University College - Chemistry Chem 112A Lecture Notes - Lecture 35: Rate Equation, Reaction Rate Constant

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limehorse889

11 Sep 2017

School

WUSTL

Department

University College - Chemistry

Course

University College - Chemistry Chem 112A

Professor

D' Arcy

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Document Summary

Study the speed of reaction (reaction rates), the factors that influence them, and the mechanisms of reactions aa + bb -> cc + dd. Each time reaction occurs a of a and b of b are consumed, and c of c and d of d are produced. Rate of change of a over time = -d[a]/dt. Units for the rate of reaction: mol rxn/l/s. In general: rate of reaction = (cid:2869) (cid:3031)[(cid:3045)(cid:3032)(cid:3030)(cid:3047)(cid:3041)(cid:3047)(cid:3046)] (cid:3031)(cid:3047) Differential rate law how the rate of reaction is changing with respect to reactant conc. Rate = k[a]n[b]m n, m orders of reaction with respect to individual species. Only equal to the coefficients for elementary (single step) reactions. Units are dependent on the orders of reaction. []0 (cid:3031)[](cid:3031)(cid:3047) (cid:3047)(cid:2868) ln[a] ln[a]0 = -kt ln[a] = ln[a]0 kt. This is the integrated first order rate law. Plot ln[a] vs t will provide a linear plot for 1st order reactions with slope -k.

Related Questions

The rate of a chemical reaction depends on the concentrations ofthe reactants. For the general reaction between Aand B,

aA+bB?cC+dD

The dependence of the reaction rate on the concentration of eachreactant is given by the equation called the rate law:

rate=k[A]m[B]n

where k is a proportionality constant called the rateconstant. The exponent m determines the reaction orderwith respect to A, and n determines the reaction orderwith respect to B. The overall reaction order equals the sum of theexponents (m+n).

For the reaction A+B+C?D+E, the initial reaction rate wasmeasured for various initial concentrations of reactants. Thefollowing data were collected:

Trial	[A] (M)	[B] (M)	[C] (M)	Initial rate (M/s)
1	0.40	0.40	0.40	1.2

For the reaction A+B+CâD+E , the initial reaction rate was measured for various initial concentrations of reactants. The following data were collected:

Trial	[A] (M )	[B] (M )	[C] (M )	Initial rate (M/s )
1	0.20	0.20	0.20	6.0Ã10^â5
2	0.20	0.20	0.60	1.8Ã10^â4
3	0.40	0.20	0.20	2.4Ã10^â4
4	0.40	0.40	0.20	2.4Ã10^â4

The rate of a chemical reaction depends on the concentrations of the reactants. For the general reaction between A and B ,

aA+bBâcC+dD

The dependence of the reaction rate on the concentration of each reactant is given by the equation called the rate law:

rate=k[A]^m[B]^n

where k is a proportionality constant called the rate constant. The exponent m determines the reaction order with respect to A , and n determines the reaction order with respect to B . The overall reaction order equals the sum of the exponents (m+n ).

Part A

What is the reaction order with respect to A ?

Express your answer as an integer.

Part B

What is the reaction order with respect to B ?

Express your answer as an integer.

Part C

What is the reaction order with respect to C ?

Express your answer as an integer.

Learning Goal:

To understand reaction order and rate constants.

For the general equation aA+bBâcC+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+cCâdD, 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.(Figure 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.(Figure 2)

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 Mâ2â sâ1 since

M/s=(Mâ2â sâ1)(M3)

For a second-order reaction, the rate constant has units of Mâ1â sâ1 because M/s=(Mâ1â sâ1)(M2). In a first-order reaction, the rate constant has the units sâ1because M/s=(sâ1)(M1).

Analyzing a specific reaction

Consider the following reaction:

2H2+2NOâN2+2H2O,rate=k[H2][NO]2

Part A

What is the overall reaction order?

Express your answer as an integer.

What are the units of the rate constant k for this reaction?

choose one

What are the units of the rate constant for this reaction?

Mâ sâ1

sâ1

Mâ1â sâ1