Solutions

Solutions

Chapter 13

Solution Process

Solution – homogeneous mixture of solute and solvent Can be gases, liquids, or solids Solvent is component present in the largest amount

(rest are solutes) Intermolecular forces are rearranged with

solutions of condensed phases. Ex. NaCl in Water – Water molecules orient

themselves on NaCl; H-bonds between waters have to be broken; NaCl dissociates; ion-dipole forces form; interaction is called solvation or hydration

The Solution Process

Energy Changes

Three steps involving energy in the formation of a solution Separation of solute molecules (∆H1) Separation of solvent molecules (∆H2) Formation of solute-solvent interactions (∆H3)

Enthalpy change in the solution process ∆Hsoln = ∆H1 + ∆H2 + ∆H3

∆Hsoln can be + or – Breaking attractive intermolecular forces is always endothermic

(+) Forming attractive intermolecular forces is always exothermic

(-)

Energy changes

Energy Changes and Solution Formation

In general, solutions form if ∆Hsoln is negative. If ∆Hsoln is too endothermic a solution will not

form. “Rule of Thumb” – Like dissolves Like

Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes

Ex. NaCl in gasoline: Only weak interactions are possible because gasoline is nonpolar; interactions do not compensate for separation of ions; NaCl does not dissolve to any great extent in gasoline.

Solution Formation:Spontaneity and Disorder

Spontaneous process occurs without outside intervention Energy of system decreases – spontaneous Tend to be exothermic Some are endothermic

Entropy is a measure of randomness or disorder of a system. Solution formation is favored by the increase in entropy

that accompanies solution formation (most cases) A solution can spontaneously mix when Esoln is not

significantly decreasing when entropy is increasing.

Solution Formation:Chemical vs. Physical

Solutions can be created physically or chemically.

Ni(s) + 2HCl(aq) NiCl2(aq) + H2(g)

Chemical form has changed (Ni NiCl2) When water is removed, no Ni is found (only

NiCl2 remains)

NaCl(s) + H2O(l) Na+(aq) + Cl-

(aq)

When water is removed from the solution, NaCl is found.

Solubility

Dissolution – formation of a solution Crystallization – “destruction” of a solution

If these processes are in equilibrium, the solution is saturated.

Solubility is the amount of solute required to form a saturated solution. Unsaturated Supersaturated

Factors affecting Solubility

Nature of the solute and solvent Intermolecular forces – favorable? Miscible vs. immiscible

Temperature Generally, as temperature increases, solubility increases. Sometimes not the cases – gases are less soluble at higher

temperatures Pressure

Solubility of a Gas is a function of the pressure of the gas over the solution.

Increase pressure, increase solubility Henry’s Law – Sg = kPg

Solubility: Pressure

Concentration

Amount of solute per amount of solvent Qualitative – dilute vs. concentrated Quantitative –

Mass percentage Mole fraction Molarity Molality

Concentration

100solution of mass total

solutionin component of masscomponent of % mass

910solution of mass total

solutionin component of masscomponent of ppb

solution of moles totalsolutionin component of molescomponent offraction Mole

solution of literssolute molesMolarity

solvent of kgsolute moles Molality, m

Molality – Molarity

Colligative Properties

Colligative properties depend on quantity of solute molecules. (E.g. freezing point depression and melting point

elevation.) Lowering Vapor Pressure

Non-volatile solutes reduce the ability of the surface solvent molecules to escape the liquid.

Therefore, vapor pressure is lowered. The amount of vapor pressure lowering depends

on the amount of solute.

Vapor Pressure


Lowering Vapor Pressure Raoult’s Law: PA is the vapor pressure with

solute, PA is the vapor pressure without solute, and A is the mole fraction of A, then

Recall Dalton’s Law:

AAA PP

totalPP AA


Lowering Vapor Pressure Ideal solution: one that obeys Raoult’s law. Raoult’s law breaks down when the solvent-

solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces.

Boiling-Point Elevation Goal: interpret the phase diagram for a solution. Non-volatile solute lowers the vapor pressure. Therefore the triple point - critical point curve is

lowered

Colligative Properties: Boiling-Point Elevation

At 1 atm (normal boiling point of pure liquid) there is a lower vapor pressure of the solution. Therefore, a higher temperature is required to teach a vapor pressure of 1 atm for the solution (Tb).

Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m:

mKT bb

Colligative Properties:Freezing Point Depression

At 1 atm (normal boiling point of pure liquid) there is no depression by definition

When a solution freezes, almost pure solvent is formed first. Therefore, the sublimation curve for the pure

solvent is the same as for the solution. Therefore, the triple point occurs at a lower

temperature because of the lower vapor pressure for the solution.

Colligative Properties:Freezing Point Depression

• The melting-point (freezing-point) curve is a vertical line from the triple point.

• The solution freezes at a lower temperature (Tf) than the pure solvent.

• Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant):

mKT ff

Osmosis

Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane.

Osmosis: the movement of a solvent from low solute concentration to high solute concentration.

There is movement in both directions across a semipermeable membrane.

As solvent moves across the membrane, the fluid levels in the arms becomes uneven.

Osmosis

Eventually the pressure difference between the arms stops osmosis.

Osmosis

Osmotic pressure, , is the pressure required to stop osmosis:

Isotonic solutions: two solutions with the same separated by a semipermeable membrane.

MRT

RTVnnRTV

Osmosis

Hypotonic solutions have a lower than a more concentrated solution.

Hypertonic solutions have a higher than a more dilute solution.

Osmosis is spontaneous.

Ex. Red blood cells are surrounded by semipermeable membranes.

Crenation: red blood cells placed in hypertonic solution

(relative to intracellular solution); there is a lower solute concentration in the cell

than the surrounding tissue; osmosis occurs and water passes through the

membrane out of the cell. The cell shrivels up.



Hemolysis: red blood cells placed in a hypotonic solution; there is a higher solute concentration in the cell; osmosis occurs and water moves into the cell. The cell bursts.

To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic.

Colloids

Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due to gravity.

Particle size: 10 to 2000 Å.

Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid.

Types of Colloids

aerosol (gas + liquid or solid, e.g. fog and smoke),

foam (liquid + gas, e.g. whipped cream), emulsion (liquid + liquid, e.g. milk), sol (liquid + solid, e.g. paint), solid foam (solid + gas, e.g. marshmallow), solid emulsion (solid + liquid, e.g. butter), solid sol (solid + solid, e.g. ruby glass).

Hydrophobic and Hydrophilic Colloids

Focus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic.

Molecules arrange themselves so that hydrophobic portions are oriented towards each other.

Hydrophobic and Hydrophilic Colloids

Typical hydrophilic groups are polar (containing C-O, O-H, N-H bonds) or charged.

Hydrophobic colloids need to be stabilized in water.

Adsorption: when something sticks to a surface we say that it is adsorbed. If ions are adsorbed onto the surface of a colloid,

the colloids appears hydrophilic and is stabilized in water.

Removal of Colloid Particles

Colloid particles are too small to be separated by physical means (e.g. filtration).

Colloid particles are coagulated (enlarged) until they can be removed by filtration.

Methods of coagulation: heating (colloid particles move and are

attracted to each other when they collide); adding an electrolyte (neutralize the surface

charges on the colloid particles).

Dialysis: Removal of Colloids

Semipermeable membrane is used to separate ions from colloidal particles.

Blood passes through a membrane immersed in a washing solution. Washing solution is isotonic with ions that must

be retained. Washing solution does not contain waste products.

Washing solution removes (dialyzes) wastes and the “good” ions remain.

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