Chemistry Chapter 13
Chemistry Chapter 13 CHM 170 002
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This 4 page Bundle was uploaded by Devin Mart on Tuesday March 29, 2016. The Bundle belongs to CHM 170 002 at Missouri State University taught by Dr. Richter in Spring 2016. Since its upload, it has received 9 views. For similar materials see General Chemistry 2 in Chemistry at Missouri State University.
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Date Created: 03/29/16
Mart 1 Chapter 13: Chemical Kinetics ● Chemical kinetics studies how the molecular world changes with time. ○ Reactions are predictions, they do not tell us what will happen, but what should or might happen. ● Chemical kinetics can be defined as the search for answers to the following questions: 1. What is the rate at which the reactants are converted into the products of a reaction? 2. What factors influence the rate of reaction? 3. What is sequence of steps, or the mechanism, by which the reactants are converted into products? ● The term rate is used to describe the change in a quantity that occurs per unit of time. ○ Δ = change in ○ [ x ] = any time brackets are used around a number the measurement is in molarity. ■ Speed = change in distance / change in time ■ Weight loss = change in weight / change in time ○ The rate of a chemical reaction is a measure of how fast the reaction occurs. ○ The rate of a chemical reaction is the change in the concentration of one of the reactants or products that occurs during a given period of time, Δt. ■ Rate = Δ[concentration] / Δt ■ Rate = + (Δ[product] / Δt) = (Δ[reactant] / Δt) ○ The calculated rate is dependent on the time points taken. ○ The rate is always positive, there can never be a negative rate. ○ The average reaction rate is the change in measured concentrations in any particular time period. ○ The instantaneous rate is the change in concentration at any one particular time. ● Reaction rates for at least one reactant or product must be measured experimentally. ○ Spectroscopy can be used to measure concentration changes if color changes occur. ○ Pressure can be used to measure concentrations of the number of moles of gas changes. ● The rate (as we’ve seen) of a reaction often depends on the concentration of one or more of the reactants. ● The Rate Law is a mathematical relationship between the rate of the reaction and the concentration of reactants. ○ Rate = k[A]ⁿ ○ The rate of a reaction is directly proportional to the concentration of each reactant raised to a power. Mart 2 ■ Rate = k[A]ⁿ[B]ⁿ ● K is the rate constant while the A and B represent concentrations of reactants. ● The values of n and m determine how the rate depends on the concentration and the reactant. ○ If n (orm) = 0, the reaction is zero order and the rate is independent of the concentration of A (or B). ○ If n (orm) =1, the reaction is first order and the rate is directly proportional to the concentration of A (or B). ○ If n (orm) =2, the reaction is second order and the rate is proportional to the square of the concentration of A (or B). ● The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation. ● Rate Laws: ○ Rate laws are always determined experimentally, they cannot be determined theoretically. ○ Reaction order is always defined in terms of reactant (not product) concentrations. ○ The order of a reactant is not relation to the stoichiometric coefficient of the reactant in the balanced chemical equation. ● One method for experimentally determining the order of a reaction is the method of initial rates. ○ The initial rate the rate for a short period of time at the beginning of the reaction this is measured by running the reaction several times, each time varying the concentration of only one reactant and measure its “initial” rate. ○ The resulting change in rate indicates the order with respect to that reactant. ○ If you are unsure about the reaction order using inspection, set up a ratio and solve it mathematically. ● When more than one reactant is present, the concentration of each is varied independently of the other. ○ Between different phases of the reaction only one of the reactants concentrations will change at a time. ● The units of the rate constant (k) vary depending on the order of the reaction. ○ 0 reaction order = M*s (or mol L¹s⁻¹) ○ 1st reaction order = 1/s (or¹) ○ 2nd reaction order = M⁻¹*s⁻¹ (or L mol¹s⁻¹) ○ 3rd reaction order = L²/mol²*s (or L²mo²s⁻¹) ● The integrated rate law is a relationship between the concentrations of reactants and time. ○ For a first order reaction, a plot of the natural log of the reactant concentration as a function of time yields a straight line. Mart 3 ○ For a second order reaction, a plot of the inverse of the reactant concentration as a function of time yields a straight line. ■ When a second order reaction is plotted using firstorder parameters, a straight line is not observed. ○ For a zero order reaction, a plot of the reactant concentration as a function of time yields a straight line. ● The halflif (t.) of a reaction is the time required for the concentration of a reactant to fall to onehalf of its initial value. ○ The halflife expression defines the dependence of halflife on the rate constant and the initial concentration. ○ Even though the concentration is changing as the reaction proceeds, the halflife is constant. ● The rates of chemical reaction are, in general, highly sensitive to temperature. ○ The temperature dependence of the reaction rate is contained in the rate “constant”. ● The activation energy is the energy necessary to initiate a reaction. ○ Not all chemical molecules in a container have the same kinetic energy. ● The frequency factor is the how many times the reactants approach the activation barrier per unit time. ○ The frequency factor is the number of times the two molecules collide in the correct orientation. ○ The frequency factor and activation energy are determined experimentally using an Arrhenius Plot. ● The Activation Energy can also be determined if you know the rate constant at two different temperatures. ● Not all reactions occur in a single step. ○ The second and third reactions are much faster than the first. ● The rate laws for chemical reactions can be explained by the following general rules. ○ The rate of any step in a reaction is directly proportional to the concentrations of the reagents consumed in that step. ○ The overall rate law for a reaction is determined by the sequence of steps, or the mechanism , by which the reactants are converted into products. ○ The overall rate law for a reaction is dominated by the rate law for the slowest step in the reaction. ● The rate limiting step in a reaction mechanism limits the overall rate of the reaction. ○ Although the rate law for an overall chemical reaction cannot be determined from the balanced equation, the rate law for an elementary step can be. ● For a reaction mechanism to be valid, the following conditions must be met: 1. The elementary steps in the mechanism must sum to the overall reaction. Mart 4 2. The rate law predicted by the mechanism must be consistent with the experimentally determined rate law. ● A catalyst increases the rate of a reaction by providing an alternative mechanism that has a smaller activation energy. ○ Catalysts lower the energy of activation, lowering the activation energy increases the rate constant, k, and thereby increases the rate of the reaction. ■ A catalyst increases the activation energy by providing a different mechanism for the reaction through a new, lower energy pathway. ○ A catalyst increases the rate of the forward and the reverse reactions. ○ A catalyst is not consumed by the reaction, typically, small amounts of catalysts affect the rate of reaction for a large amount of products. ● Catalytic reactions also occur in the atmosphere. ○ Significant ozone depletion has been observed around the globe, especially in the Antarctic. ● Catalytic converts also involve catalysis. ○ Homogeneous catalysts exist in the same phase as the reactants, while heterogeneous catalysts exist in a different phase than the reactants.
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