Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site. Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H 2 O(l) → Mg(OH) 2 (s) + H 2 (g)Reaction 1 Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body. Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation. Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2 Solubility equilibrium of Mg(OH) 2 in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings. They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1. The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1 Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -Magnesium hydroxide begins to precipitate onto the magnesium metal surface when which of the following expressions is true?

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Quizplus · Accepted Answer

11ecba2d_11a7_b19c_a3bf_4be7aec0e334_MD0008_00 The equilibrium constant of any chemical reaction describes the molar ratio of products to reactants once equilibrium is achieved. The solubility product constant K_sp is a special equilibrium constant used for slightly soluble compounds in solution. It does not include a denominator because the reactant (magnesium metal, in this case) is a solid. The amount of solid present does not affect the equilibrium. When the product of the concentrations of ions in solution is equal to K_sp, the dissolution and precipitation rates are equal. Under these conditions, the solution is said to be saturated, and any increase in ion concentration will result in precipitation. The equilibrium expression in this case is given by 11ecba2d_11a7_d8ad_a3bf_3998b19e8e5d_MD0008_00 K_sp = 8.9 × 10⁻¹² = [Mg²⁺][OH⁻]² When the product of [Mg²⁺][OH⁻]² is greater than 8.9 × 10⁻¹², magnesium hydroxide will begin to precipitate. (Choices B and C) The equilibrium expression must account for all ions in the reaction, in this case Mg²⁺ and OH⁻. Also notice that the hydroxide concentration is squared in the K_sp expression because for every 1 mole of Mg(OH)₂ that dissolves, 2 moles of OH⁻ are produced. (Choice D) The formation of precipitate is not dependent on whether the concentrations of ions are equal to each other but on when the product of [Mg²⁺][OH⁻]² exceeds K_sp. Educational objective: The solubility product constant K_sp describes the concentrations of slightly soluble compounds in solution at equilibrium. At equilibrium, the rates of dissolution and precipitation are equal, and the solution is saturated with the ions of the reaction. At concentrations above K_sp, the ions combine and precipitate.

Passage Bone Growth and Remodeling During Fracture Healing Are Often Slow