Rate Of Reaction

Question

For the reaction in which nitrogen reacts with hydrogen to form ammonia, how does the rate of disappearance of hydrogen relate to the rate of formation of ammonia?

RankGuru · Accepted Answer

The rate of a reaction expressed through any species must be divided by that species' stoichiometric coefficient so that a single rate value describes the whole reaction. For N_2 + 3H_2 → 2NH_3, the unique rate is -d[N_2]/dt = -(1/3)d[H_2]/dt = +(1/2)d[NH_3]/dt. Equating the hydrogen and ammonia terms, -(1/3)d[H_2]/dt = (1/2)d[NH_3]/dt, so the rate of disappearance of hydrogen equals (3/2) times the rate of formation of ammonia, that is 1.5 times faster. The option claiming equality ignores the coefficients. The option that ammonia forms faster reverses the ratio. The option of three times faster compares hydrogen only to nitrogen, not to ammonia. This stoichiometric treatment of rate is a foundational NCERT concept. Such questions reward conceptual clarity, since a student who truly grasps rate expression can solve many superficially different variants with the same approach. A common JEE pitfall is to ignore the role of rate expression, yet it is exactly this factor that distinguishes the correct answer from the tempting alternatives. Plausibility check: three molecules of hydrogen are consumed for every two molecules of ammonia produced, so hydrogen must change faster by the ratio 3:2, confirming 1.5 times.

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