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Rate Of Reaction

Mediumchemistry

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?

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About This Question

Subject
chemistry
Chapter
chemical kinetics
Topic
rate of reaction
Difficulty
Medium
Year
2025
Tags
rate expressionstoichiometric coefficientammonia synthesisrate ratioreaction rate

Solution

Correct Answer:

Hydrogen disappears 1.5 times faster

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.

This medium difficulty chemistry question is from the chapter chemical kinetics, covering the topic of rate of reaction. It appeared in the 2025 exam.

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