Solutions Review Questions As seawater moves through the intestine, it flows past cells that line the digestive tract, which consist of largely fluid interiors surrounded by membranes. Although cellular fluids themselves contain dissolved ions, including sodium and chloride, the fluids are more dilute than seawater. Nature's tendency towards mixing (which tends to produce solutions of uniform concentration), together with the selective permeability of the cell membranes (which allow water to flow in and out, but restrict the flow of dissolved solids), cause a flow of solvent out of the body's cells and into the seawater. 12.2
A solution is a homogeneous mixture of two or more substances. A solution has at least two components. The majority component is usually called the solvent and the minority component is usually called the solute.
12.3
A substance is soluble in another substance if they can form a homogeneous mixture. The solubility of a substance is the amount of the substance that will dissolve in a given amount of solvent. Many different units can be used to express solubility, including grams of solute per 100 grams of solvent, grams of solute per liter of solvent, moles of solute per liter of solution, and moles of solute per kilogram of solvent.
12.4
Ideal gases do not interact with each other in any way (that is, there are no significant forces between their constituent particles). When the two gases mix their potential energy remains unchanged, so this does not drive the mixing. The tendency to mix is related, rather, to a concept called entropy. Entropy is a measure of energy randomization or energy dispersal in a system. Recall that a gas at any temperature above 0 K has kinetic energy due to the motion of its atoms. When the gases are separated, their kinetic energies are also confined to those regions. However, when the gases mix the kinetic energy of each gas becomes spread out or dispersed over a larger volume. Therefore, the mixture of the two gases has greater energy dispersal, or greater entropy, than the separated components. The pervasive tendency for all kinds of energy to spread out, or disperse, whenever they are not restrained from doing so is the reason that two ideal gases mix.
12.5
Entropy is a measure of energy randomization or energy dispersal in a system. When two substances mix to form a solution there is an increase in randomness, due to the fact that the components are no longer segregated to separate regions. This makes the formation of a solution energetically favorable, even when it is endothermic.
12.6
Whether two substances will spontaneously mix to form a homogeneous solution is dependent on a number of different types of intermolecular forces including dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole forces.
12.7
A solution always forms if the solvent-solute interactions are comparable to, or stronger than, the solvent-solvent interactions and the solute-solute interactions.
12.8
The statement "like dissolves like" means that similar kinds of solvents dissolve similar kinds of solutes. Polar solvents, such as water, dissolve many polar or ionic solutes, and nonpolar solvents, such as hexane, dissolve many nonpolar solutes.
457
Chapter 12 Solutions 12.30
461
(a)
water, methanol, ethanol; dispersion, dipole-dipole, hydrogen bonding
(b)
water, acetone, methanol, ethanol; dispersion, ion-dipole
(c)
hexane, toluene, or CCl^; dispersion forces
(d)
water, acetone, methanol, ethanol; dispersion, ion-dipole
HOCH2CH2CH2OH would be more soluble in water because it has -OH groups on both ends of the molecule, so it can hydrogen bond on both ends. 12.32
CH2C12 would be more soluble in water because it is a polar molecule and can exhibit dipole-dipole interactions with the water molecules. CCLj is a nonpolar molecule.
12.33
(a^
water; dispersion, dipole-dipole, hydrogen bonding
(b)
hexane; dispersion forces
(c)
water; dispersion, dipole-dipole
(d)
water; dispersion, dipole-dipole, hydrogen bonding
(a)
hexane; dispersion forces
(b)
water; dispersion, dipole-dipole, hydrogen bonding
(c)
hexane; dispersion forces
(d)
water; dispersion, dipole-dipole, hydrogen bonding
12.34
Energetics of Solution Formation 12.35 i
(a)
endothermic
(b)
The lattice energy is greater in magnitude than the heat of hydration.
(c)
*
AH,
AH= -lattice energy r NH/(a AHsoin then mol —» g 101. 11 g
Solution: AH^ = AHsoIute + AHhydration where AHsolute = -AHlattice so AHsoln = AHhydration - AHlattice AHsoln = [-155.5 kcal/mol - (-163.8 kcal/mol)](4. 184 kj/kcal) = 34. 7 kj/ mol then g = n AHsoIn.
