IEMJEE Chemistry preparation: Chemical first-order half-lives — practice worksheet with answers

How to use this worksheet

This independent worksheet checks chemical first-order half-lives with a new problem, an explanation task and an interleaved related skill. It is not an official IEM question paper, a difficulty forecast or a prescribed timed examination.

Write an attempt first, then compare your method with the visible answers. Record whether a miss came from interpretation, a missing concept, execution or a condition you overlooked.

Question 1: apply the topic

How many half-lives reduce concentration to one quarter?

Answer and checking point

Two half-lives.

Check atom and charge bookkeeping, units, signs and the identity of the species to which a statement applies. For an organic transformation, retain the substrate and conditions. A familiar reagent or periodic trend is not enough to prove every proposed product or comparison.

Question 2: diagnose an approach

Explain why the following warning matters and identify the correction it calls for: The constant half-life result is specific to the first-order model, not all reaction orders.

Model checkpoint: In a first-order kinetic model, each halving takes the same time even though the amount consumed shrinks. Governing rule: t₁/₂=ln2/k.

Question 3: transfer to vsepr electron domains

Why is NH₃ not described as a planar three-bond molecule in VSEPR?

Answer: Nitrogen has three bonds and one lone pair: four domains, with a trigonal-pyramidal atom arrangement.

This is an interleaved related-skill task, not a claim that the two topics have identical methods. Explain which idea changes when switching from the main topic to this one.

Review the attempt

Separate correct answers reached with a justified method from lucky guesses. Rework missed steps without looking at the answer, then use a fresh problem from an appropriate primary textbook or official paper. Do not use a small worksheet result as an admission cutoff, rank or readiness guarantee.

Official sources and verification

Sources checked on 26 October 2026. A source’s date, programme and campus limit which claims it supports. Official pages may change after review.

1. UEM — IEMJEE examination information

   Historical examination pattern; verify the current cycle on the central portal.

2. NTA — explicitly labelled JEE Main 2026 syllabus

   Historical PCM syllabus reference. Lessons and exercises are independently authored, not official IEMJEE papers or verified 2027 paper predictions.

3. NCERT — official textbook catalogue

   Primary textbook reference for checking school-level concepts. This directory does not certify the independent lesson or establish IEM examination weightage.

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