Pre Class Lecture Chapter 12 3 Rate Laws and Orders CHM 1046 General Chemistry II 1 Learning Outcomes Expectations 12 3 Rate Laws and Orders Differentiate between reaction rate and reaction rate constant k Write the general form of the rate law for a given reaction Describe the relationship between variables in rate law Translate reaction and overall order i e first second etc into a rate law equation Determine the overall reaction order from reactant orders Determine the experimental rate law from concentration rate relationships Regarding units and k Determine the units on k from the rate law Determine the overall reaction order from the units of k 2 3 Experimental Determination of red brown Rates Colorless Br2 aq HCOOH aq 2Br aq 2H aq CO2 g D Br2 a D Absorption Time 4 Experimental Determination of Rates Rate changes with conc rate a Br2 rate D Br2 Dt Br2 final Br2 initial tfinal tinitial We need a more general way to quantify how fast 5 Experimental Determination of Rates rate constant constant of the proportionality between the reaction rate and the concentration of reactant General descriptor for the rate of a reaction that is independent of conc and is constant at a given temperature rate a Br2 y m x b rate k Br2 0 k rate Br2 rate constant 3 50 x 10 3 s 1 6 Rate Laws Br2 aq HCOOH aq 2Br aq 2H aq CO2 g rate a Br2 rate k Br2 Can describe the rate of a complex reaction with one simple equation that is dependent on reactants using the Rate Law The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers aA bB products The rate law for a general reaction takes the form 7 Rate Laws aA bB cC dD The rate law for a general reaction takes the form The rate law is defined in terms of reactant concentrations not products x and y are the order of the reaction The power coefficients x and y are not the same as the stoichiometric coefficients a and b For rate laws x and y are determined experimentally 8 Reaction Order aA bB cC dD The rate law for a general reaction takes the form Reaction order specify the relationship between the concentrations of reactants A and B and the reaction rate Reaction is xth order in A Reaction is yth order in B Reaction is x y th order overall If a reagent is zero order it is not included in the rate law Helps to explain the mechanism of the reaction Order tells you what molecules are involved in the rate limiting step and how of each molecule are needed 9 Reaction Order aA bB cC dD Important Note the power coefficients x and y are not the same as the stoichiometric coefficients a and b F2 g 2ClO2 g 2FClO2 g rate k F2 ClO2 1 1 k F2 ClO2 The reaction is rate determined by the collision and reaction of 1 F2 and 1 ClO2 But when each reaction is complete 2 ClO2 and 1 F2 are consumed Reaction order x and y must be determined experimentally 10 Determining Rate Laws by inspection F2 g 2ClO2 g 2FClO2 g Method of Initial Rates 1 Vary concentrations 2 Measure rate 3 Tabulate the data rate k F2 x ClO2 y Comparing rxn 1 and 2 F2 constant ClO2 quadruples rate quadruples y 1 rate a ClO2 11 Determining Rate Laws by inspection F2 g 2ClO2 g 2FClO2 g Method of Initial Rates 1 Vary concentrations 2 Measure rate 3 Tabulate the data rate k F2 x ClO2 y Comparing rxn 1 and 2 F2 constant ClO2 quadruples rate quadruples y 1 rate a ClO2 Comparing rxn 1 and 3 ClO2 constant F2 doubles rate doubles rate a F2 x 1 12 Determining Rate Laws by inspection F2 g 2ClO2 g 2FClO2 g Method of Initial Rates 1 Vary concentrations 2 Measure rate 3 Tabulate the data rate k F2 1 ClO2 1 k F2 ClO2 Calculate the rate constant k Can use any experiment rate1 1 2 x 10 3 M s k 0 1 M 0 01 M k 1 2 M 1 s 1 Measure conc over time find rate law calculate rate constant 13 Determining Rate Laws by algebra rate k NO x H2 y Ratio of rxn 1 and 2 Ratio of rxn 2 and 3 2x 4 2 x 2 y 1 2y 14 Rate Constant and Order s Mxs Mx s 1 rate A M M M M s 1 M M M s must cancel until the right side equals M s x m n 1 Order Rate Law Units on k 0 1 2 3 rate k rate k A rate k A 2 rate k A B rate k A 3 rate k A 2 B rate k A B C M s 1 s 1 M 1 s 1 M 2 s 1 15 Rate Expression vs Rate Law For a general reaction aA bB cC dD Rate expression Rate law rate k A x B y From the chemical equation Determined experimentally Tells you how concentrations change relative to each other over time Describes both reactants a products Gives you the rate constant Tells you how rate changes with concentration change Rate is conc dependent Only describes reactants 16 Summary Reaction rates can change as the reactant concentration changes A rate law describe the relationship between the rate of a chemical reaction and the concentration of its reactants not products The relationship rate law is dependent on the rate constant k of the reaction k is independent of concentration and constant at a given temperature Reaction rates are also dependent on the order of the reaction reactants In the rate law the exponents on the reactants are the reaction orders Overall reaction order is the sum of the reactant orders Reaction orders must be determined experimentally Typically by the method of initial rates Can be done by inspection or algebraic proportionality The units on k lend insight into the reaction order and vice versa 18
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