Chapter 13 Worksheet 3 (ws13.3)

Rate Laws – The Effect of Concentration of Reactants on Reaction Rate

Rate laws (or rate equations)

A rate law describes the dependence of the (forward) rate on the concentrations of reactants.

  1. For an elementary reaction(and ONLY for an elementary reaction), the rate law can be determined by simply looking at the balanced equation and using common sense. Below is a list of every possible elementary reaction. Write the rate law for each reaction.

UnimolecularA = productsRate =

BimolecularA + B = productsRate =

Bimolecular2A = productsRate =

TermolecularA + B + C = productsRate =

Termolecular2A + B = productsRate =

Termolecular3A = product Rate =

  1. For a reaction with two reactants (A and B), the rate law is:

Rate = k[A]x[B]y

k is the rate constant

x is the order of the reaction with respect to A

y is the order of the reaction with respect to B

x + y is the overall order of the reaction.

Most reactions are zeroth, first, or second order with respect to a given reactant.

Complete the table for the elementary reactions above:

Elementary Reaction / Order with respect to . . . / Overall Order
A / B / C
A = products / ------ / ------
A + B = products / ------
2A = products / ------ / ------
A + B + C = products
2A + B = products / ------
3A = product / ------ / ------
  1. What information is provided by the overall order of an elementary reaction?

  1. List the 4 factors that affect the rate of a homogenous reaction and the one additional factor that affects the rate of a heterogeneous reaction that contains a solid.
  1. Which of the factors above affect the magnitude of the rate constant for a reaction (k)?
  1. The unit for a rate constant depends on the overall order of a reaction. Complete the following table assuming that the unit for rate is M-s-1.

Overall order / Unit for k
Zeroth
First
Second
Third
  1. For a reaction with one reactant, the rate equation is: Rate = k[A]x. Sketch graphs of

rate vs. [A] for zeroth, first, and second order reactions.

  1. Recall that the rate of a reaction is identical to the rate of the slowest elementary step in the mechanism. For any reaction, there are many possible mechanisms. The actual mechanism must be determined experimentally.

Below are two proposed mechanisms for the destruction of ozone.

O3(g) = O2(g) + O(g) SLOWO3(g) + O3(g) = O2(g) + O4(g)SLOW

O(g) + O3(g) = O2(g) + O2(g) FASTO4(g)= O2(g) + O2(g)FAST

Determine the net reaction for both mechanisms above.

Identify the intermediate(s) in mechanism 1. ______

Identify the intermediate(s) in mechanism 2. ______

What is the molecularity of the rate limiting step in the above mechanisms?

Mechanism 1 ______

Mechanism 2 ______

Write the rate law for each of the net reactions based on the given mechanisms.

Mechanism 1 ______

Mechanism 2 ______

The experimentally determined rate law for the destruction of ozone is as follows : Rate = k [O3].

Which mechanism is consistent with the experimental rate law? ______

  1. True or false (Explain your answer. THIS IDEA IS VERY IMPORTANT!!)

For the generalized reaction, aA + bB = products, the rate equation is: Rate = k[A]a[B]b.

  1. Determine the rate law for the net reaction based on the given mechanism.

H2(g) + ICl(g) → HClI(g) + H(g) (Slow)

H(g) + ICl(g) → HCl(g) + I(g) (Fast)

HClI(g) → HCl(g) + I(g) (Fast)

I(g) + I(g) → I2(g) (Fast)

Net:

  1. The experimentally determined rate law for the reaction, 2NO2 + F2 = 2NO2F, is:

rate = k[NO2][F2]

Write a two step mechanism that is consistent with this rate law. (Make the first step rate-limiting.)

  1. How is it possible for a reaction to be zeroth order with respect to a reactant?

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