δ = = ] [ ] [ ] [ dt A d t A R t ] [ ] [ 0 = Δ Δ = > Δ L2: Rates: instantaneous and initial
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Instantaneous vs average rxn rate NO 2 + CO - > NO + CO 2 Average rate: Instantaneous rate: t NO t t NO NO R i f i f Δ Δ = = ] [ ] [ ] [ dt NO d t NO R t ] [ ] [ 0 = Δ Δ = > Δ
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NO 2 + CO NO + CO 2 dt CO d dt NO d R ] [ ] [ 2 = = time Concentration [NO] [CO] Define: Then: 2 [] dNO dCO R dt dt = = The instantaneous rate changes in the course of the reaction; it decreases at later time because of the decrease in concentration of the reactants Insert link: http://www2.wwnorton.com/college/chemistry/gilbert/tutorials/ch14.htm Section 4
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2NO 2 2NO+O 2 dt O d R ] [ 2 = dt NO d R ] [ 2 1 2
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Ch13_RateNotes115b07 - Ch. 13. Chemical Kinetics - How fast...

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