April 8 Activity 20 Key PDF

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TRANSITION METAL LATIMER AND FROST DIAGRAMS – ACTIVITY 19 THE PLAN: Redox chemistry can be summarized into Latimer or Frost diagram and utilized to understand chemical properties of transition metals. We are going to use all three to understand chromium in the environment. WHY? As this is an inorganic chemistry class, we are mostly concerned about understanding the valence state of the metal and how that can change based upon the experimental conditions. These diagrams can help you do that quickly, without scrolling through a large list of standard cell potentials. INTRODUCTION (10 minutes) PART 1: LATIMER DIAGRAMS (20 minutes) PART 3: FROST DIAGRAMS (20 minutes) THE SCRIBE’S SHEET THE SCRIBE’S SHEET (ONE PER GROUP)

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Chromium is a transition metal that can occur in environmental systems in a variety of oxidation states. Understanding the speciation and oxidation state of chromium is important as it impacts the mobility in environmental systems and its toxicity (Cr(III) is non-toxic, Cr(VI) is toxic). PART 1: LATIMER DIAGRAMS (20 minutes) You will be utilizing the following diagrams to answer questions in Part 1

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1. Based on the diagram, write two possible sets of reactions for the reduction of Cr(VI) to Cr(III) in an acidic solution. ½ Cr2O7-2 + e- + 7 H+  Cr+5 + 7/2 H2O Cr+5 + e-  Cr+4 Cr+4 + e-  Cr+3 ½ Cr2O7-2 + 3e- + 7 H+  Cr+3 + 7/2 H2O 2. Calculate the ΔG° for this reaction using both reactions. ΔGshort = -3 * (96485 C/mol) * 1.38 V = -3.99*105 J/mol = -399 kJ / mol ΔGlong = -3 * (96485 C/mol) * (((1 * 0.55 + 1 * 1.34 + 1 * 2.10) V) / 3) = -3.85*105 J/mol = -385 kJ / mol These results are not exactly the same because the values for each step come from different experiments, each with their own associated margin of error.

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3. Write out the reaction for the reduction of Cr(VI) to Cr(III) in a basic solution for both possible species. CrO4-2 + 3 e- + 4 H2O  Cr(OH)3 + 5 OHCrO4-2 + 3 e- + 4 H2O  Cr(OH)4- + 4 OH-

4. Calculate the ΔG° for these reactions. ΔGtop = -3 * (96485 C/mol) * -0.11 V = 3.18*104 J/mol = 31.8 kJ / mol ΔGbottom = -3 * (96485 C/mol) * -0.13 V = 3.76*104 J/mol = 37.6 kJ / mol

PART 2: FROST DIAGRAMS (20 minutes) 5. We are going to turn our attention to Frost diagrams. What are plotted on the x and y axes of a Frost diagram? The x-axis lists the oxidation state from most reduced on the left to most oxidized on the right side. The y axis is ΔG/F = nE°. So it is plotting the standard electrode potential times the number of electrons transferred in the reduction reaction, which is also equal to Gibbs free energy of the reduction reaction divided by Faraday’s constant. Note that this is all relative to the zero oxidation state that is set to zero on the y axis. 6. Using the Latimer Diagram given in Part 1, graph the Frost diagram for chromium in an acidic solution. Stepwise Oxidation Stepwise reduction electon Calculated potential compared State potential (V) transfer to Cr(0) (V) 0 0 0 0 2 -0.9 2 -1.8 3 -0.424 1 -2.22 4 2.1 1 -0.12 5 1.34 1 1.22 6 0.55 1 1.77

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7. What is the most stable oxidation state of chromium under acidic conditions? How do you know? Chromium (III) We are looking for the lowest point on the Frost diagram because that signifies the lowest Gibbs Free energy of the system. 8. Do any of the species undergo disproportionation? If they do, write them down below. Yes. Chromium (IV) will disproportionate to chromium (III) and chromium (V), and chromium (V) itself will disproportionate to chromium (IV) and chromium (VI). Therefore any mixture of chromium (IV) and/or chromium (V) will eventually disproportionate to a mixture of chromium (III) and chromium (VI).

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