Do half-lives shrink, stay or grow?
"Do half-lives shrink, stay or grow?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the NCERT section "Half-life and pseudo first order" in Chemical Kinetics. Successive half-lives drawn as rungs for zero, first and second order: they shrink, stay equal or grow, following t½ ∝ [R]₀^(1−n) NEET has asked about this idea 3 times in 10 years of papers. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Half-life and pseudo first order in Chemical Kinetics. Change the values and watch what happens.
What it shows
NCERT §3.3.3
Successive half-lives drawn as rungs for zero, first and second order: they shrink, stay equal or grow, following t½ ∝ [R]₀^(1−n)
Controls
4 to try
- Segmentedorder 0 / 1 / 2
- Slider[R]₀ 0.1–2 mol L⁻¹
- Sliderk (units follow the order)
- Readouts1st, 2nd, 3rd half-life, time to 99.9%
Try this
Find the order for which each successive half-life is twice the previous one, then the order for which doubling [R]₀ leaves t½ unchanged.
Common mistake
Using t½ = 0.693/k for every order.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: Chemical Kinetics, Chemical Kinetics (chapter)
In JEE Main
42 shifts analysed
48 questions from Chemical Kinetics across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §3.3.3
- Half-life t½ is the time for the concentration of a reactant to fall to half its starting value.
- Zero order: t½ = [R]₀/2k. It is proportional to the starting concentration, so each successive half-life is half as long as the one before.
- First order: t½ = 0.693/k, independent of concentration. Every half-life of a first-order reaction takes the same time.
- Example: for k = 5.5 × 10⁻¹⁴ s⁻¹, t½ = 0.693/(5.5 × 10⁻¹⁴) = 1.26 × 10¹³ s.
- For first order, 99.9% completion takes (2.303/k) log 1000 = 6.909/k, which is about 10 half-lives; 99% completion takes 4.606/k, about 6.6 half-lives.
- After n half-lives of a first-order reaction, the fraction remaining is (½)ⁿ.
- A pseudo first order reaction is truly of higher order, but one reactant is in such large excess that its concentration barely changes. Its term is absorbed into k.
- Acid hydrolysis of ethyl acetate in a large excess of water behaves as first order: with 0.01 mol ester and 10 mol water, water only drops to 9.99 mol when the ester is gone. Rate = k′[CH₃COOC₂H₅], where k′ = k[H₂O].
- Inversion of cane sugar in acid (C₁₂H₂₂O₁₁ + H₂O → glucose + fructose) is another pseudo first order reaction: rate = k[C₁₂H₂₂O₁₁].
Questions
5 answers
- What does the "Do half-lives shrink, stay or grow?" simulation show?
- Successive half-lives drawn as rungs for zero, first and second order: they shrink, stay equal or grow, following t½ ∝ [R]₀^(1−n)
- What mistake does the "Do half-lives shrink, stay or grow?" simulation fix?
- A common mistake it clears up: using t½ = 0.693/k for every order.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 times in 10 years of papers.
- Does JEE Main ask questions from Chemical Kinetics?
- Yes. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Half-life and pseudo first order" in Chemical Kinetics (Class 12 Chemistry).
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