The reaction, 2 NO(g) + O2(g) → 2 NO2(g), was found to be first order in each of the two reactants and second order overall. The rate law should therefore be written as

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The reaction, 2 NO(g) +  O2(g) →  2 NO2(g), was found to be first order in each of the two reactants and second order overall. The rate law should therefore be written as

BlackTom AI · Accepted Answer

To begin analyzing the rate law, we note the overall reaction order and the given information. The reaction 2 NO(g) + O2(g) → 2 NO2(g) is described as first order in each reactant and second order overall. This directly means the rate law must have first-order dependence on both NO and O2, and the exponents must sum to 2.
Option a: rate = k[NO][O2]. This matches the required first-order dependence on each reactant and sums to 1 + 1 = 2, so it is consistent with the given data.
Option b: rate = k[NO]^2. Here NO has a second-order dependence and O2 has zero dependence, giving an overall order of 2, but it ignores O2 entirely, contradicting the statement that the rate is first order in each reactant. Therefore this is incorrect.
Option c: rate = k[NO]^2[O2]^2. This would be fourth-order overall (2 + 2 = 4), which contradicts the given second-order total order. It overestimates sensitivity to both reactants.
Option d: rate = k([NO][O2])^2. This is equivalent to rate = k[NO]^2[O2]^2, also fourth-order overall. The squared product form unnecessarily doubles the order for both species, so it does not fit the data.
Option e: rate = k[NO]^2[NO]^-2[O2]^-1. This expression contains negative exponents and nonsensical fractional powers that do not correspond to a physically meaningful rate law for this reaction. It is incorrect on multiple grounds.
In summary, only option a provides the correct first-order dependence on each reactant with the correct total order of two, while all other options misrepresent the reaction order or introduce invalid dependencies.

CHM1051 MUM S2 2025 CHM1051 Practice Exam 2

The reaction, 2 NO(g) + O2(g) → 2 NO2(g), was found to be first order in each of the two reactants and second order overall. The rate law should therefore be written as

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