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. -If researchers wanted to replicate physiological conditions by increasing the SBF temperature to 37°C, how would this change affect the solubility product constant and equilibrium pH of Mg(OH)₂ dissolution?

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11ecba2d_11aa_70c2_a3bf_bfb3c4afee4b_MD0008_00 The passage states that dissolution of Mg(OH)₂ is an endothermic process. For endothermic reactions, heat can be treated as one of the reactants because it is required for the reaction. Heat can also be treated as a product for exothermic reactions. Therefore, adding heat to an endothermic reaction drives it toward the products according to Le Châtelier's principle. In the case of a dissolving salt, this change is reflected as an increase in the solubility product constant K_sp. As heat is added, more Mg(OH)₂ will dissolve. Because Mg(OH)₂ produces OH⁻ ions when dissolved in water, it leads to an increase in OH⁻ ions in solution. Therefore, the pH of the solution at equilibrium will increase. (Choice A) The K_sp will increase, not decrease, upon addition of heat to the system. (Choices C and D) The equilibrium constant K_sp of a system changes with a change in temperature. Furthermore, if K_sp were to stay the same, the amount of OH⁻ released would also remain unchanged, and so would the equilibrium pH. Educational objective: Adding heat to an endothermic reaction shifts the equilibrium toward the products according to Le Châtelier's principle; the opposite is true of exothermic reactions. For dissolving salts, changes in temperature change the solubility product constant K_sp.

Passage Bone Growth and Remodeling During Fracture Healing Are Often Slow