Limiting Reagent

Question

In a reaction between nitrogen and hydrogen gases to form ammonia ($\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$), if 2 mol of $\text{N}_2$ and 3 mol of $\text{H}_2$ are mixed, which substance is the limiting reagent?

RankGuru · Accepted Answer

The limiting reagent is the reactant that is completely consumed first, thereby determining the maximum yield of product. The balanced equation $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ shows that 1 mol of $\text{N}_2$ reacts with 3 mol of $\text{H}_2$. Stoichiometric check: to consume 2 mol of $\text{N}_2$, we need $2 \times 3 = 6$ mol of $\text{H}_2$. But only 3 mol of $\text{H}_2$ is available, which is only enough to react with $3/3 = 1$ mol of $\text{N}_2$. Hence $\text{H}_2$ is exhausted first and is the limiting reagent. $\text{N}_2$ is incorrect because 1 mol of $\text{N}_2$ remains unreacted after all $\text{H}_2$ is consumed. $\text{NH}_3$ is a product, not a reagent, so it cannot be the limiting reagent. Both cannot be limiting simultaneously in a standard stoichiometric scenario. This is a fundamental NCERT concept on limiting reagents in chemical reactions. Plausibility check: only $2 \times (3/6) = 1$ mol $\text{N}_2$ is consumed and $2 \times 1 = 2$ mol $\text{NH}_3$ is produced, confirming $\text{H}_2$ is consumed first.

Limiting Reagent

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