Ch.21 - Transition Elements and Coordination Chemistry
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- The percent iron in iron ore can be determined by dissolving the ore in acid, then reducing the iron to Fe2+, and finally titrating the Fe2+ with aqueous KMnO4. The reaction products are Fe3+ and Mn2+. (a) Write a balanced net ionic equation for the titration reaction. (b) What is the mass % Fe in the iron ore if titration of the Fe2+ from a 1.265-g sample of ore requires 34.83 mL of 0.051 32 M KMnO4 to reach the equivalence point? (c) Use the E° values in Appendix D to calculate ΔG° (in kilojoules) and the equilibrium constant for the reaction. (d) Does the paramagnetism of the solution increase or decrease as the reaction proceeds? Explain.
Problem 136
- The complete reaction of 2.60 g of chromium metal with 50.00 mL of 1.200 M H2SO4 in the absence of air gave a blue solution and a colorless gas that was collected at 25 °C and a pressure of 735 mm Hg. (a) Write a balanced net ionic equation for the reaction. (b) How many liters of gas were produced? (c) What is the pH of the solution? (d) Describe the bonding in the blue-colored ion, using both the crystal field theory and the valence bond theory. Include the appropriate crystal field d-orbital energy-level diagram and the valence bond orbital diagram. Identify the hybrid orbitals used in the valence bond description.
Problem 137
- In acidic aqueous solution, the complex trans-[Co(en)2Cl2]+ undergoes the following substitution reaction:
Problem 138
The reaction is first order in trans-[Co(en)2Cl2]+, and the rate constant at 25 °C is 3.2 × 10⁻⁵ s⁻¹. (a) What is the half-life of the reaction in hours? (b) If the initial concentration of trans-[Co(en)2Cl2]+ is 0.138 M, what is its molarity after a reaction time of 16.5 h? (c) Devise a possible reaction mechanism with a unimolecular rate-determining step. - Chromium forms three isomeric compounds A, B, and C with percent composition 19.52% Cr, 39.91% Cl, and 40.57% H2O. When a sample of each compound was dissolved in water and aqueous AgNO3 was added, a precipitate of AgCl formed immediately. A 0.225-g sample of compound A gave 0.363 g of AgCl, 0.263 g of B gave 0.283 g of AgCl, and 0.358 g of C gave 0.193 g of AgCl. One of the three compounds is violet, while the other two are green. In all three, chromium has a coordination number of 6. (a) What are the empirical formulas of A, B, and C? (b) What are the probable structural formulas of A, B, and C? Draw the structure of the cation in each compound. Which cation can exist as diastereoisomers? (c) What are the approximate freezing points of 0.25 m solutions of A, B, and C, assuming complete dissociation?
Problem 139
- Consider the following reaction, and assume that its equilibrium constant is 1.00 * 10^14:
Problem 141
(a) Write the equilibrium equation for the reaction, and explain why CrO4^2- ions predominate in basic solutions and Cr2O7^2- ions predominate in acidic solutions. (b) Calculate the CrO4^2- and Cr2O7^2- concentrations in a solution that has a total chromium concentration of 0.100 M and a pH of 4.000. (c) What are the CrO4^2- and Cr2O7^2- concentrations if the pH is 2.000? - An alternative to cyanide leaching of gold ores is leaching with thiocyanate ion, which forms a square planar gold(III) complex, Au(SCN)4^3-. (a) If the formation constant for Au(SCN)4^3- is Kf = 10^37, what is the equilibrium concentration of Au^3+ in a 0.050 M solution of Au(SCN)4^3-?
Problem 142