Igot the correct answer, but i don't know why. could someone explain this to me?

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Option 2

Explanation:-

In a multi step reaction, the overall reaction is as fast as the slowest step only.

So the rate of a multistep reaction is the same as the rate of its slowest step.

The gas phase reaction was found to be second order wrt NO2.

So the slowest step must have 2 NO2 since the dependence is second order on NO2.

It was also found to be zero order wrt CO.

So the slowest step must not contain any CO since it’s presence has no effect on the reaction (zero order).

Among all options, only number 2 meets the two criterion

Thus the steps are

NO2 + NO2 – NO3 + NO (Slow)

NO3 + CO – NO2 + CO2 (Fast)

the answer is barium and copper bromide