JEE Main 2025 · 24 January · evening · Single correct · +4 / −1

For the reaction,
H₂( g)+I₂( g)⇌ 2HI( g)
Attainment of equilibrium is predicted correctly by :

  1. (a)
    Option (a)
  2. (b)
    Option (b)
  3. (c)
    Option (c)
  4. (d)
    Option (d)
Answer and solution

Answer (c)

Solution

Chemistry ma’am · 3 steps

All four graphs flatten out, so check directions: both reactants must fall together while HI rises.

  1. Flat is not enough. At equilibrium no concentration changes any more, so every curve must end up horizontal. All four graphs do this, so this test alone cannot pick the answer.
  2. Follow the coefficients. In H₂ + I₂ ⇌ 2HI, every 2 mol of HI formed uses 1 mol of H₂ and 1 mol of I₂. So H₂ and I₂ always change by equal amounts in the same direction, and HI moves the opposite way.
    GraphH₂I₂HIPossible?
    (a)risesrisesrisesNo: reactants and product cannot all increase
    (b)risesfallsfallsNo: H₂ and I₂ move apart
    (c)fallsfallsrises from zeroYes
    (d)fallsrisesrisesNo: H₂ and I₂ move apart
  3. So. Only graph (c) shows H₂ and I₂ being used up together while HI builds up from zero, with all three becoming constant. The answer is (c).

The idea to keep. For a concentration vs time graph, read the equation: species on the same side move together, species on opposite sides move in opposite directions, and every curve goes flat at equilibrium.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Chemical Equilibrium

Asked about 90 times in JEE Main since 2020. This question is from 2025.