GENERAL CHEMSTRY II
GENERAL CHEMSTRY II CHEM 178
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This 4 page Class Notes was uploaded by Columbus Torphy on Sunday September 27, 2015. The Class Notes belongs to CHEM 178 at Iowa State University taught by Staff in Fall. Since its upload, it has received 10 views. For similar materials see /class/214546/chem-178-iowa-state-university in Chemistry at Iowa State University.
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Date Created: 09/27/15
AVERAGE RATES 1 Writing average rates in terms of changing concentration 0 The average rate of a reaction is the change of concentration of a reactant during a given change in time 0 You may be asked to write out the average rate in terms of the change in concentration of a reactant or product For example 2N0 g 2H2 g gt N2 g 2H20 g l ANO 1 AH2 rate 7 rate 77 2 At 2 At These two are the rates of disappearance of the reactants hence the negative signs We put the 12 in front because of the stoichiometric coef cients of 2 AN2 1 AHZO At At These are the rates of appearance of products so they are positive Notice that the coef cients match the stoichiometric coef cients in the balanced equation rate rate 2 Comparing rates of appearance and disappearance All four of the above expressions are equal so a potential test question might ask you to nd the rate of appearance of something given the rate of disappearance of something else For example look in the textbook at problem 1419 We are given that A H A O 085 Ms negative because it is disappearing We want to nd the rate of disappearance of oxygen We can write the average rates like we did above me MHz 2 A02 2 At At Then we plug in what we know to solve A O M l085Ms 043 Ms At 2 So the rate of disappearance of oxygen is 043 Ms What we did 0 List known information 0 Identify that the problem is asking about rates of appearance andor disappearance of speci c reactants or products Identify what you re looking for Write expressions using deltas for relevant reactants and products Set the expressions for rate equal to each other and plug in numbers to solve for desired quantity 3 Calculating average rate from tabulated concentration data You ll be given a reaction and a table of values 2H2 02 9 2HzO Time s Hle 0 10 10 9 20 7 30 6 40 6 50 5 Let s say you want to nd the average rate of the reaction between 20s and 30s First write the expression for average rate in terms of concentration of Hz Then plug in numbers from the table 1AH2 1Hf Hi l6M 7M 2 At 2 t 4 2 30s 20s rate 005 Ms RATE LAWS to practice exercises like these do problems 1431 in the book or the SI worksheet from 828 1 Finding rate law and value of rate constant k from initial concentrations 0 What the problem looks like You ll have a table that gives values for initial concentrations of two reactants as well as the initial rate for several different experiments 0 First pick two rows of data where the concentrations of one of the reactants are equal Then plug it into the equation ratel A Elly rate A B 0 Now pick two rows of data where the concentrations are equal for the other reactant and solve for the other unknown Then plug them into the rate law rate kAxBy 0 To solve for k pick any row of data they all should give you the same answer Then plug them into the equation being careful that you get the correct units You should get cancellation and you ll be able to solve for x or y rate All Bly 2 Determining the order of the reaction 0 To determine the overall order of the reaction sum the exponents in the rate law 0 To determine the order of the reaction with respect to one reactant simply nd the exponent related to the concentration of that reactant This is the order of the rate with respect to that reactant INTEGRATED RATE LAWS 1 Determining graphically whether a reaction is lst or 2quotd order and nding k 0 You will either have a list of time and concentration data or else a graph First try to graph lnAt vs t If it is a straight line with a negative slope the reaction is first order with respect to A and the rate constant k is the negative of the slope 0 Try graphing 1 A vs t If it is a straight line with a positive slope the reaction is second order with respect to A and the value of k is the slope 2 Determining concentrations at certain times for first and second order reactions 0 If you know that a reaction is lst or 2quotd order and are given 3 of the 4 following pieces of information initial concentration concentration at time t time rate constant you can find the fourth by using the integrated rate equations o 151 order 1nAt kt lnA0 o 2 d order 1At kt 1A0 0 Plug the values you do know into the equation to solve for the unknown 3 Determining the half life of a first order reaction 0 The half life is the time it takes for the concentration to decrease to half of the original concentration For a rst order reaction t1 2 0693k If you know k and you want to find the time it takes for a reaction to proceed to a certain point figure out how many half lives it would take divide the original concentration by 2 until you obtain the final concentration The number of times you had to divide is the number of half lives ARRHENIUS E zUATION k1 Ea 1 1 ln k2 R T2 TI This is the most useful form of the Arrhenius equation Problems with the Arrhenius equation usually involve plugging in known numbers to solve for one unknown Here are some things to remember when doing this 0 Temperature must ALWAYS be in Kelvin for this equation 0 R 8314 JmolK and because ofwith this Ea must be in Jmol 0 To solve for something within the natural log remember that elquotx X 0 For practice with these problems see 1445 1458 in the text Graphing with the Arrhenius equation 0 If you see a graph with UT on the XaXis and lnk on the yaXis the slope ofthe graph is equal to iEa R To find the activation energy first find the slope and then multiply by 7R If you see a graph with energy versus reaction progress or time the activation energy will be the difference in energy between the reactants on the left and the maximum energy the reaction reaches REACTION MECHANISMS Elementary reaction 7 a single step for a multistep reaction that illustrates how the molecules actually collide Reaction mechanism 7 process by which a reaction occurs the elementary steps that make the overall reaction happen Molecularity 7 corresponds to the overall reaction order 1 Identifying catalysts a Catalysts are a reactant in one of the elementary step that is recreated as a product in a later step b it will be crossed out in the overall reaction 2 Identifying intermediates a Intermediates are formed as a product in an early step of a mechanism and consumed in a later step b it will be crossed out in the overall reaction 3 Determining the overall reaction from a series of elementary steps a Cross out any molecule that appears as a product in one reaction in the mechanism and as a reactant in another step Find the overall reaction by summing what is left over b Make sure that the states solid liquid gas and stoichiometric constants match 4 Determining the rate law from a given mechanism a Identify which step is the slow step b Write the rate law based on the stoichiometric constants of the reactants for that step If your rate law contains molecules that are not reactants in the overall reaction ie intermediates replace the concentrations of these species in your rate law with the concentrations of the species from the step where that intermediate was formed 0