Chm 104 1st Edition Exam 1 Study Guide January 26 Lecture Kinetics The SPEED of a reaction in chemistry reactions common units are M s or molarity per second Questions to Think About How fast does the products reactants react How fast are they consumed made How do we start a reaction What is the route mechanism Rate Rate Change in something change in time Consider the Following Reaction A B C The rate at which A is being consumed is shown as Rate A time Take notice There is a negative sign because the change of concentration of a reactant is always DECREASING But the RATE will always be positive Now an actual reaction 2Ce 4 C2C4 2 2Ce 3 2CO2 In this example we must use what we just learned with some stoichiometry Rate 1 2 Ce 4 time OR Rate 1 2 Ce 3 time The rate of the reactant Ce 4 is negative AND halved because its concentration is decreasing and twice the rate of C2C4 For every 2 moles of Ce 4 there is one mole of C2C4 Since all rates are equal the only way to make Ce 4 s rate equal the rate of C2C4 is to multiply it by SOME CONCEPTS These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The RATE of a reactant is the same as the RATE of the product In A B C A t B t In the reaction 2A B2 2AB reactant A is consumed twice as fast as B so in order to balance it out and make the rates equal the change of A s concentration must be halved The RATE of a reaction IS ALWAYS POSITIVE only the change in concentrations can vary Average Rate of Change The rate something time of a reaction or of a reactant product over a time interval In the reaction X Y over a 40 second interval the Rate equation looks like Average Rate X 40 seconds x start 40 seconds Average rate is usually not the best option because reactions happen very quickly Instantaneous Rate of Change Rate of a reaction reactant product at a SPECIFIC moment in time not a time interval The Rate Law Effect of Concentration on Rate Rate k A n Reaction Order n o Reaction is is in zero order meaning the rate is independent of A n 1 means the reaction is in 1st order the rate is directly proportional to A n 2 means the reaction is in 2nd order rate is proportional to the square of A 2 Zero Order Reaction Rate k A 0 k First Order Reaction Rate k A 1 Second Order Reaction Rate k A 2 Determining the Order of the Reaction ONLY can be done by experiment A Initial Rate M s 10 M 0 015 20 M 030 40 M 060 With a data table and knowing the relationships between order concentration and rate For example if the concentration doubles and the rate also doubles this is a direct relationship Therefore we know that this is a first order reaction In the table above the concentration doubles form 10 M to 20 M At the same time the rate is doubled from 015 M s to 0 030 M s Since they both double this represents a first order reaction In other words the n is 1 If the relationship is not this simple this equation is used Rate 2 Rate 1 k A n2 k A n1 A Rate M s 1 M 015 2 M 060 4 M 240 With this data this new equation is needed 240 M s 06 M s k 4 n k 2 n 4 0 4 20 n 4 0 2n Log 4 nlog 2 N 2 Order for Many Reactants For the Equation aA bB cC dD Rate k A m B n Overall Order m n Integrated Rate Laws Relationship between concentrations of reactants and time For the equation A products First Order 1 Rate k A K A A t Rate A t 2 Natural Log of both sides and rearrange into y mx b form Ln A t kt ln A 0 Half Life The time it takes for the initial concentration to be halved First Order 1 The integrated rate law is Ln A t kt ln A 0 2 Replace A t for A 0 3 Solve for t T1 2 ln2 k DO THIS SAME PROCESS FOR THE 0 AND 2ND ORDER REACTIONS for the integrated rate law equation and half life equations Review Order and rate law are determined experimentally Rate Law rate and concentration of reactants included Integrated Rate Law Concentration of reactants in respect to time T1 2 is the time for reactant to be half of initial 1st order t1 2 is independent of initial concentration Half lives of 0 order and 2nd order depend on initial concentration February 4th Lecture The Effect of Temperature on Reaction Rate Temperature dependence in Rate k A n is contained in the rate constant Arrhenius Equation K A e Ea RT K Rate constant A Frequency Factor Ea Activation Energy R Gas constant 8 314 J mol K Activation Energy Energy barrier hump that must be reached for reactants to turn into products K A e Ea RT Natural log of both sides and arrange into an y mx b equation Arrhenius Plot Equation Lnk Ea R 1 T ln A Slope Ea R Y intercept lnA February 6 Lecture Mechanism pathways for the reactants to turn into products If a reaction is done in ONE STEP the coefficients in front of each part is the order in the rate law Most reactions are not done in one step because the chance of everything meeting at the same place and time is very improbable Reaction Br2 2NO 2BrNO Steps Br2 NO Br2NO Br2NO 2BrNO Add these two steps together to get the overall reaction Intermediate Br2NO The component involved in the reaction but has no part of the final product Elementary Steps Different steps that add up to the overall Elementary Step Rate Law Molecularity A x Rate k A Unimolecular Step A A Q A B X Rate k A 2 Rate k A B Bimolecular X 2y z Rate k x y 2 Termolecular Slowest Elementary Step Rate Determining Step RATE LAW RATE DETERMINING STEP RATE LAW FOR ENTIRE REACTION Catalyst Make reaction go faster Gives causes a new mechanism ABCD Q P with a lower activation energy Equilibrium Most equations are reversible aA bB cC dD Equilibrium exists when rate forward rate reverse Kc product coefficeient reactant coefficient Kc C c D d A a B b If K is larger than 1 equilibrium LIES to the right because there are more products than reactants If K is smaller than 1 the equilibrium lies to the left favors reactants Kforward A B kreverse C D When k is larager the activation energy is lower inversely related Kforward 1 Kreverse Reaction Quotient Q determines if reaction …