1st Edition CHM 104 Lecture 3 Outline of Last Lecture I Chemical Kinetics and Rate II Rate of Change of Chemical Concentration Outline of Current Lecture II Rate Law Equation III Rate Constant IV Reaction Order Current Lecture Rate Laws Effect of Concentrations on Reaction Rates A Products Exploring values of n Exploring values of n 1 2 8 2 7 n 0 6 3 0 0 100 Time s A M A M 10 Rate k A n n 0 n 1 0 0 100 Time s 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 Exploring values of n n 0 n 1 A M 1 2 0 0 100 Time s Example 3A 2B C 4D How does D change What is the rate of D change As long as the reaction proceeds forward A Products then we can say the rate Rate k A n k rate constant it s a proportionality constant unique to the reaction system units can be M s 1 s 1 Ms depending on the reaction n reactant order value of n determines how the rate depends on reactant common values for n are 0 1 2 n 0 or zeroth order rate is independent of reactant rate k A 0 A 0 1 rate k n 1 or first order rate directly proportional to reactant rate k A 1 n 2 or second order rate directly proportional to the square of reactant rate k A 2 n 0 no change in rate as A decreases slope gives you a way to directly determine rate constant Rate k unit k M s n 1 as A decreases the rate slope decreases Rate k A 1 unit k 1 s n 2 as A decreases the rate slope decreases Rate k A 2 unit k 1 Ms The more flat the curve the more the reactant influences the rate of the reaction How to determine the order of a reaction must be done experimentally 2 strategies Strategy 1 Initial Rate Method Rate is measured for a short time at the beginning of the reaction Reaction is formed several times each time using different starting concentrations of reactants Example N2O5 g 2 NO2 g O2 g The rate is directly proportional to N2O5 0 therefore n 1 we can then solve for k by inserting the info from either experiment 1 or experiment 2 into the rate law equation Rate k N2O5 10 Experiment 1 0 020 M s k 0 100M 1 k 0 20 1 s or k 0 20 1 s If n 0 N2O5 0 Initial Rate Exp 1 0 100M 0 020 M s Exp 2 0 200M 0 020 M s Rate is not dependent on reactant If n 2 N2O5 0 Initial Rate Exp 1 0 100M 0 020 M s Exp 2 0 200M 0 080 M s When reactant doubles then rate increases by a factor of 4 22 If the numbers aren t obvious then substitute any 2 initial concentrations and corresponding rates into a ratio of the rate laws and solve for n Example aA bB cC dD Rate k A m B n Example Rate k NO2 m Cl2 n Divide rate law from experiment 2 by rate law from experiment 1 to determine n For determining M Rate k NO2 1 Cl2 1 6 3 M s k 0 21 M 1 0 70M 1 k 43 1 Ms First order with respect to NO2 First order with respect to Cl2 Second order for overall reaction m n Rate law shows relationship between reaction rate and reactant Sometimes we want to know the relationship between reactant and time Predict for example how much of a reactant is left after a certain period of time