AP Chemistry - Reaction Mechanisms

Consider the following mechanism:

A + B -> R + C (slow)

A + R -> C (fast)

There are no intermediates

Explanation

R is the intermediate. It is formed in Step 1 and consumed in Step 2.

Which of the following is true?

If we know that a reaction is an elementary reaction, then we know its rate law.

The rate-determining step of a reaction is the rate of the slowest elementary step of its mechanism

Since intermediate compounds can be formed, the chemical equations for the elementary reaction in a multistep mechanism do not always have to add to give the chemical equation of the overall process.

In a reaction mechanism an intermediate is identical to an activated complex

All of the above

Explanation

All of the above describe elementary reactions and how they give an overall mechanism.

For the reaction NO2 (g) + CO (g) -> NO (g) + CO2 (g), the reaction was experimentally determined to be Rate = k\[NO2\]2. If the reaction has the following mechanism, what is the rate limiting step, and why?

Step 1: 2 NO2 -> NO3 + NO (slow)

Step 2: NO3 + CO -> NO2 + CO2 (fast)

Step 1 is limiting because the NO2 is a reactant.

Step 2 is limiting because the fast step determines how quickly the reaction can occur.

Step 1 is limiting because the reaction can not go faster than its slowest step.

Step 2 is limiting because the NO3 intermediate has to be formed before the reaction can occur.

Not enough information

Explanation

The reaction can never go faster than its slowest step.

A possible mechanism for the overall reaction Br2 (g) + 2 NO (g) -> 2 NOBr(g) is

The rate law for the formation of NOBr based on this mechanism is rate = .

$k_1[NO]^{1/2}$

$k_1[Br_2]^{1/2}$

$(k_2k_1/k_{-1}) [NO]^2[Br_2]$

$(k_1/k_{-1})^2[NO]^2$

$(k_2k_1/k_{-1}) [NO][Br_2]^2$

Explanation

Based on the slowest step the rate law would be: Rate = k2 \[NOBr2\] \[NO\], but one cannot have a rate law in terms of an intermediate (NOBr2).

Because the first reaction is at equilibrium the rate in the forward direction is equal to that in the reverse, thus:

$k1 [NO][Br_2] = k_{-1}[NOBr_2]$ and: $[NOBr_2 ] = \frac{k_1 [NO][Br_2 ]}{k_{-1} }$

Substitution yields: $Rate = \frac{k_1 k_2 [NO]^2 [Br_2 ]}{k_{-1} }$

Based on the figure above, what arrows corresponds to the activation energy of the rate limiting step and the energy of reaction? Is the reaction endo- or exothermic?

Exothermic

Endothermic

Explanation

Since the products are higher in energy than the reactions, the reaction is endothermic.

Reaction Mechanisms

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