CHEM 1212: Chapter 14
CHEM 1212: Chapter 14 CHEM1212
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Brittany Ariana Borzillo
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This 8 page Class Notes was uploaded by Brittany Ariana Borzillo on Tuesday September 20, 2016. The Class Notes belongs to CHEM1212 at University of Georgia taught by Donald Wayne Suggs in Fall 2015. Since its upload, it has received 5 views. For similar materials see Freshman Chemistry II in Chemistry at University of Georgia.
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Date Created: 09/20/16
CHAPTER 14 Factors that Influence Reactivity Chemical kinetics o the study of reaction mechanisms and the rates of chemical reactions under various conditions For a reaction to occur to an appreciable amount, it must be both thermodynamically and kinetically favored o Thermodynamic controls the enthalpy change for the reaction the entropy change for the reaction the temperature at which the reaction takes place o kinetic controls the manner in which the reaction takes place (its mechanism) the energy barrier that must be overcome to convert reactants to products (the activation energy) the concentration of the species present the temperature at which the reaction takes place Collision Theory conditions o The reacting species come into contact (they collide) When more collisions between reacting species occur, the reaction will proceed faster o The collision has enough energy to overcome the activation energy, the energy barrier necessary to initiate the reaction only some fraction of reacting species will have enough energy to react at a given temperature As the temperature increases, the number of species with enough energy to react will increase the high-temperature sample will have more energetic collisions and will experience a faster reaction rate a greater fraction of reactant molecules have enough energy to overcome a small activation energy than to overcome a large activation energy o The reacting species collide in an orientation that allows the necessary bond breaking and bond forming needed to transform reactants to products to take place Average Rate vs Reaction Stoichiometry Reaction rate o the speed at which a reaction progresses, as a ratio of change in concentration over change in time o Because reactants decrease over time, the change in its concentration is negative Average Reaction Rate o the change in concentration of a reactant or product over a defined time interval Stoichiometry o relative rate at which reactants are consumed and products are formed is directly related to the reaction stoichiometry Instantaneous and Initial Rates instantaneous rate o the rate of a reaction at that point in time, is equal to the slope of a line tangent to the concentration–time curve at a given point in time Initial Rate o the rate of the reaction at the beginning of the reaction, when t=0 o start of chemical reaction change over time o concentration versus change is plotted on a graph to see the disappearance of reactant related to the creation of product Concentration and Reaction Rate rate law o shows the quantitative relationship between reaction rate and the concentration of species involved in the chemical reaction catalyst o a species that speeds up the rate of a chemical reaction but does not undergo any permanent chemical change components of rate laws o rate constant, k the proportionality constant in the rate equation value of k, which is determined from experiments, is unique to a given reaction and varies with temperature o Concentrations of reacting species (in this example A, B, and the catalyst, C) includes the concentrations of reacting species in moles per liter (for species in solution) or pressure units (for gases) Not all species that participate in a reaction will always appear in the rate law, and, as is true for catalysts, it is possible for a species that does not appear in the overall reaction to appear in the rate law o The order of the reaction with respect to the reacting species (in this example, m, n, and p) Reaction order expresses the order of the reaction with respect to each reactant, and it shows the effect a reacting species has on the rate of the reaction Reaction orders are determined from experiments; they are not taken from the stoichiometric coefficients in the balanced equation Overall reaction order Sum of the individual reaction orders Determining Rate Law using Initial Rates Method of initial rates o a reaction under investigation is repeated multiple times, and each experiment has a different set of initial concentrations for the reactants o initial rate is measured for each experiment, and the results are used to determine the order of the reaction with respect to each reacting species o the concentration of only one reacting species is changed between sets of experiments while the other concentrations do not change rate 2(concentration in 2 nd experiment) z st rate1 (concentration in 1 experiment) the entire right side is raised to the power of z z=reaction order Integrated Rate Laws derived by integration of a rate law equation Graphical Determination of Reaction Order Integrated rate laws for zero-, first-, and second-order reactions can be rewritten in the form of an equation for a straight line, y=mx+b Steps o Collect concentration–time data for a reaction at some temperature. o Plot the concentration–time data three different ways: Concentration versus time Natural log of concentration versus time 1/(concentration) versus time o If one of the three plots shows that the data fall on a straight line, the reaction is either zero, first, or second order in that reactant. If the concentration versus time plot is linear, the reaction is zero order with respect to the reactant. If the natural log of concentration versus time plot is linear, the reaction is first order with respect to the reactant. If the 1/(concentration versus time plot is linear, the reaction is second order with respect to the reactant. o The slope of the straight-line plot is related to the rate constant for the reaction Reaction Half Life the amount of time required for the concentration of a reactant to fall to one half of its initial value a first-order reaction has a half-life that is independent of the initial concentration of reactant zero- and second-order reactions, the half-life depends on both the rate constant and the initial concentration of the reactant Radioactive Decay All radioactive isotopes decay via first-order reactions Along with other applications, radioactive decay and half-life are used in radioactive dating, the determination of the age of organic objects, such as those made from plant-based materials, and inorganic objects, such as the rocks and minerals found in meteorites Reaction Coordinate Diagrams a plot that shows energy (on the y-axis) as a function of the progress of the reaction from reactants to products (on the x-axis) o the conversion of reactants to products passes through a high-energy state where an activated complex, or high-energy transition state, is formed o energy required to reach this transition state is the reaction’s activation energy, E . A Activation energy is always a positive quantity and, for the reaction where reactants are converted to products, is equal to the energy difference between the reactant energy and the energy of the activated complex Catalysts and temperature manipulation can change the activation energy activation barrier o an energy that must be overcome for reactants to proceed to form products, and changing the temperature affects the ability of reactants to overcome this barrier Arrhenius Equation frequency factor, A, is a measure of the number of collisions that take place with the correct orientation during a reaction frequency factor is a measure of the number of collisions that take place with the correct orientation during a reaction as EAincreases, the other e term decreases and the rate constant decreases; and as T increases, the e term increases and the rate constant increases Graphical Determination of Activation Energy Arrhenius equation can also be used to graphically determine the activation energy for a reaction using multiple temperature–rate constant data points (second version above) Components of Reaction Mechanism Reaction mechanism o detailed description at the molecular level of steps by which reactants are converted to products o cannot be calculated or predicted with any certainty o elementary step A simple event in which some chemical transformation occurs; one of a sequence of events that form the reaction mechanism Gives overall equation for the reaction Molecularity of elementary step tells us how many species react in the step unimolecular o involving a single reacting species bimolecular o involving the collision of two reacting species termolecular o three species collide simultaneously o does not occur very often o bond breaking step involves a single species breaking apart into two or more species o bond-forming step involves the combination of two or more species to form a single product o rate-determining step rate of a reaction can be no greater than the rate of the slowest step in the mechanism concerted process o involves more than one chemical process happening simultaneously Multistep Mechanisms A chemical reaction that occurs in a single step has only one elementary step, which is the same as the balanced equation for the chemical reaction Intermediates can be involved with multistep mechanisms o species that do not appear in the overall net reaction o species that is formed in one step in the mechanism and then consumed in a later step o A catalyst, a species that speeds up a chemical reaction, is consumed in one step and then produced, in its original form, in a later step in the mechanism o often short-lived chemical species that are very difficult to detect Reaction Mechanisms and the Rate Law the generalized rate law we wrote earlier for a chemical reaction, the reaction order (the exponents in the rate law) were unrelated to the reaction stoichiometry the rate law for any elementary step in a mechanism is directly related to the stoichiometry of the elementary step because elementary steps describe the molecular-level collisions that are occurring during the reaction rate law for each elementary step is equal to the rate constant for the elementary step multiplied by the concentration of each reactant raised to the power of its stoichiometric constant (dis)proving a mechanism o Proposing a mechanism, including identifying the rate-determining step. o Predicting the rate law using the stoichiometry of the rate-determining step. o Determining the experimental rate law for the reaction using one of the methods covered earlier in this unit. If the predicted and experimental rate laws do not match, then the proposed mechanism is incorrect. If they match, then the proposed mechanism might be correct. However, the proposed mechanism might not be correct because different mechanisms can lead to the same predicted rate law. Further experimentation is required to confirm whether the mechanism is correct or not. If both the forward and reverse reactions are fast and occur at the same rate, a state of dynamic equilibrium is formed in which both reactants and products are present in the reaction flask Advanced techniques for determining mechanisms o Detection of an intermediate using spectroscopy Detection can be done using spectroscopic methods if the intermediate absorbs light in the infrared, ultraviolet, or visible region of the electromagnetic spectrum o Trapping experiments experiments are used to chemically detect intermediates by having the intermediate react with a secondary reagent to produce an identifiable product o Isotopic labelling one of the reactants is synthesized containing a particular isotope of one of the reacting atoms location of the labeled atom can be tracked using a variety of detection methods Catalysis he technique of using a catalyst to influence the rate of a reaction. Catalysts serve one of two functions: making reactions faster or making them more “selective” the catalyst provides an alternative mechanistic pathway for the reaction, which changes the activation energy and changes the reaction rate selectivity o A measure of the tendency of a reaction to form one set of products over another o often a chemical reaction can proceed by many different pathways, often with similar activation energies, which results in the formation of many different products homogeneous o often a chemical reaction can proceed by many different pathways, often with similar activation energies, which results in the formation of many different products heterogeneous o not in the same phase as the compounds undergoing reaction
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