ch13 MSJ jlm - Chapter 13 Chemical Kinetics Rates of...

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1 Chemical Kinetics: Chemical Kinetics: Rates of Reactions Rates of Reactions Chapter 13 Chapter 13 Svante A. Arrhenius 1859-1927.* Developed concept of activation energy; asserted solutions of salts contained ions.
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2 Chemical Kinetics Chemical Kinetics Kinetics is the study of how fast chemical reactions occur. There are 3 important factors which affect rates of reactions: reactant concentration [molarity or pressure] temperature action of catalysts Goal: to understand chemical reactions at the molecular level.
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3 Chemical Kinetics Chemical Kinetics Reaction Rates Reaction Rates Speed of a reaction is measured by the change in concentration with time [brackets means concentration, units of molarity, M ] For a reaction A B At the beginning of the reaction (t = 0), let us assume that [A] = 1.0 M and [B] = 0. Average rate = change in conc. of B change in time = Δ[B] Δ t
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4 Reaction Rates Reaction Rates A B Conc (M) [ ] [ ] Concentration of reactant and product as function of time
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5 Reaction rates Reaction rates The rate for this reaction depends on the time. At beginning of reaction, rate is fast, and the rate slows down as the reaction proceeds. At any time, the rate is given by either : (a) change in [A] with change in time, or… (b) change in [B] with change in time . There are two ways of calculating rates: (a) average rate (rough and imprecise) (b) instantaneous rate (more exact ) Average rate = change in conc. of B change in time = Δ[B] Δ t - Δ[A] Δ t =
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6 Average rate is rate between two time intervals At t=10 min, [B]=0.26 M; at t=30 min, [B]=0.60 M Average rate = [B]/ t = (0.60-0.26)/(30-10) = (.34 M / 20 min) = 0.017 M/min conc(M) [ ] [ ] What is rate between 10 min and 30 min?
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7 The instantaneous rate is more precise than the average rate. It is the rate at some specific instance in time; the concept is derived from the calculus. It is the slope of a curve at a specific instance in time. This is obtained by constructing a tangent to the curve at the indicated point. “rise” = Δy “run” = Δx Δ y Δx M time Slope =
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8 0.6 M 38 min What is instantaneous rate at t = 20 min? Conc (M) = 0.96 M/s Rate = Δ [B] Δ t = 0.6 M 38 min 0.016 M/min =
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9 Reaction Rates Reaction Rates A B Conc (M) [ ] [ ] When do we use (+) and (-) signs for rate?
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