Chemical Kinetics.pdf - Chapter 19 Chemical Kinetics Key...

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Red paint from the pond in Van Gogh's Wheat Stack Under a Cloudy Sky has degraded to grey/white over time. Chapter 19: Chemical Kinetics Key topics: Reaction Rates Factors That Affect Reaction Rates Reaction Mechanisms How fast does a reaction occur? e.g. , many famous painting are changing color
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(from ibchem.com) Also see page 434. Collision Theory of Chemical Reactions A very simple picture of a chemical reaction between two molecules is that 1. They must collide with enough energy to react 2. They must collide in the correct orientation to react The collision (kinetic energy) can be transferred into vibrational energy , which can lead to bond breaking . This is the first step in a chemical reaction. The minimum energy required to initiate a reaction is called the activation energy , E a . If the collision is < E a , it is ineffective and the reactants simply bounce off each other. Temperature has a large effect on having energy > E a .
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Orientation dependence on a simple reaction. The correct orientation is also needed for an effective reaction:
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The transition state (activated complex) is a transient species of high energy that falls apart to generate the products of the reaction. Transition state theory (TST), also known as Activated Complex Theory, is a more sophisticated theory that has two main assumptions 1) The transition state is in equilibrium with the reactions. 2) One (special) vibrational motion results in the breakup of the transition state into the products. Measuring Reaction Progress & Expressing Reaction Rate We can express the reaction rate as either: o Rate of disappearance of reactants (decrease or –ve) o Rate of appearance of products (increase or +ve)
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Reaction progress with time. The rate changes over time. Reaction progress can be monitored using a spectrometer. We can follow a color change, conductance change, pressure change, or whatever is convenient . Consider the reaction A B. average rate = - Δ [ A ] Δ t = Δ [ B ] Δ t
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The instantaneous rate is given by the tangent line. average rate = - Δ [Br 2 ] Δ t = - [Br 2 ] final - [Br 2 ] initial t final - t initial The average rate decreases over time (see Table 19.1). The instantaneous rate is given by the tangent line: From Table 19.1: Time (s) [Br 2 ] Rate ( M /s) 50 0.0101 3.52 x 10 -5 250 0.00500 1.75 x 10 -5 We can see that when [Br 2 ] is halved, the rate is halved. Therefore rate / [Br 2 ], or rate = k [Br 2 ] where k is the proportionality constant. k = rate constant (does depend on the temperature)
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Value of k ? k = rate / [Br 2 ] at t = 50 s , k = 3 . 52 10 - 5 M s - 1 0 . 0101 M = 3 . 49 10 - 3 s - 1 at t = 250 s , k = 1 . 75 10 - 5 M s - 1 0 . 00500 M = 3 . 50 10 - 3 s - 1 (same within experimental error) Consider the reaction A 2B. rate = - Δ [ A ] Δ t = 1 2 Δ [ B ] Δ t In general for the reaction aA + bB ! cC + dD 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 e.g. , Write the balanced equation corresponding to the following rate expression rate = - 1 2 Δ [N 2 O 5 ] Δ t = 1 2 Δ [N 2 ] Δ t = 1 5 Δ [O 2 ] Δ t Solution: -ve sign indicates reactants and +ve sign indicates products.
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  • Spring '13
  • PetravanKoppen
  • Rate equation

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