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7.1 Postlab

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    Kinetics Data Treatment

    1. Plot \(ln(A-A_{infty})\) versus t in order to obtain k in each kinetic run. Express \(k\) in \(s^{-1}\). Include the least-squares line and correlation coefficient on each plot.

    2. Determine the activation energy for the aquation reaction over the temperature range you studied by plotting the various rate constants you obtained at different temperatures versus 1/T. Include the least-squares line and correlation coefficient on the plot. (Remember that T should be in K and be careful about J versus kJ)

    3. Using the value for the activation energy, predict the rate constant for the aquation reaction at 22 °C. How long you would have to monitor the reaction at 22 °C in order to reach one half-life? How much error is generated by room temperature aquation in the experiments in which you determineε for \([Co(NH_3)_5Cl]^{2+}\) at 550 nm? How much \([Co(NH_3)_5Cl]^{2+}\) is converted into \([Co(NH_3)_5(H_2O)]^{3+}\) during the time required to make up a solution and measure the absorption for \([Co(NH_3)_5Cl]^{2+}\) ?

    4. Calculate the amount of starting material remaining after five half-lives (essentially "completion") of a first-order reaction.

    5. Show that plotting \(ln(A-A_{infty})\) versus t is the same as plotting \(ln{[Co(NH_3)_5Cl]^{2+}}\) versus t by turning the equation \(ln(A-A_{infty}) = - kt + C_1\) into \(ln{[Co(NH_3)_5Cl]^{2+}}= - kt + C_2\), where \(C_1\) and \(C_2\) are constants. (\(C_2\) is actually \([Co]_o\), the concentration of \([Co(NH_3)_5Cl]^{2+}\)  at time t = 0. What is \(C_1\)?)

    6. You could have chosen to follow the reaction at a different wavelength. What determines a good choice of wavelength? What determines a poor choice of wavelength? What other wavelengths would be good choices for this experiment? What wavelengths would be poor choices?

    7. Where did you observe isosbestic points in your spectra? What is true in terms of absorption by the reactant and the product at these wavelengths?


    Other Questions

    8. Explain why Co(II) complexes are readily substituted, in contrast to Co(III) complexes.

    9. How is carbonate bound to cobalt in \([Co(NH_3)_4(CO_3)]NO_3\)?

    10. Rationalize why nitrate is a poorer ligand than carbonate, i.e., why \([Co(NH_3)_4(CO_3)]NO_3\) is not actually \([Co(NH_3)_4(NO_3)]CO_3\).

    11. What is the role of hydrogen peroxide in the first experiment?

    12. The colors of the cobalt complexes can be assigned to relatively weak ("not allowed") "dd" transitions. If \((t_{2g})^6(e_g)^0\) is the ground state of a given complex, what is the excited state configuration after absorption of a photon?




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