chemistryfinal - Chapter 12 Chemical Kinetics Chemical...

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Chapter 12 Chemical Kinetics ·Chemical Kinetics is the area of chemistry concerning rates ·Why is this important? ·Spontaneity has nothing to do with how fast a reaction occurs but rather its inherent tendency to occur ·Must take place in a reasonable amount of time to be useful · Reaction rate is the change of concentration of a reactant or product over time. ·So… Ex. 2NO 2 2NO + O 2 ·Table 12.1 shows concentrations of reactants and products as a function of time and this data is plotted in figure 12.1 · Reaction Rate = -∆[reactants or product adjusted for moles] ∆T ·Typically show reactant rates in terms of reactants, not products ·Instantaneous rates are calculated by calculating the slope of a line tangent to a curve at t = 100 ·Rate laws – an expression that shows mathematically how the reaction rate depends on the concentration of reactant(s) ·Must assume there is no reverse reaction ·Ignore products that are formed ·Assume they are negligible ·Therefore we only study reactions at a point soon after the reactions are mixed ·Ex. 2NO 2 2NO + O 2 ·Reaction rate only depends on NO 2 therefore rate = k[NO] 2 ·K = rate constant (determined experimentally) ·N= the order of the reaction ·2 Types of Rate Laws 1. Differential Rate Law the rate law that expresses how the rate depends on reaction concentration. Often called “the rate law” ex. -∆[NO 2 ] = k[NO 2 ] n ∆T 2. Integrated Rate Law expresses how the concentrations depend on time ·Later we will see that if the differential rate law is known we automatically know the form of the integrated rate law and vice-versa ·Which rate law we choose to depend on depends on the data given ·Determining the form of the rate law ·The first step in understanding how a reaction occurs ·A reaction is said to be n-order ·Method of Initial Rates ·The form of a rate law is determined by this method ·We will measure the rates just after t=0 (that is why it is called initial rate) ·Initial Rate instantaneous rate at t=0 ·Table 12.4 gives initial rates from 3 experiments involving different concentrations of reactions 12.4 Integrated Rate Law ·Integrated Rate Law ·Used when one wants to express reactant concentrations as a function of time ·We will only examine reactions with a single reactant in which the differential rate law is known ·Rate = - ∆[A] = k[A] n Where A = products ∆T 1. First order Integrated Rate Law ·n=1 in differential rate law so rate = k[A]
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·First order Integration = Ln[A] = -kt + ln[A] o ·This expresses the reactant concentration as a function of time ·If the initial concentration and rate constant are known, we can find [A] at any time t ·This equation is in the form y = mx+b and is therefore a straight line ·This can also be written Ln( [A o ]) =kt [A] ·Slope = -k ·Half life- ·The time required for a reactant to reach half it’s original concentration (t 1/2 ) ·The Half life formula for a first order reaction = T 1/2 = ln2 K 2.
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chemistryfinal - Chapter 12 Chemical Kinetics Chemical...

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