Chapter 13 Worksheet 2 (ws13



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.

Unimolecular A = products Rate = k[A]

Bimolecular A + B = products Rate = k[A][B]

Bimolecular 2A = products Rate = k[A]2

Termolecular A + B + C = products Rate = k[A][B][C]

Termolecular 2A + B = products Rate = k[A]2[B]

Termolecular 3A = product Rate = k[A]3

2. 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 |First |--------------------- |--------------------- |First |

|A + B = products |First |First |--------------------- |Second |

|2A = products |Second |--------------------- |--------------------- |Second |

|A + B + C = products |First |First |First |Third |

|2A + B = products |Second |First |--------------------- |Third |

|3A = product |Third |--------------------- |--------------------- |Third |

3. What information is provided by the overall order of an elementary reaction?

The overall order tells you the molecularity. The overall order for a reaction that occurs in multiple steps tells you the number of molecules involved in the rate-determining step and all of the steps preceding the rate-determining step. (You’ll see this later.)

4. 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. The properties of reactants and products – in particular, their molecular structure and bond strengths

2. The concentrations of reactants

3. The temperature

4. The presence of a catalyst and its concentration (more catalyst = faster reaction)

The rate of a heterogeneous reaction with a solid phase is affected by the surface area of the solid phase. The rate increases as the surface area increases.

5. Which of the factors above affect the magnitude of the rate constant for a reaction (k)?

All except number 2. Notice that the effect of concentration is shown explicitly in the rate law. The other effects are hiding in the rate constant.

6. 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 |M-s-1 |

|First |s-1 |

|Second |M-1-s-1 |

|Third |M-2-s-1 |

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7. 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) SLOW O3(g) + O3(g) = O2(g) + O4(g) SLOW

O(g) + O3(g) = O2(g) + O2(g) FAST O4(g) = O2(g) + O2(g) FAST

2O3(g) = 3O2(g) 2O3(g) = 3O2(g)

Determine the net reaction for both mechanisms above.

Identify the intermediate(s) in mechanism 1. ____ O(g)____

Identify the intermediate(s) in mechanism 2. ____ O4(g)____

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

Mechanism 1 ________unimolecular____________

Mechanism 2 _________bimolecular___________

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

Mechanism 1 _________ rate = k[O3] ___________

Mechanism 2 _________ rate = k[O3]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? ___mechanism 1____

Note: A mechanism must be consistent with the experimentally determined rate law. However, this does not prove that a mechanism is correct.

8. 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.

FALSE! The rate law for a reaction is the same as the rate law for the rate determining (slowest) step in the mechanism. The reaction in problem 6 is a good example. If the statement above was true, then the reaction would be second order with respect to ozone. In reality it is first order because the slowest step in the mechanism involves only one molecule of ozone rather than two molecules. RATE CONSTANTS AND REACTION ORDERS MUST BE DETERMINED EXPERIMENTALLY! COEFFICIENTS IN BALANCED EQUATIONS CORRESPOND TO REACTION ORDERS ONLY IF THE REATION IS AN ELEMENTARY REACTION!!

9. 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: H2(g) + 2 ICl(g) → 2 HCl (g) + I2(g)

rate = k[H2][ICl]

Notice that the reaction is first order with respect to ICl (not second order) because the slow step involves only one molecule of ICl.

For all of the mechanisms in this worksheet, the first step is the rate determining step. This certainly is not always the case. Read about the hydrogen iodide reaction on pages 590-591 in your textbook to learn how to handle mechanisms with a fast initial step.

10. 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.)

NO2(g) + F2(g) → NO2F(g) + F(g) (slow)

NO2(g) + F(g) → NO2F(g) (fast)

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

A reaction can be zeroth order with respect to a reactant if that reactant is not involved in the rate determining step.

Note: If the first step is not the slowest step, then, to be zeroth order, a reactant must also not be involved in any of the steps that precede the rate determining step.

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