14.4.2 Determining the Rate Laws of Elementary Reactions

Question 1

Determining the Rate Laws of Elementary Reactions

Answer 1

• If a reaction aA + bB -> cC + dD is an elementary step, the stoichiometric coefficients a and b are the values of the orders m and n in the rate law.
• The reaction rate of the overall reaction is determined by the rate of the slowest elementary step, called the rate-determining step.
• If a proposed reaction mechanism is consistent with the experimentally-determined rate law, the proposed mechanism is possible; if a proposed mechanism is inconsistent with the rate law, the mechanism is definitely incorrect.

Question 2

note

Answer 2

• If a reaction aA + bB cC + dD is an elementary step, the stoichiometric coefficients a and b are the values of the orders m and n in the rate law.
• For example, the rate law for the unimolecular elementary step A products is rate = k[A].
• The rate law for the bimolecular elementary
  step A + A products is rate = k[A] 2 .
• The rate law for the bimolecular elementary
  step A + B products is rate = k[A][B].
• The reaction rate of the overall reaction is determined by the rate of the slowest elementary step, called the rate-determining step.
• For example, if the slowest step at a grocery store is the checkout line, the rate of customers leaving the store will be determined by the rate of customers passing through the checkout line.
• The rate law for a multistep reaction is the rate law for the rate-determining step.
• If a proposed reaction mechanism is consistent with the experimentally-determined rate law, the proposed mechanism is possible; if a proposed mechanism is inconsistent with the rate law, the mechanism is definitely incorrect.
• For example, the experimentally-determined rate law for the reaction of ozone (O 3 ) with nitrogen dioxide (NO 2 ) is rate = k[O 3 ][NO 2 ]. Since the rate law for the proposed reaction mechanism is the same as the experimentally-determined rate law, the mechanism is consistent with reality, and it is possible that this mechanism is correct.
• However, if the second step was the slow step in the proposed mechanism, the rate law would not be the same as the experimentally-determined rate law, and the mechanism would definitely be incorrect.

Question 3

What parameter must remain the same for k to be a constant in the rate law equation?

Answer 3

Temperature

Question 4

For the reaction of
O3(g) + 2NO2(g) → N2O5(g) + O2(g)
The experimentally determined rate law is rate = k [O3 ] [ NO2 ]2
What can we definitely say about the reaction?

Answer 4

The experimentally determined rate law is consistent with the reaction taking place as a single elementary step

Question 5

How does a decrease in temperature affect a chemical reaction?

Answer 5

The reaction slows down.

Question 6

Which of the following is the general form for a rate law equation for a reaction with two reactants?

Answer 6

k[A]^m [B]^n

Question 7

In which of the following cases would the coefficients from a balanced chemical reaction be used in the rate law expression for the reaction?

Answer 7

The reaction is an elementary step

Question 8

In the formula for the rate of a chemical reaction, which expression changes with temperature?

rate = k[A]^m [B]^n

Answer 8

k

Question 9

CH3Cl3(g) + Cl2(g) → HCl(g) + CCl4(g)

Step 1: Cl2(g) ↔ 2 Cl − (g) fast
Step 2: Cl − (g) + CH3Cl3(g) → HCl(g) + CCl3− (g) slow
Step 3: CCl3− (g) + Cl − (g) → HCl(g) + CCl4(g) fast

In the above proposed reaction mechanism, which of the following is an intermediate product?

Answer 9

CCl3−

Question 10

Which of the following is the rate law expression for the following chemical reaction? This reaction occurs in one step and has no reaction intermediates.

2 NO(g) + O2(g) → 2 NO2(g)

Answer 10

k[ NO]^2 [O2 ]

Question 11

What is the slowest step in a series of chemical reactions called?

Answer 11

rate limiting step

Question 12

According to the collision theory, the rate of a reaction does not depend on what?

Answer 12

pressure