The reaction is first order with respect to $H_2$ and second order with respect to NO. The rate equation is

Arate

= k[H_2][NO]

Brate

= k[H_2]^2[NO]

Crate

= k[H_2][NO]^2

Drate

= k[H_2]^2[NO]^2

Explanation: Each concentration is raised to the power of its own order, giving an overall order of three.

Derived from ZIMSEC Chemistry Paper 2, November 2008, Q2

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