CHM102_Chapter 13 class notes

CHM102_Chapter 13 class notes - 10/18/2010 Chapter 13 Rates...

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10/18/2010 1 Chapter 13 Rates of Reactions The questions posed in this chapter will be: s How is the rate of a reaction measured? s What conditions will affect the rate of a reaction? s How do you express the relationship of rate to the variables affecting the rate? s What happens on a molecular level during a chemical reaction? Chemical Kinetics
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10/18/2010 2 Chemical kinetics is the study of reaction rates, including how reaction rates change with varying conditions and which molecular events occur during the overall reaction. Chemical Kinetics Which conditions will affect the rate of a reaction? Four variables affect the rate of reaction: 1. The concentrations of the reactants 2. The concentration of the catalyst 3. The temperature at which the reaction occurs 4. The surface area of the solid reactant or catalyst
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10/18/2010 3 Chemical Kinetics Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time ( M /s). A B rate = - Δ [A] Δ t rate = Δ [B] Δ t Δ [A] = change in concentration of A over time period Δ t Δ [B] = change in concentration of B over time period Δ t Because [A] decreases with time, Δ [A] is negative . – speeds, or rates, of which a chemical reaction occurs. A B rate = - Δ [A] Δ t rate = Δ [ B ] Δ t time s The above rates are average rates
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10/18/2010 4 Br 2 ( aq ) + HCOOH ( aq ) 2Br - ( aq ) + 2H + ( aq ) + CO 2 ( g ) average rate = - Δ [Br 2 ] Δ t = - [Br 2 ] final – [Br 2 ] initial t final - t initial slope of tangent slope of tangent slope of tangent instantaneous rate = rate for specific instance in time instantaneous rates (always decrease with time) rate α [Br 2 ] rate = k [Br 2 ] k = rate [Br 2 ] = rate constant = 3.50 x 10 -3 s -1 (instantaneous rate) s Reaction rate depends on concentration ± The higher the concentration the higher the rate
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10/18/2010 5 Reaction Rates and Stoichiometry a A + b B c C + d D rate = - Δ [A] Δ t 1 a = - Δ [B] Δ t 1 b = Δ [C] Δ t 1 c = Δ [D] Δ t 1 d 2 N 2 O 5 4NO 2 + O 2 Rate of O 2 formation = - Δ [N 2 O 4 ] Δ t 1 2 In general: Write the rate expression for the following reaction: CH 4 ( g ) + 2O 2 ( g ) CO 2 ( g ) + 2H 2 O ( g ) rate = - Δ [CH 4 ] Δ t = - Δ [O 2 ] Δ t 1 2 = Δ [H 2 O] Δ t 1 2 = Δ [CO 2 ] Δ t Suppose that , at a particular moment during the reaction, CH 4 is reacting at the rate of 0.024 M/s. At what rate is H 2 O being formed? 0.048 M /s
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10/18/2010 6 The Rate Law s The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers. a
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CHM102_Chapter 13 class notes - 10/18/2010 Chapter 13 Rates...

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