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MCQEasyJEE 2026Rate of Reaction

JEE Chemistry 2026 Question with Solution

A→ProductA \rightarrow \text{Product} (First order reaction). Three sets of experiments were performed for a reaction under similar experimental conditions: Run 1⇒100 mL of 10 M solution of reactant A\text{Run 1} \Rightarrow 100 \, \text{mL of } 10 \, \text{M solution of reactant A} Run 2⇒200 mL of 10 M solution of reactant A\text{Run 2} \Rightarrow 200 \, \text{mL of } 10 \, \text{M solution of reactant A} Run 3⇒100 mL of 10 M solution of reactant A+100 mL of H2O\text{Run 3} \Rightarrow 100 \, \text{mL of } 10 \, \text{M solution of reactant A} + 100 \, \text{mL of H}_2\text{O} The correct variation of rate of reaction is

  • A

    Run 3<Run 1=Run 2\text{Run 3} < \text{Run 1} = \text{Run 2}

  • B

    Run 1=Run 2=Run 3\text{Run 1} = \text{Run 2} = \text{Run 3}

  • C

    Run 1<Run 2<Run 3\text{Run 1} < \text{Run 2} < \text{Run 3}

  • D

    Run 3<Run 1<Run 2\text{Run 3} < \text{Run 1} < \text{Run 2}

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The reaction is first order, so the rate law is based on the concentration of reactant AA. Find: The correct order of rates in Run 1, Run 2, and Run 3.

From the solution:

Rate=k[A]\text{Rate} = k[A]

For Run 1, the concentration of AA is 10 M10 \, \text{M}.

Rate1∝10\text{Rate}_1 \propto 10

For Run 2, the volume is doubled but the concentration remains 10 M10 \, \text{M}. The extracted solution states that the total number of moles of AA is doubled, so the overall reaction rate is higher.

Rate2>Rate1\text{Rate}_2 > \text{Rate}_1

For Run 3, dilution occurs:

New concentration=100×10200=5 M\text{New concentration} = \frac{100 \times 10}{200} = 5 \, \text{M}

Hence,

Rate3∝5\text{Rate}_3 \propto 5

Comparing all three:

Rate3<Rate1<Rate2\text{Rate}_3 < \text{Rate}_1 < \text{Rate}_2

Therefore, the correct option is D.

Common mistakes

  • Assuming that doubling the volume always doubles the rate. This is wrong because for a first order reaction the rate law depends on concentration of AA, not volume directly. Check how concentration changes before comparing rates.

  • Ignoring dilution in Run 3. Adding 100 mL100 \, \text{mL} of water changes the concentration from 10 M10 \, \text{M} to 5 M5 \, \text{M}, so the rate decreases. Always recalculate concentration after mixing.

  • Confusing rate per unit volume with the extracted solution's comparison of overall reaction rate across runs. Read the interpretation used in the solution carefully and then compare the runs on that same basis.

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