Textbook Question
Two nonpolar organic liquids, hexane (C6H14) and heptane (C7H16), are mixed. (a) Do you expect ∆Hsoln to be a large positive number, a large negative number, or close to zero? Explain.
Ch.13 - Properties of Solutions
Chapter 13, Problem 23
The solubility of Cr1NO323 # 9 H2O in water is 208 g per 100 g of water at 15 °C. A solution of Cr1NO323 # 9 H2O in water at 35 °C is formed by dissolving 324 g in 100 g of water. When this solution is slowly cooled to 15 °C, no precipitate forms. (b) You take a metal spatula and scratch the side of the glass vessel that contains this cooled solution, and crystals start to appear. What has just happened?
Verified step by step guidance1
Step 1: Understand the concept of supersaturation. A supersaturated solution contains more dissolved solute than it would under normal circumstances at a given temperature. This occurs when a solution is prepared at a higher temperature and then cooled without the solute precipitating out.
Step 2: Recognize that at 35 °C, the solution of Cr(NO3)3·9H2O in water is supersaturated when cooled to 15 °C. This means it contains more solute than the solubility limit at 15 °C, but no crystals form initially because the solution is stable in this state.
Step 3: Identify the role of scratching the glass vessel. Scratching provides a site for nucleation, which is the initial step in the crystallization process. It disrupts the stability of the supersaturated solution, allowing solute particles to gather and form a solid crystal.
Step 4: Explain the process of nucleation. Nucleation is the process where solute molecules or ions dispersed in the solvent start to gather into clusters, leading to the formation of a new phase or a new structure, such as a crystal.
Step 5: Conclude that the act of scratching the glass vessel provides the necessary disturbance to overcome the energy barrier for nucleation, leading to the rapid formation of crystals from the supersaturated solution.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Solubility and Saturation
Solubility refers to the maximum amount of a solute that can dissolve in a solvent at a given temperature. A saturated solution contains the maximum amount of solute, while an unsaturated solution can still dissolve more. In this scenario, the solution at 35 °C was saturated with Cr(NO3)3·9H2O, but upon cooling to 15 °C, it remained unsaturated, which is why no precipitate formed initially.
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Solubility Rules
Supersaturation
A supersaturated solution occurs when a solution contains more solute than it can theoretically hold at a given temperature. This can happen when a saturated solution is cooled slowly without allowing the excess solute to crystallize. The solution remains in a metastable state until disturbed, such as by scratching the glass, which provides nucleation sites for crystal formation.
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Nucleation
Nucleation is the process by which solute particles begin to aggregate and form a new phase, such as crystals. In this case, scratching the glass introduces disturbances that act as nucleation sites, allowing the dissolved Cr(NO3)3·9H2O to start crystallizing from the supersaturated solution. This is a critical step in the crystallization process, leading to the formation of solid crystals.
Related Practice
Textbook Question
Two nonpolar organic liquids, hexane (C6H14) and heptane (C7H16), are mixed. (b) Hexane and heptane are miscible with each other in all proportions. In making a solution of them, is the entropy of the system increased, decreased, or close to zero, compared to the separate pure liquids?
Textbook Question
KBr is relatively soluble in water, yet its enthalpy of solution is + 19.8 kJ/mol. Which of the following statements provides the best explanation for this behavior? (a) Potassium salts are always soluble in water. (b) The entropy of mixing must be unfavorable. (c) The enthalpy of mixing must be small compared to the enthalpies for breaking up water–water interactions and K–Br ionic interactions. (d) KBr has a high molar mass compared to other salts like NaCl.
Textbook Question
By referring to Figure 13.15, determine whether the addition of 40.0 g of each of the following ionic solids to 100 g of water at 40 °C will lead to a saturated solution:(c) K2Cr2O7
Textbook Question
By referring to Figure 13.15, determine whether the addition of 40.0 g of each of the following ionic solids to 100 g of water at 40 °C will lead to a saturated solution: (d) Pb(NO3)2.
Textbook Question
By referring to Figure 13.15, determine the mass of each of the following salts required to form a saturated solution in 250 g of water at 30 °C: (b) Pb(NO3)2,