Kinetics unit test
A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.
Rate law
Steady-state intermediates
Rate-determining step
Reaction rate
Energy profile diagram
Arrhenius equation
Reaction mechanism
Zero-order kinetics
Second-order kinetics
Activation energy
Catalysis
Why a catalyst does not shift equilibrium
Short answer 1. Define or explain: Why order cannot be read from coefficients
3 ptsShort answer 2. Define or explain: Method of initial rates
3 ptsShort answer 3. Define or explain: Pseudo-first-order conditions
3 ptsShort answer 4. Define or explain: First-order kinetics
3 ptsFree response
10 ptsAnswer the following questions about the chromate ion, CrO4²−, and the dichromate ion, Cr2O7²−. FIGURE 1 shows one possible Lewis diagram for CrO4²−: the chromium atom is joined to each of four oxygen atoms by a single bond, and each oxygen carries three lone pairs. This places a formal charge of −1 on each oxygen and +2 on the chromium. FIGURE 2 is an empty box in which a resonance structure is to be drawn. When reacted with a strong acid like HNO3(aq), CrO4²−(aq) can be converted to Cr2O7²−(aq) and H2O(l) in a reversible reaction. In acidic solutions, Cr2O7²−(aq) reacts to form CrO3(aq): Equation 1: Cr2O7²−(aq) + 2 H3O+(aq) → 2 CrO3(aq) + 3 H2O(l) CrO3(aq) can be electrolyzed to plate objects with a thin layer of chromium metal: Equation 2: CrO3(aq) + 6 H+(aq) + 6 e− → Cr(s) + 3 H2O(l) E° = −1.32 V The molar mass of Cr is 52.00 g/mol; the Faraday constant is 96,485 C/mol e−. FIGURE 3 — an iron wire is placed directly in 0.50 M Cr2O7²−(aq) at pH 1.99, and [Cr2O7²−] is recorded over time: t (min) 0 4 8 12 16 20 24 28 32 36 40 44 48 [Cr2O7²−] (M) 0.50 0.44 0.38 0.33 0.29 0.25 0.22 0.19 0.17 0.148 0.128 0.110 0.097 FIGURE 4 — a plot of ln[Cr2O7²−] against time for a trial starting at 0.50 M: t (min) 0 3 6 9 12 15 18 ln[Cr2O7²−] −0.70 −0.80 −0.90 −1.00 −1.10 −1.20 −1.30 The points lie on a straight line. A single additional point for a second trial has been plotted at (0, −1.39).
(a)(i) Based on VSEPR theory, predict the molecular geometry of the CrO4²− ion.
(a)(ii) On Figure 2 a complete Lewis diagram for a resonance structure of CrO4²− is to be drawn, one that results in a formal charge of zero on two of the O atoms and −1 on the other two, using the same number of valence electrons as Figure 1. Describe that structure: state the bonding to each oxygen and the nonbonding electrons on every atom.
(b)(i) Write a balanced net ionic equation for the reaction between CrO4²−(aq) and a strong acid.
(b)(ii) Is that reaction a redox reaction? Justify your answer based on the oxidation number of Cr.
(c) Is the reaction represented by Equation 2 thermodynamically favorable or unfavorable under standard conditions? Justify your answer with a calculation of ΔG°.
(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.
(e) Explain how the data in Figure 3 support the conclusion that the reaction is first order with respect to Cr2O7²−.
(f) Calculate the value of the rate constant, k, for the reaction. Report your answer in units of min−1.
(g) The student runs a second trial under identical conditions but with an initial concentration of 0.25 M Cr2O7²− instead of 0.50 M. Describe the plot of ln[Cr2O7²−] against time that would be expected for the second experiment: state its intercept, its slope, and how it relates to the first line.