Notes 13-1 obj 13.1, 13.2

A.) The solution process Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent.

B.) The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

C.) As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates (hydrates), them.solvates

D.) If an ionic salt is soluble in water, it is because the ion- dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

E.) Three processes affect the energetics of solution: – separation of solute particles, – separation of solvent particles, – new interactions between solute and solvent.

Why doesn’t NaCl dissolve in nonpolar solvents such as hexane (C 6 H 14 )? The enthalpy change of the overall process depends on  H for each of these steps.

Which substance below is miscible with water? a.CH 3 OH b.CH 4 c.C 6 H 6 d.CH 3 CH 2 OCH 2 CH 3

Considering the substances given below, predict which will be soluble in water , 3 1, 2, 3

F.) Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered. © 2009, Prentice-Hall, Inc.

G.) Why Do Endothermic Processes Occur? Yet we know that in some processes, like the dissolution of NH 4 NO 3 in water, heat is absorbed, not released. © 2009, Prentice-Hall, Inc.

H.) Enthalpy Is Only Part of the Picture The reason is that the spreading of energy (known as entropy) of a system tends to lower the energy of the system. © 2009, Prentice-Hall, Inc.

I.) Enthalpy Is Only Part of the Picture So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered. © 2009, Prentice-Hall, Inc.

J.) entropy-degree of randomness in a system, sometimes referred to as a Energy Spreading (thermodynamic quality) 1.) Process in which the energy content of the system decreases tend to occur spontaneously. 2.) Processes occurring at a constant temperature in which the randomness or dispersal in space (entropy) of the system increases tend to occur spontaneously.

When two nonpolar organic liquids such as hexane and heptane are mixed, the enthalpy change is generally quite small. – A.) Use the energy diagram to explain why – B.) Given that the change in enthalpy of the solution is close to 0, explain why hexane and heptane spontaneously form a solution

Note!! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved. © 2009, Prentice-Hall, Inc.

Note!! Dissolution is a physical change — you can get back the original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted. © 2009, Prentice-Hall, Inc.

Sample Exercise 13.1 Assessing Entropy Change In the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is Essentially all of the water vapor in the closed container is consumed in this reaction. If we consider our system to consist initially of Na 2 SO 4 (s) and 10 H 2 O(g), (a) does the system become more or less ordered in this process, and (b) does the entropy of the system increase or decrease?

Sample Exercise 13.1 Assessing Entropy Change Does the entropy of the system increase or decrease when the stopcock is opened to allow mixing of the two gases in this apparatus? Practice Exercise

13.2 Saturated Solutions and Solubility A.) Crystallization is the opposite of dissolving. B.) To reach equilibrium, rate of crystallization and dissolving is the same solute + solvent solution

C.) Saturated – In a saturated solution, the solvent holds as much solute as is possible at that temperature. – Dissolved solute is in dynamic equilibrium with solid solute particles. © 2009, Prentice-Hall, Inc.

D.) Unsaturated – If a solution is unsaturated, less solute than can dissolve in the solvent at that temperature is dissolved in the solvent. © 2009, Prentice-Hall, Inc.

E.) Supersaturated – In supersaturated solutions, the solvent holds more solute than is normally possible at that temperature. – These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask. © 2009, Prentice-Hall, Inc.