9.1: Solutions

A solution is another name for a homogeneous mixture. A mixture as a material composed of two or more substances. In a solution, the combination is so intimate that the different substances cannot be differentiated by sight, even with a microscope.?

The major component of a solution, called the solvent(溶劑), is typically the same phase as the solution itself. Each minor component of a solution (and there may be more than one) is called the solute(溶液).

Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water).

Solutions form because a solute and a solvent experience similar intermolecular interactions.

9.2: Concentration?濃度

There is usually a limit to how much solute will dissolve in a given amount of solvent. This limit is called the solubility(溶解度) of the solute. Some solutes have a very small solubility, while other solutes are soluble in all proportions. (Solubilities vary with temperature)

If a solution contains so much solute that its solubility limit is reached, the solution is said to be saturated(飽和的).

Under special circumstances, more solute can be dissolved even after the normal solubility limit is reached; such solutions are called supersaturated(過飽和的)?and are not stable. If the solute is solid, excess solute can easily recrystallize(再結(jié)晶). If the solute is a gas, it can bubble out of solution uncontrollably (like what happens when you shake a soda can and then immediately open it).

Precipitation(沉淀) from Supersaturated Solutions

Dilutions(稀釋)

Percent Composition:?

1. The mass/mass percent (% m/m) is defined as the mass of a solute divided by the mass of a solution times 100.

2. For gases and liquids, volumes are relatively easy to measure, so the concentration of a liquid or a gas solution can be expressed as a volume/volume percent (% v/v): the volume of a solute divided by the volume of a solution times 100.

3. Again, the units of the solute and the solution must be the same. A hybrid concentration unit, mass/volume percent (% m/v). It is defined as the mass in grams of a solute, divided by volume in milliliters of solution times 100.

Molarity: Number of moles of solute per liter of solution.

Molarity is abbreviated M (often referred to as “molar”), and the units are often abbreviated as mol/L.?

9.3: The Dissolution Process

When a solute dissolves, its individual particles are surrounded by solvent molecules and are separated from each other.

In the case of molecular solutes like glucose, the solute particles are individual molecules.?

However, if the solute is ionic, the individual ions separate from each other and become surrounded by solvent particles. That is, the cations and anions of an ionic solute separate when the solute dissolves. This process is referred to as?dissociation.

The dissociation of soluble ionic compounds gives solutions of these compounds an interesting property: they conduct electricity. Because of this property, soluble ionic compounds are referred to as electrolytes(電解質(zhì)). Many ionic compounds dissociate completely and are therefore called strong electrolytes(強(qiáng)電解質(zhì)), the compound in vinegar, is a weak electrolyte. Solutes that dissolve into individual neutral molecules without dissociation do not impart additional electrical conductivity to their solutions and are called nonelectrolytes.

9.4: Properties of Solutions

Colligative Properties:

Solutes affect some properties of solutions that depend only on the concentration of the dissolved particles. These properties are called colligative properties. Four important colligative properties that we will examine here are vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure.

1. Vapor Pressure Depression:?

All liquids evaporate. In fact, given enough volume, a liquid will turn completely into a vapor. If enough volume is not present, a liquid will evaporate only to the point where the rate of evaporation equals the rate of vapor condensing back into a liquid. The pressure of the vapor at this point is called the vapor pressure of the liquid.

2. Boiling Point and Freezing Point Effects:?

A related property of solutions is that their boiling points are higher than the boiling point of the pure solvent. Because the presence of solute particles decreases the vapor pressure of the liquid solvent, a higher temperature is needed to reach the boiling point. This phenomenon is called boiling point elevation. For every mole of particles dissolved in a liter of water, the boiling point of water increases by about 0.5°C.

(The presence of solute particles has the opposite effect on the freezing point of a solution.) When a solution freezes, only the solvent particles come together to form a solid phase, and the presence of solute particles interferes with that process. Therefore, for the liquid solvent to freeze, more energy must be removed from the solution, which lowers the temperature. Thus, solutions have lower freezing points than pure solvents do. This phenomenon is called freezing point depression. For every mole of particles in a liter of water, the freezing point decreases by about 1.9°C.

3.?Osmotic Pressure (滲透壓)

Osmotic Pressure: The tendency for solvent molecules to move from the more dilute solution to the more concentrated solution until the concentrations of the two solutions are equal.

9.5: Chemical Equilibrium

Chemical equilibrium can be attained whether the reaction begins with all reactants and no products, all products and no reactants, or some of both.?

Chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after equilibrium has been reached. Because the rates of the reactions are the same, there is no change in the relative concentrations of reactants and products.

Equilibrium Constant (化學(xué)平衡常數(shù))

Reaction Quotient?(反應(yīng)商)

9.6: Le Chatelier's Principle

Le Chatelier's Principle:

When a chemical system that is at equilibrium is disturbed by a stress, the system will respond in order to relieve the stress. Stresses to a chemical system involve changes in the concentrations of reactants or products, changes in the temperature of the system, or changes in the pressure of the system.?

Effect of Concentration:

A change in concentration of one of the substances in an equilibrium system typically involves either the addition or the removal of one of the reactants or products.

If the concentration of one substance in a system is increased, the system will respond by favoring the reaction that removes that substance.?In other words, the amount of each substance is different but the ratio of the amount of each remains the same.

Effect of Temperature:

Increasing or decreasing the temperature of a system at equilibrium is also a stress to the system.

Effect of Pressure:

Changing the pressure of an equilibrium system in which gases are involved is also a stress to the system. A change in the pressure on a liquid or a solid has a negligible effect.?

pressure increase?->?reaction produces fewer gas molecules

pressure decrease -> reaction produces more gas molecules

9.7: Osmosis and Diffusion

1.?Osmosis(滲透)

Osmosis is the diffusion of water molecules across a semipermeable membrane from an area of lower concentration solution (i.e., higher concentration of water) to an area of higher concentration solution (i.e., lower concentration of water). Water moves into and out of cells by osmosis.

2.?Diffusion(擴(kuò)散)

Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration.?

Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient". The end result is an equal concentration, or equilibrium, of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop. At equilibrium, there is equal movement of materials in both directions.

References:

LibreTexts

Labster Theory