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# Learning Goal: To understand reaction order and rate constants. For the general equation aA+bB?cC+dD, the rate...

Learning Goal: To understand reaction order and rate constants. For the general equation aA+bB?cC+dD, the rate law is expressed as follows: rate=k[A]m[B]n where m and n indicate the order of the reaction with respect to each reactant and must be determined experimentally and k is the rate constant, which is specific to each reaction. Order For a particular reaction, aA+bB+cC?dD, the rate law was experimentally determined to be rate=k[A]0[B]1[C]2=k[B][C]2 This equation is zero order with respect to A. Therefore, changing the concentration of A has no effect on the rate because [A]0 will always equal 1. This equation is first order with respect to B. This means that if the concentration of B is doubled, the rate will double. If [B] is reduced by half, the rate will be halved. If [B] is tripled, the rate will triple, and so on. This equation is second order with respect to C. This means that if the concentration of C is doubled, the rate will quadruple. If [C] is tripled, the rate will increase by a factor of 9, and so on. . Overall reaction order and rate-constant units The sum of the individual orders gives the overall reaction order. The example equation above is third order overall because 0+1+2=3. For the units of rate to come out to be M/s, the units of the rate constant for third-order reactions must be M?2?s?1 since M/s=(M?2?s?1)(M3) For a second-order reaction, the rate constant has units of M?1?s?1 because M/s=(M?1?s?1)(M2). In a first-order reaction, the rate constant has the units s?1 because M/s=(s?1)(M1). Analyzing a specific reaction Consider the following reaction: NO+O3?NO2+O2,rate=k[NO][O3] Part A What is the overall reaction order? Express your answer as an integer. SubmitHintsMy AnswersGive UpReview Part Part B What are the units of the rate constant k for this reaction? What are the units of the rate constant for this reaction? M?s?1 s?1 M?2?s?1 M?1?s?1 M?3?s?1 SubmitHintsMy AnswersGive UpReview Part Part C What would happen to the rate if [NO] were doubled? The rate would What would happen to the rate if were doubled? stay the same. double. triple. quadruple. SubmitHintsMy AnswersGive UpReview Part Part D What would happen to the rate if [O3] were doubled? The rate would What would happen to the rate if were doubled? stay the same. quadruple. triple. double

given reaction:

NO+O3 ---->NO2 +O2

and ratek[NO][O3]

part A) For any given rxn:

aA+bB ----->cC+dD

ratek[A]^m [B]^n ,where m,n are called or taken as the order of rxn with respect to A and B respectively

And overall order is the sum total of all the orders

ovrall orderm+n

For the given rxn, order with respect to NO is 1 and order with respect to O3 is also 1.

overall order1+12

part B) As the overall order is 2 , the given reaction is second order rxn.Units of rate constant is M^-1s^-1

part C) If [NO] is doubled

new ratek(2*[NO])[O3]2*{k[NO][O3]}2*old rate

Thus, the rate is also doubled

part D) If [O3] is doubled

new ratek[NO](2*[O3])2*{k[NO][O3]}2*old rate

Thus, the rate is also doubled

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