2. Answer the following questions about the chromate ion, CrO4 2-, and the dichromate ion, Cr2O7 2-. — Lewis Structures, Electrochemistry, Kinetics Chemistry Question

The bonds between chromium and oxygen in CrO₄ 2- can be represented as covalent bonds. One possible Lewis diagram for the CrO₄ 2- ion is shown in Figure 1.

i. Based on VSEPR theory, predict the molecular geometry of the CrO₄ 2- ion.

ii. On Figure 2, draw a complete Lewis diagram for a resonance structure of CrO₄ 2- that results in a formal charge of zero on two of the O atoms and a formal charge of 1- on the other two O atoms. Your diagram should contain the same number of valence electrons as in Figure 1.

When reacted with a strong acid like HNO₃(aq), CrO₄ 2-(aq) can be converted to Cr₂O₇ 2-(aq) and H₂O(l) in a reversible reaction.
i. Write a balanced net ionic equation for the reaction between CrO₄ 2-(aq) and a strong acid.

ii. Is the reaction a redox reaction? Justify your answer based on the oxidation number of Cr.

The following information applies to parts C and D.
In acidic solutions, Cr₂O₇ 2-(aq) reacts to form CrO₃(aq) according to Equation 1.
Equation 1: Cr₂O₇ 2-(aq) + 2 H⁺(aq) → 2 CrO₃(aq) + H₂O(l)
CrO₃(aq) can be electrolyzed to plate objects with a thin layer of chromium metal according to Equation 2.
Equation 2: CrO₃(aq) + 6 H⁺(aq) + 6 e^- → Cr(s) + 3 H₂O(l)

C. Is the reaction represented by Equation 2 thermodynamically favorable or unfavorable under standard conditions? Justify your answer with a calculation of ΔG°. Show the work that leads to your answer.

D. Calculate the number of grams of Cr(s) that could be plated onto the surface of a steel rod when 15.0 A of current is applied for 3250 seconds. Show the work that leads to your answer.

In a separate experiment, an iron wire is placed directly in a solution of 0.50 M Cr₂O₇ 2-(aq) with pH = 1.99. The concentration of Cr₂O₇ 2-(aq) remaining is recorded over time, and the results are presented in Figure 3.

E. Explain how the data in Figure 3 support the conclusion that the reaction is first order with respect to Cr₂O₇ 2-.

The following information applies to parts F and G.
The student creates a plot of ln[Cr₂O₇ 2-] versus time for a trial with an initial concentration of 0.50 M Cr₂O₇ 2-(aq), as shown in Figure 4.

F. Calculate the value of the rate constant, k, for the reaction. Report your answer in units of min^-1. Show the work that leads to your answer.

G. The student then runs a second trial of the experiment under identical conditions but uses an initial concentration of 0.25 M Cr₂O₇ 2-(aq) instead of 0.50 M Cr₂O₇ 2-(aq). On Figure 4, carefully draw the plot of ln[Cr₂O₇ 2-] versus time that would be expected for the second experiment. The point at time zero has already been plotted.