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For the reaction A → products, the half-life is 120 min when the initial concentration of A is 8.0 × 10⁻² M. If the starting concentration is changed to 4.0 × 10⁻² M, the half-life increases to 240 min. What is the reaction order?
- zero
- one
- two
- 0.5
Correct answer: two
Solution
t-half is proportional to [A]^(1-n). Here halving [A] doubled t-half, so t-half ~ 1/[A], meaning 1-n = -1, giving n = 2 (second order).
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