14.4.3 Calculating the Rate Laws of Multi-step Reactions Flashcards
Calculating the Rate Laws of Multi-step Reactions
- If a proposed reaction mechanism is consistent with the experimentally-determined rate law, the proposed mechanism might be correct; if a proposed mechanism is not consistent with the experimentally-determined rate law, the proposed mechanism is incorrect.
- If the first elementary step of the reaction mechanism is not the rate-determining step, the derived rate law will often include reaction intermediates.
- The rate law must be written in terms of known concentrations.
- If an elementary step is fast, the reverse of the elementary step will also be fast.
note
- A reaction mechanism consists of a series of elementary steps. One of the elementary steps will be the rate-determining step.
- Reaction mechanisms are hypothesized. If a proposed mechanism is consistent with the experimentally-determined rate law, the proposed mechanism might be correct; if a proposed mechanism is not consistent with the experimentally-determined rate law, the proposed mechanism is definitely incorrect.
- If the first elementary step of a reaction mechanism is not the rate-determining step, the derived rate law will often include reaction intermediates.
- For example, a proposed mechanism for the reaction between nitrogen monoxide (NO) and bromine (Br 2 ) has a proposed rate law rate = k 2 [NO][NOBr 2 ]. The NOBr 2 is a reaction intermediate.
- However, since NOBr 2 is produced and consumed
simultaneously throughout the course of the reaction, it is impossible to measure its concentration. The proposed rate law must be written in terms of known concentrations. - If an elementary step is fast, the reverse of the elementary step will also be fast. The rate of the forward reaction is the same as the rate of the reverse reaction.
- This can be used to express the concentrations of reaction intermediates (such as NOBr 2 ) in terms of known concentrations.
- For example, since the forward rate of the first elementary step in the proposed mechanism is the same as the reverse rate, k1[NO][Br 2 ] = k–1[NOBr 2 ]. This can be rearranged to express [NOBr 2 ] in terms of [NO] and [Br 2 ].
- Plugging this formula for [NOBr 2 ] into the proposed rate law yields a new rate law in terms of known concentrations. The three rate constants (k 2 , k 1 , and k –1 ) merge into one rate constant, k obs or simply k. This new proposed rate law is the same as the observed rate law, so the proposed mechanism is consistent with reality, and might be correct.
In a proposed two-step mechanism for the reaction, CO(g) + NO2(g) → CO2(g) + NO(g), the second, fast, step is NO3 + CO → NO2 + CO2. What is the expected rate law of this reaction?
rate of reaction = k[ NO2 ]^2
In the reaction, 2NO(g) + O2(g) → 2NO2(g), the first step, 2NO(g) ↔ N2O2(g), is fast. Which is a true equation for this first step of the reaction mechanism?
k1 [ NO]^2 = k_−1 [ N2O2 ]
Which best describes a rate-determining step?
An elementary process that occurs slower than all others and specifies the speed of the overall reaction.
The rate law for the reaction, 2NO(g) + O2(g) → 2NO2(g), is found to be: rate of reaction = k [ NO]2 [O2 ]2.
The first, fast step is 2NO(g) ↔ N2O2(g).
What is the value of Δ[ N2O2 ] / Δt ?
0
What is the slow step of the reaction mechanism called?
The rate-determining step
What is referred to as dynamic equilibrium?
The rate of the forward reaction equals the rate of the reverse reaction.
Which best defines a reaction mechanism?
A description of a chemical reaction presented as a series of one-step changes.
Which best defines an elementary process?
Any molecular event that significantly alters a molecule’s energy.
Which is not an important characteristic of elementary processes?
One elementary process may occur much more quickly than all the others in the reaction mechanism, and in all cases, determine the rate of the overall reaction.
Which of the following statements is TRUE?
The slow step determines the rate of the overall reaction.
