JEE Main 2026 — Jan 23, Shift 2 — Chemistry solution

JEE Main2026Jan 23, Shift 2Chemistry

Observe the following reactions at $T (K)$

I. $A \to$ products.

II. $5 B r^{-} (a q) + B r O_{3}^{-} (a q) + 6 H^{+} (a q) \to 3 B r_{2} (a q) + 3 H_{2} O (l)$

Both the reactions are started at $10.00$ am. The rates of these reactions at $10.10$ am are same. The value of $- \frac{Δ [ B r ^{-} ]}{Δ t}$ at $10.10$ am is $2 \times 1 0^{- 4}$ mol $L^{- 1}$ Min $^{- 1}$ . The concentration of A at $10.10$ am is $1 0^{- 2}$ mol $L^{- 1}$ . What is the first order rate constant (in min $^{- 1}$ ) of reaction I?

A
$2 \times 1 0^{- 3}$
B
$1 0^{- 3}$
C
$1 0^{- 2}$
D
$4 \times 1 0^{- 3}$

Solution

The rate of reaction II in terms of the overall stoichiometry:

$Rate_{I I} = - \frac{1}{5} \frac{Δ [ B r ^{-} ]}{Δ t} = \frac{1}{5} \times (2 \times 1 0^{- 4}) = 4 \times 1 0^{- 5}$ mol $L^{- 1}$ min $^{- 1}$

Given the rates are equal at $10 : 10$ am, $Rate_{I} = 4 \times 1 0^{- 5}$ mol $L^{- 1}$ min $^{- 1}$ .

For first-order reaction I: $Rate_{I} = k [A]$

$k = \frac{4 \times 1 0 ^{- 5}}{1 0 ^{- 2}} = 4 \times 1 0^{- 3}$ min $^{- 1}$ .

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