solutions

For each compound, would you expect greater solubility in water or in hexane? Indicate the kinds of intermolecular forces that would occur between the solute and the solvent in which the molecule is most soluble. a. toluene b. sucrose c. isobutene d. ethylene glycol

(a)hexane; dispersion forces (b)water; dispersion, dipole-dipole, hydrogen bonding (c)hexane; dispersion forces (d)water; dispersion, dipole-dipole, hydrogen bonding

When lithium iodide (LiI) is dissolved in water, the solution becomes hotter. Is the dissolution of lithium iodide endothermic or exothermic? What can you conclude about the relative magnitudes of the lattice energy of lithium iodide and its heat of hydration? Why does the solution form? What drives the process?

1. exothermic 2. The lattice energy is smaller in magnitude than is the heat of hydration. 3. The solution forms because chemical systems tend toward lower energy and greater entropy.

Explain why the lower vapor pressure for a solution containing a nonvolatile solute results in a higher boiling point and lower melting point compared to the pure solvent.

A nonvolatile solute lowers the vapor pressure of a solution relative to that of the pure solvent. The vapor pressure lowering occurs at all temperatures, which shifts the vaporization curve in the phase diagram. This means that the temperature must be raised above the pure solvent normal boiling point for the solution vapor pressures to be raised to 1 atm. This shift also results in a lowering of where the vapor pressure curve intersects the solid-gas curve. The net effect is that the solution has a lower melting point and a higher boiling point than the pure solvent.

Explain how the relative strengths of solute-solute interactions, solvent-solvent interactions, and solvent-solute interactions affect solution formation.

A solution always forms if the solvent-solute interactions are comparable to, or stronger than, the solvent-solvent interactions and the solute-solute interactions.

what is a solution? what are the solute and solvent?

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.

What does it mean to say that a substance is soluble in another substance? Which units are used in reporting solubility?

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.

endothermic

Absorbs heat; overcoming all attractions among solute and solvent particles

How does the solubility of a solid in a liquid depend on temperature? How is this temperature dependence exploited to purify solids through recrystallization?

Although there are exceptions, the solubility of most solids in water increases with increasing temperature. Recrystallization is a common technique to purify a solid. In this technique, the solid is put into water (or some other solvent) at an elevated temperature. Enough solid is added to the solvent to create a saturated solution at the elevated temperature. As the solution cools, it becomes supersaturated and the excess solid begins to come out of solution. If the solution cools slowly, the solid forms crystals as it comes out of solution. The crystalline structure tends to reject impurities, resulting in a purer solid. The solvent is chosen so that the solubility of the impurities is high at the lower temperature, reducing the tendency to co-precipitate.

Explain the difference between an ideal and a nonideal solution.

An ideal solution follows Raoult's law at all concentrations for both the solute and the solvent. A nonideal solution exhibits deviations from Raoult's law in the vapor pressure of a component as the mole fraction of this component decreases from 1 (i.e., the pure component).

which molecule would you expect to be more soluble in water: CCL4 or CH2CL2

CH2Cl2 would be more soluble in water because it is a polar molecule and can exhibit dipole-dipole interactions with the water molecules. CCl4 is a nonpolar molecule.

What are colligative properties? What is osmosis? What is osmotic pressure?

Colligative properties depend on the amount of solute and not the type of solute. Examples of colligative properties are vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. Osmosis is defined as the flow of solvent from a solution of lower solute concentration to one of higher solute concentration through a semipermeable membrane—a membrane that selectively allows some substances, but not others, to pass through. The osmotic pressure is the pressure required to stop the osmotic flow and is given by the following equation

what is entropy? why is entropy important in discussing the formation of solutions

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 because the components are no longer segregated to separate regions. This makes the formation of a solution energetically favorable, even when it is endothermic.

Scuba divers breathing air at increased pressure can suffer from oxygen toxicity—too much oxygen in their bloodstream—when the partial pressure of oxygen exceeds about 1.4 atm. What happens to the amount of oxygen in a diver's bloodstream when he or she breathes oxygen at elevated pressures? How can this be reversed?

Henry's law says that as pressure increases, oxygen will more easily dissolve in blood. To reverse this process, divers should ascend to lower pressures or breathe special gas mixtures with lower oxygen levels.

Why do two ideal gases thoroughly mix when combined? What drives the mixing?

Ideal gases do not interact with each other in any way (i.e., 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 two ideal gases mix.

What is the effect on vapor pressure of a solution with particularly strong solute-solvent interactions? With particularly weak solute-solvent interactions?

If the solute-solvent interactions are particularly strong (stronger than solvent-solvent interactions), the solute tends to prevent the solvent from vaporizing as easily as it would otherwise and the vapor pressure of the solution will be less than that predicted by Raoult's law. If the solute-solvent interactions are particularly weak (weaker than solvent-solvent interactions), the solute tends to allow more vaporization than would occur with just the solvent and the vapor pressure of the solution will be greater than predicted by Raoult's law.

when is the enthalpy of a solution zero?

If the sum of the endothermic terms is approximately equal in magnitude to the exothermic term

when is the enthalpy of a solution positive?

If the sum of the endothermic terms is greater in magnitude than the exothermic term, then ΔHsolnΔHsoln is positive and the solution process is endothermic.

when is the enthalpy of a solution negative?

If the sum of the endothermic terms is smaller in magnitude than the exothermic term, then ΔHsolnΔHsoln is negative and the solution process is exothermic.

Explain dynamic equilibrium with respect to solution formation. What is a saturated solution? An unsaturated solution? A supersaturated solution?

In any solution formation, the initial rate of dissolution far exceeds the rate of deposition. But as the concentration of dissolved solute increases, the rate of deposition also increases. Eventually, the rate of dissolution and deposition become equal—dynamic equilibrium has been reached. A saturated solution is a solution in which the dissolved solute is in dynamic equilibrium with the solid (or undissolved) solute. If you add additional solute to a saturated solution, it will not dissolve. An unsaturated solution is a solution containing less than the equilibrium amount of solute. If you add additional solute to an unsaturated solution, it will dissolve. A supersaturated solution is a solution containing more than the equilibrium amount of solute. Such solutions are unstable and the excess solute normally precipitates out of the solution. However, in some cases, if left undisturbed, a supersaturated solution can exist for an extended period of time.

Exothermic

Releases heat; form new attractions among solute particles and solvent molecules

What are the three steps involved in evaluating the enthalpy changes associated with solution formation?

Step 1: Separate the solute into its constituent particles. This step is always endothermic (positive ) because energy is required to overcome the forces that hold the solute together. Step 2: Separate the solvent particles from each other to make room for the solute particles. This step is also endothermic because energy is required to overcome the intermolecular forces among the solvent particles. Step 3: Mix the solute particles with the solvent particles. This step is exothermic because energy is released as the solute particles interact with the solvent particles through the various types of intermolecular forces.

what does the statement like dissolves like mean with respect to solution formation?

The expression "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.

How does the solubility of a gas in a liquid depend on pressure? How does this pressure dependence account for the bubbling that occurs upon opening a can of soda?

The higher the pressure of a gas above a liquid, the more soluble the gas in the liquid. In a sealed can of soda pop, for example, the carbon dioxide is maintained in solution by a high pressure of carbon dioxide within the can. When the can is opened, this pressure is released and the solubility of carbon dioxide decreases, resulting in bubbling.

Solubility

The maximum amount of solute that can be dissolved in a given amount of solvent

how does the solubility of a gas in a liquid depend on temperature?

The solubility of gases in liquids decreases with increasing temperature.

a solution contains 32 g of KNO3 per 100.0 g of water at 25 °C. Is the solution unsaturated, saturated, or supersaturated?

The solution is almost saturated because we are dissolving 32 g of KNO3 per 100 g of water and the solubility from the figure is ∼36gKNO3per100g of water at 25∘C.

What is the effect of a nonvolatile solute on the vapor pressure of a liquid? Why is the vapor pressure of a solution different from the vapor pressure of the pure liquid solvent?

The vapor pressure of the solution is lower than the vapor pressure of the pure solvent. The simplest explanation for the lowering of the vapor pressure of a solution relative to that of the pure solvent is related to the concept of dynamic equilibrium. In dynamic equilibrium, the rate of vaporization is equal to the rate of condensation. When a nonvolatile solute is added, however, the solute particles interfere with the ability of the solvent particles to vaporize because they occupy some of the surface area formerly occupied by the solvent. The rate of vaporization is therefore diminished compared to that of the pure solvent. The change in the rate of vaporization creates an imbalance in the rates; the rate of condensation is now greater than the rate of vaporization. The net effect is that some of the molecules that were in the gas phase condense into the liquid. As they condense, the reduced number of molecules in the gas phase causes the rate of condensation to decrease. Eventually, the two rates become equal again, but only after the concentration of molecules in the gas phase has decreased, which means a lower vapor pressure for the solution compared to the pure solvent.

what kind of intermolecular forces are involved in solution formation

Whether two substances will spontaneously mix to form a homogeneous solution is dependent upon a number of different types of intermolecular forces, including dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole forces.

Explain the meaning of the van't Hoff factor and its role in determining the colligative properties of solutions containing ionic solutes.

a ratio of moles of particles in solutions to moles of formula units.The measured van't Hoff factor is generally less than the theoretical due to ion pairing in solution. Therefore, the measured van't Hoff factors often cause the ΔT to be smaller than one might expect.

When ammonium chloride (NH4Cl) is dissolved in water, the solution becomes colder. Is the dissolution of ammonium chloride endothermic or exothermic?

endothermic

What is osmosis? What is osmotic pressure?

osmosis is defined as the flow of solvent from a solution of lower solute concentration to one of higher solute concentration through a semipermeable membrane- a membrane that selectively allows some substances but not others to pass through. the osmotic pressure is the pressure required to stop the osmotic flow and is given by the following equation pi=MRT

How are parts by mass and parts by volume used in calculations?

parts by mass and parts by volume are ratios of masses and volume, respectively. A parts by mass concentration is the ratio of the mass of the solute to the mass of the solution, all multiplied by a multiplication factor, where percent by ass is the desired unit, the factor = 100; where parts per million ppm is the desired unit the factor =10^6 and for ppb the factor is 10^9 the size of the multiplication factor depends on the concentration of the solution

A KCl solution containing 42 g of KCl per 100.0 g of water is cooled from 60 °C to 0 °C. What happens during cooling?

precipitate out of solution

What is Raoult's law? For what kind of calculations is Raoult's law useful?

quantifies the relationship between the vapor pressure of a solution and its concentration as Psolution = Xsolvent Psolvent, where Psolution is the vapor pressure of the solution, Xsolvent is the mole fraction of the solvent, and Psolvent is the vapor pressure of the pure solvent. This equation allows you to calculate the vapor pressure of a solution or to calculate the concentration of a solution, given the vapor pressure of the solution

miscible

two liquids that are mutually soluble

insoluble

when one substance does not dissolve in another ex: oil is insoluble in water