Chapter 13 Notes

Chapter 13 Notes - Chapter 13 Chemical Kinetics: Rates of...

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Chapter 13 Chemical Kinetics: Rates of Reactions
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Chemical Kinetics The study of speeds of reactions and the nanoscale pathways or rearrangements by which atoms and molecules are transformed from reactants to products. Chemical kinetics is also called reaction kinetics .
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Reaction Rate Factors affecting the speed of all reactions: • Properties of reactants and products • especially their structure and bonding • Concentrations of reactants ( and products) • Temperature • Catalysts • and if present, their concentration Reactions are either: Homogeneous Heterogeneous - species in multiple phases
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Reaction Rate Heterogeneous reaction rates may also depend upon surface area. Combustion of Fe(s) powder : Small surface area. Fe burns slowly. Finely divided Fe burns rapidly.
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Reaction Rate Change in [reactant] or [product] per unit time. rate = = change in concentration of Cv + elapsed time Δ [Cv + ] Δ t Cresol violet (Cv + ; a dye) decomposes in NaOH(aq): Cv + (aq) + OH - (aq) CvOH(aq)
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Time, t [Cv + ] Average rate (s) (mol / L) (mol L -1 s -1 ) 0.0 5.000 x 10 -5 10.0 3.680 x 10 -5 20.0 2.710 x 10 -5 30.0 1.990 x 10 -5 40.0 1.460 x 10 -5 50.0 1.078 x 10 -5 60.0 0.793 x 10 -5 80.0 0.429 x 10 -5 100.0 0.232 x 10 -5 13.2 x 10 -7 9.70 x 10 -7 7.20 x 10 -7 5.30 x 10 -7 3.82 x 10 -7 2.85 x 10 -7 1.82 x 10 -7 0.99 x 10 -7 Sample calculation Avg. rate = Δ [Cv + ] Δ t = (3.68 x10 -5 – 5.00 x10 -5 ) mol/L (10.0 – 0.0) s = -1.32 x 10 -6 mol L -1 min -1 Table shows positive rates – explained soon… Average rate of the Cv + reaction can be calculated: Reaction Rate
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Stoichiometry: Loss of 1 Cv + → Gain of 1 CvOH Rate of Cv + loss = Rate of CvOH gain Another example: 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) Negative rate Positive rate Rate of loss of N 2 O 5 divided by -2, equals rate of gain of O 2 Loss of 2 N 2 O 5 Gain of 1 O 2 Rate of N 2 O 5 loss = 2 x (rate of O 2 gain ) Cv + (aq) + OH - (aq) CvOH(aq) Reaction Rates & Stoichiometry
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For any general reaction: a A + b B c C + d D The overall rate of reaction is: Reactants decrease with time. Negative sign. Products increase with time. Positive sign Rate = = = = 1 a Δ [A] Δ t 1 b Δ [B] Δ t + 1 c Δ [C] Δ t + 1 d Δ [D] Δ t Reaction Rates & Stoichiometry
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For: H 2 (g) + I 2 (g) → 2 HI (g) the rate of loss of I 2 is 0.0040 mol L -1 s -1 . What is the rate of formation of HI? Rate = = = Δ [H 2 ] Δ t Δ [I 2 ] Δ t + 1 2 Δ [HI] Δ t Rate = = − ( -0.0040) = Δ [H 2 ] Δ t + 1 2 Δ [HI] Δ t Δ [HI] Δ t So = +0.0080 mol L -1 s -1
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Average rate depends on interval chosen. Green and grey triangles have different slopes. Graphical view of Cv
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Chapter 13 Notes - Chapter 13 Chemical Kinetics: Rates of...

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