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Unformatted text preview: Chemical Kinetics How fast is it? Consider a general reaction: A + 2 B 3 C → Kinetics is all about the rate at which a reaction occurs. How fast are the reactants turned into products? Kinetics is all about the rate at which a reaction occurs. How fast are the reactants turned into products? Consider a general reaction: A + 2 B 3 C → There are a number of equivalent ways of looking at the rate of the reaction A + 2 B 3 C → If I want to measure the rate, I need to look at the change in something. A + 2 B 3 C → If I want to measure the rate, I need to look at the change in something. I have 3 somethings: A + 2 B 3 C → If I want to measure the rate, I need to look at the change in something. I have 3 somethings: I could look at how fast A disappears. I could look at how fast B disappears. I could look at how fast C appears. All 3 representations of the rate should be equivalent! Rate The rate of a reaction is how fast it is occurring. It is a measure of the change in concentration of the chemical species involved. Rate as a function of time For my generic reaction: A + 2 B 3 C → I have 3 different chemical species: A, B, and C. I can measure the rate in terms of ANY OF THE 3 COMPOUNDS Rate as a function of time A + 2 B 3 C → Rate is Δ concentration/Δ time Or Δ[A]/Δt – the change in [A] per unit time Δ[B]/Δt – the change in [B] per unit time Δ[C]/Δt – the change in [C] per unit time Relationship between the different rates A balanced equation has stoichiometry. This stoichiometry has meaning. A + 2 B 3 C → If 1 mole of A reacted, 2 moles of B MUST have reacted also. If 1 mole of A reacted, 3 moles of C MUST have been produced. The Algebra of rates So, applying stoichiometry, for a given unit of time: Δ[C] = 3 Δ[A], Δ[B] = 2 Δ[A], 2 Δ[C] =3 Δ [B] Or, in terms of the rates, themselves: 2 Δ[A]/Δt = Δ[B]/Δt (or, Δ[A]/Δt = ½ Δ[B]/Δt) 3 Δ[A]/Δt = Δ[C]/Δt (or, Δ[A]/Δt = 1/3 Δ[C]/Δt) 3 Δ[B]/Δt = 2 Δ[C]/Δt (or, Δ[B]/Δt = 2/3 Δ[C]/Δt) The negative sign is because C appears as A or B disappears. Practical Considerations You can monitor the concentration of any species that is convenient to measure. The rate is then calculated by taking the change in concentration divided by the change in time: final concentration – initial concentration final time – initial time The rate is always a positive quantity, so you add a negative sign if discussing the rate in terms of one of the reactants that is decreasing. Is the rate of a reaction constant?...
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 Fall '08
 LANGNER
 ΔT, Arrhenius Equation

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