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Chapter 13

# Chapter 13 - Kinetics is the study of the rates of chemical...

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Kinetics is the study of the rates of chemical reactions Rate is the change of concentration per unit time Units are in mol/L•s rate = c t Δ Δ
Square brackets are used to denote molar concentration Rate is expressed either as rate of appearance of product or rate of disappearance of reactant The rate of reaction is ALWAYS a positive number, regardless of which species is measured rate = Δ c Δ t = Δ product [ ] Δ t = ± Δ reactant [ ] Δ t

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An average rate is a change in concentration measured over a non- zero time interval

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The concentration of a reactant changes from 0.49 M to 0.31 M during the time interval from 20.0 s to 40.0 s (line (c) in the graph). What is the average rate over this interval?
The average reaction rate depends on the size of the time interval As Δ t approaches zero, the average reaction rate approaches a value equal to the slope of the line drawn tangent to the curve at time t This value is called the instantaneous rate of the reaction

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The relative rates of consumption of reactants and formation of products depend on the reaction stoichiometry For the reaction 2HBr (g) H 2 (g) + Br 2 (g) two moles of HBr are consumed for every one mole of H 2 which is formed [ ] [ ] = Δ Δ Δ Δ HBr t H t 2 2

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For any reaction a A + b B c C + d D the reaction rate is given by rate = ± Δ [A] a Δ t = ± Δ [B] b Δ t = Δ [C] c Δ t = Δ [D] d Δ t
In a powerful laser beam water decomposes into its elements, producing hydrogen at the rate of 0.12 mol/L•s. 2 H 2 O >>> 2 H 2 + O 2 a) What is the rate of appearance of oxygen b) What is the rate of disappearance of water? c) What is the reaction rate ?

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Experimental Rate Law : the rate of a reaction is proportional to the product of the concentrations of the reactants raised to some power. For a reaction a A + b B products the rate law is the equation rate = k[A] x [B] y
rate = k[A] x [B] y x and y are the orders of the reaction in [A] and [B] respectively the overall order of the reaction is x + y x and y are usually small integers, but may be zero, negative, or fractions k is the specifc rate constant

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The reaction orders are determined by noting the effect that concentration changes in each reactant have on the rate k is evaluated once the orders in the rate law are known
The initial rate method measures the time, during which a known small fraction of the reactants are consumed Experiments are performed in which [A] and [B] are individually varied The time period of measurement is small enough that the average rate is approximately the instantaneous rate

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Initial rates are given below for the reaction F 2 + 2ClO 2 2FClO 2 Determine the rate law Trial [F 2 ], M [ClO 2 ], M Init. rate, M /s 1 0.10 0.010 1.2 x 10 -3 2 0.10 0.040 4.8 x 10 -3 3 0.20 0.010 2.4 x 10 -3
It is helpful to express concentrations and rates on a relative scale, by dividing the entries in each column by the smallest value.

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Chapter 13 - Kinetics is the study of the rates of chemical...

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