6. Consider the following reaction: [S 02, ex 3] Cr^3+(aq) + SCN^-(aq) --> CrSCN

Question

6. Consider the following reaction: [S 02, ex 3] Cr^3+(aq) + SCN^-(aq) --> CrSCN^2+(aq) In the presence of 0.500 M SCN- (a large excess), the reaction appears to be first order with a rate constant of 2.0 X 10^-6 s^-1 at a particular temperature. In order to determine the actual rate law and the true rate constant, the reaction was run again with [SCN^-] = 0.700 M and the [Cr^3+] measured as a function of time. A plot of ln[Cr^3+] vs. time gives a straight line with a slope of -2.8 X 10^-6 s^-1. What is the true rate law and rate constant for this reaction? (a) rate = k[Cr^3+], k = 2.8 X 10^-6 s^-1 (b) rate = k[Cr^3+][SCN^-], k = 5.6 X 10^-6 M^-1 s^-1 (c) rate = k[Cr^3+][SCN^-]^2, k = 8.0 X 10^-6 M-2 s^-1 (d) rate = k[Cr^3+][SCN^-], k = 4.0 X 10-6 M^-1 s^-1 (e) rate = k[SCN^-]^2, k = 5.6 X 10^-6 M^-2 s^-1

Explanation / Answer

as we are getting -ve slope when [SCN-] Vs time that means it is a first order rate of reaction
so w.r.t. [SCN-] is 1st order
w.r.t [cr+3] is 1 st order

overal rate law

rate = K[Cr+3]^1[SCN-]1

so the option b ic corrcet

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