In the presence of excess thiocyanate ion, SCN^-, the following reaction is first-order in chromium (III) ion, Cr^3+; the rate constant is 9.0 × 10^-5 s^-1. Cr^3+(aq) + SCN^-(aq) → Cr(SCN)^2+(aq). If

Question

In the presence of excess thiocyanate ion, SCN^-, the following reaction is first-order in chromium (III) ion, Cr^3+; the rate constant is 9.0 × 10^-5 s^-1. Cr^3+(aq) + SCN^-(aq) → Cr(SCN)^2+(aq). If 80.0% reaction is required to obtain a noticeable colour from the formation of the Cr(SCN)^2+ ion, how many hours are required? (ln 5 = 1.62)

Prashant Kotian · Accepted Answer

Correct answer: 5. For a first-order reaction: t = (1/k) ln(100 / (100 - x)). t = (1 / (9.0 × 10^-5)) ln(100 / 20) = (1 / 9.0 × 10^-5) ln(5) = 1.62 / (9.0 × 10^-5) = 1.8 × 10^4 s. Converting to hours: (1.8 × 10^4) / 3600 = 5 hours. Therefore, correct answer is 5.

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