5.9 Steady-State Approximation

Multiple Choice

$$Br_2(g)+2\ NO(g)\ →\ 2\ NOBr(g)$$

The reaction represented by the equation above has the following proposed mechanism.

Step 1: $$NO(g)+Br_2(g)\ ⇌\ NOBr_2(g)$$ fast equilibrium)

Step 2: $$NOBr_2(g)+NO(g)\ →\ 2\ NOBr(g)$$ (slow)

Based on the information, which of the following is the initial rate law for the reaction?

• Rate=k[Br2][NO]

• Rate=k[Br2][NO]2

• Rate=k[Br2]2[NO]

• Rate=k[NOBr2][NO]

Multiple Choice

2 NO(g) + O₂(g)  → 2 NO₂(g)

Consider the following mechanism for the reaction represented above.

Step 1: 2 NO   ⇄  N₂O₂              (fast reversible)

Step 2: N₂O₂ + O₂   →   2 NO₂         (slow)

Which of the following statements is true?

• Step 1 represents a unimolecular reaction.

• Increasing the concentration of NO will decrease the overall rate of the reaction.

• Raising the temperature will have no effect on the numerical value of the rate constant.

• The rate law that is consistent with the mechanism is rate = k[NO]²[O₂].

Multiple Choice

Step 1: $$NO_2(g)+F_2(g)⇄NO_2F_2(g)$$ (fast)

Step 2: $$NO_2F_2(g)\ →\ NO_2F(g)+F(g)$$ (slow)

Step 3: $$F(g)+NO_2(g)→NO_2F(g)$$ (fast)

A proposed mechanism for the chemical reaction $$2NO_2(g)+F_2(g)→2NO_2F(g)$$ is shown above. Which of the following rate laws is consistent with this mechanism?

• $$Rate=k[NO_2][F_2]$$

• $$Rate=k[NO_2]^2[F_2]$$

• $$Rate=k[NO_2][F_2]^2$$

• $$Rate=k[NO_2]^2[F_2]^2$$

Multiple Choice

Step 1. N₂H₂O₂ ⇄ N₂HO₂^– + H⁺       (fast equilibrium)

Step 2. N₂HO₂^– → N₂O + OH^–        (slow)

Step 3. H⁺ + OH^– → H₂O                (fast)

Nitramide, N₂H₂O₂, decomposes slowly in aqueous solution. This decomposition is believed to occur according to the reaction mechanism above. The rate law for the decomposition of nitramide that is consistent with this mechanism is given by which of the following?

• Rate = k[N₂H₂O₂]

• Rate = k[N₂H₂O₂] [H⁺]

• $$Rate=k\frac{[N_2H_2O_2]}{[H^+]}$$

• $$Rate=k\frac{[N_2H_2O_2]}{[N_2HO_2]}$$

• Rate = k[N₂H₂O₂] [OH^-]