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Kinetics Part II: Rate Laws & Order of Reaction. Jespersen Chap. 14 Sec 3. Dr. C. Yau Spring 2014. 1. Rate of Rxn vs . Rate Law. Rate of reaction is based on one component (reactant or product) of the reaction: disappearance of a reactant or formation of a product.

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Kinetics part ii rate laws order of reaction

Kinetics Part II:Rate Laws & Order of Reaction

Jespersen Chap. 14 Sec 3

Dr. C. Yau

Spring 2014


Rate of rxn vs rate law
Rate of Rxn vs. Rate Law

Rate of reaction is based on one component (reactant or product) of the reaction: disappearance of a reactant or formation of a product.

Rate law is a rate expression that includes all reactants.



The rate law depends on the concentrations used
The Rate Law Depends On The Concentrations Used

Rate= k [reactant]order

  • k is a reaction rate constant, a measure of time efficiency(not to be confused with “Rate”).

  • High values of k mean high efficiency.(Reaction goes fast.)

  • k must be determined experimentally.

  • Each experiment has its own rate law.

  • Rate law must be determined experimentally.

A b products
A + B products

Rate = k [A]m[B]n

where m and n are the "orders of reaction" and are found by experiment, NOT based on the coefficients of the chemical equation,

and k is the "rate constant."

This expression is called the "rate law."

H 2 seo 3 6i 4h se 2i 3 3h 2 o
H2SeO3 + 6I- +4H+ Se + 2I3- + 3H2O

Rate = 5.0x105 L5mol-5 s-1 [H2SeO3][I-]3[H+]2

5.0x105 mol-5 s-1 is the rate constant (k).

We speak of the reaction as being…

first orderwith respect to H2SeO3,

third orderwith respect to I-(Nothing to do with 6 in eqn)

second orderwith respect to H+, and

the overall order of reaction is 6 (sum of all the orders).

Learn this terminology!

What is the unit of Rate in the equation shown above?

Do Practice Exercises 7, 8 & 9 on p.648.

What is a rate law used for?

Rate changes with concentrations. The rate law allows us to determine the rate for various concentrations of the reactants.


The rate law for the reaction 2A +B→3C is

Rate = 0.045M-1s-1 [A][B]

If the concentration of A is 0.2M and that of B is 0.3M, what will be the reaction rate?

rate = 0.045 M-1 s-1 [0.2M][0.3M]

rate = 0.0027 M/s

Do Practice Exercises 5 & 6 p.646

Chlorine dioxide, ClO2, is a reddish-yellow gas that is soluble in water. In basic solution it gives ClO3- and ClO2- ions.

ClO2(aq) + OH(aq) ClO3(aq) + ClO2(aq) + H2O (l)

The rate law is Rate=k[ClO2]2[OH-]. What is the value of the rate constant given that when [ClO2]=0.060M, [OH-] = 0.030M, the reaction rate is 0.0248 M/s

  • 0.02 M-1 /s

  • 0.02 M/s

  • 0.02 s-

  • None of these

2.3x102 M-2 s-1


  • are indicated for each reactant,

  • the overallreaction order is the sum of individual reactant orders,

  • may be zero, negative, fractional or integers, but in this course we will usually encounter positive integers, and

  • must be determined from experimental data.

Determining the rate law
Determining The Rate Law:

  • Run reaction under the same conditions, varying only the concentrations of reactants (not the temperature).

  • A ratio of rate laws for each experiment allows us to determine the order of each reactant.

  • The rate law is unique to temperature and concentration conditions. Therefore, when a rate law is stated, it must include the temperature at which it is determined.

Use rate laws to determine orders
Use Rate Laws To Determine Orders :

2NO(g) + O2(g)→ 2NO2(g)

Select 2 rate laws that vary in concentration for only one of the substances (NO).

Hint: Write the fractions with the larger R on top.

Use rate laws to determine orders1
Use Rate Laws To Determine Orders :

2NO(g) + O2(g)→ 2NO2(g)

Next choose 2 rate laws where the concentration for the other component

(O2) changes.

x=2, y = 1


Rate = k [NO]2[O2]

Determining the value of k
Determining The Value Of k

Finally we can solve for k. Use any rate law and the orders that we have determined.

rate = k[NO]2[O2]

0.048M/s =k [0.015M]2[0.015M]

1.4×104 M-2s-1 =k

Do Example 14.5, 14.6 p.651, Exercises 10 thru 14p.651+.

Determine the rate law from given data
Determine The Rate Law From Given Data





Note that changing the concentration of C had no effect on the rate. We say it is “zero order with respect to C.”

Effect of order of rxn on rate
Effect of Order of Rxn on Rate

Consider Rate = k[A]n

If n = 0, change in conc has no effect on rate.

If n = 1, Rate = k[A]1 and when conc is 2x,

rate is 2x.

If n = 2, Rate = k[A]2 and when conc is 2x,

rate is 4x

If n = 2, when conc is tripled, rate is …?

rate is 9x

If n = 3, and conc is doubled, rate is…?

rate is 8x

Visual determination of reaction order
Visual Determination of Reaction Order

  • Once you understand how you can predict effect of a change in concentration on rates (as in the previous slide), you can often determine the rxn order visually without doing complicated calculations.

  • HOWEVER, that is only if the conc were neatly doubled or tripled, etc.

    (See next 2 slides.)

p. 648

What is the rate law?

Rate = k[A]?[B]?

Rate = k[A]1[B]2

For the following data, determine the order of NO2 in the reaction at 25°C

2 NO2(g) + F2(g)→ 2 NO2F(g):

  • 0

  • 1

  • 2

  • 3

  • not enough information given

When visual determination fails
When Visual Determination Fails...

We cannot always determine the rxn order visually.

For example, if we ended with


How do we determine what x is?

In high-level chemistry courses, x might even be a fraction!