Lecture notes Chapter 15 - Bakersfield College

Chemistry B2A
Chapter 15
Solutions
Solutions: solutions have some properties:
1. The distribution of particles in a solution is uniform. Every part of the solution has exactly
the same composition and properties as every other part (a single phase-homogenous).
2. Solutions are almost always transparent (solid solutions are exceptions).
3. A solution cannot be separated into its components by filtration.
4. The components of a solution do not separate on standing.
5. Solutions can be separated into pure components (distillation, chromatography).
Note: we have different types of solutions: gas in gas (air), liquid in liquid (alcohol in water),
solid in liquid (sugar in water), solid in solid (alloys), and gas in liquid (gas in cokes).
Note: All mixtures of gases are solutions. Because gas molecules are far apart from each
other and much empty space separates them. Whenever we mix solids, we almost always get a
heterogeneous mixture.
Note: a solution consists of two parts:
1. Solvent: the component present in the greater amount in a solution (when one liquid is
dissolved in another).
2. Solute: the component(s) present in the smaller amount in a solution (when one liquid is
dissolved in another).
Miscible: some liquids are completely soluble in other liquids to form a solution (no matter
what quantities of each are mixed) (for example: methanol and water).
Immiscible: some liquids cannot mix together and they produce the different phases (for
example: oil and water).
Saturated solution: when a solution contains all the solute it can hold at a given temperature,
we call the solution saturated.
Unsaturated solution: any solution containing a lesser amount of solute than a saturated
solution at a given temperature is unsaturated (so we can dissolve more solute in the solvent).
Supersaturated solution: when a solution contains more solute in the solvent than it can
normally hold at a given temperature under equilibrium conditions. A supersaturated solution
is not stable; when disturbed in any way, such as by stirring or shaking, the excess solute
precipitates.
Solubility: the maximum amount of a solute that will dissolve in a given amount of a
particular solvent (at a given temperature).
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Note: The more similar two compounds are (similar in term of polarity), the more likely that
one will be soluble in the other. Like dissolves like. Polar compounds dissolve in polar
solvents, and nonpolar compounds dissolve in nonpolar solvents. For example, water
dissolves NaCl (two polar compounds) and CCl 4 dissolves C 6 H 14 (two nonpolar compounds).
Note: For most solids and liquids that dissolve in liquids, solubility increases with increasing
temperature (except for gases, solubility in liquids almost always decreases with increasing
temperature).
Henry’s law: the solubility of a gas in a liquid is directly proportional to the pressure (the
higher the pressure, the greater the solubility of a gas in a liquid). Pressure has little effect on
the solubility of liquids or solids.
Concentration: amount of a solute dissolved in a given quantity of solvent. Many methods of
expressing concentration exist. We learn the three most important here:
1. Percent concentration:
Weight solute
Weight / Volume (W / V) % =
× 100
Volume of solution (mL)
Weight solute
Weight / Weight (W / W) % =
× 100
Weight of solution
Volume solute (mL)
V olume / Volume (V / V) % =
× 100
Volume of solution (mL)
2. Molarity: the number of moles of solute dissolved in 1 liter (L) of solution. The units of
molarity are moles per liter.
or
moles solute (n)
Molarity (M) =
volume of solution (L)
Molarity (M) × V = number of moles (n)
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Note: we can prepare a solution of a given molarity (with a known volume).
Standard Solution: a standard solution is a solution whose concentration is accurately
known. When the appropriate solute is available in pure form, a standard solution can be
prepared by weighing out a sample of solute, transferring it completely to a volumetric flask
(a flask of accurately known volume), and adding enough solvent to bring the volume up to
the mark on the neck of the flask.
3. Pert per million (ppm) and part per billion (ppb): for very dilute solutions.
ppm =
ppb =
g solute
g solvent
g solute
g solvent
× 10
× 10
Dilution: we frequently prepare solutions by diluting concentrated solutions (stock solutions)
rather than by weighing out pure solute.
When we add only solvent during dilution, the number of moles of solute remains unchanged:
M 1 V 1 = moles before dilution
M 2 V 2 = moles after dilution
Therefore,
6
9
M 1 V 1 = M 2 V 2
% 1 V 1 = % 2 V 2 (using percent concentrations)
Note: All nitrates (NO 3
-) and acetate (CH 3 COO - ) are soluble in water.
Note: Most chlorides (Cl - ) and sulfates (SO 4
2-) are soluble in water (except: AgCl, BaSO 4 ,
PbCl 2 , Hg 2 Cl 2, and PbSO 4 ).
Note: Most carbonates (CO 3
2-), phosphates (PO 4
3-) and hydroxides (OH - ) are insoluble in
water (except: NaOH, LiOH, KOH, and NH 3 ).
Hydrate and hydration: when a solid ionic compound is added to water, water molecules
surround the ions at the surface of the crystal. Water is a polar molecule. The negative ions
(anions) attract the positive poles of water molecules, and the positive ions (cations) attract
the negative poles of water molecules. Each ion attracts multiple water molecules and the ion
remove from the crystal. We say ions are hydrated and this phenomenon is called hydration (a
more general term, covering all solvents, is solvated).
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Electrolyte: substances that conduct an electric current when dissolved in water or when in
the molten state are called electrolytes. These substances can be ionized and produce ions.
The positively charged ions (cations) migrate to the negative electrode (cathode) and the
negatively charged ions (anions) migrate to the positive electrode (anode). The movement of
ions constitutes an electric current.
Note: compounds that dissociate (ionize) completely are called strong electrolytes (most of
the ionic compounds and some acids). Compounds that dissociate partially are called weak
electrolytes (such as CH 3 COOH). Compounds that do not dissociate (do not conduct
electricity) are called nonectrolytes (such as distilled water).
Solubility of covalent compounds in water: some acids are soluble in water. Covalent
compounds will dissolve in water if they can form hydrogen bonds with water. In general,
they should have no more than three C atoms for each O or N atom. For example, acetic acid,
CH 3 COOH, is soluble in water, but benzoic acid, C 6 H 5 COOH, is not. The exception to this
generalization is the rare case where a covalent compound reacts with water-for instance,
HCl.
Colloids: in a colloid, the diameter of the solute particles ranges from about 1 to 1000 nm
(this diameter is under 1 nm in a rue solution). The colloids are not uniform and transparent
(they appear cloudy and milky). Colloidal systems are stable and their components do not
separate on standing (for example: milk, butter, smoke, and fog).
Emulsion: a mixture of immiscible substances (liquid-liquid). Emulsion is a type of the
colloidal systems (usually as droplets of larger than colloidal size). The emulsion systems are
usually stable. Milk and mayonnaise are two examples of the emulsion systems.
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Tyndall effect: if light passes through a colloidal system, we can see the pathway of the light
without seeing the colloidal particles themselves (they are too small to see). This method is
used to distinguish a colloid from a solution (because we cannot see the pathway of the light
in a solution).
Brownian motion: colloidal particles are in constant motion in a solvent (randomly). For
example, the motion of the dust particles dispersed in air. This motion creates favorable
conditions for collisions between particles. Why do colloidal particles remain in solution
despite all the collisions
1. Most colloidal particles carry a large solvation layer. They do not actually touch each
other; instead, only their solvent layers collide.
2. All colloids in a particular solution acquire the same kind of charge. Therefore, the
like charges repel each other.
Suspension: when the diameter of the solute particles is greater than 1000 nm, we have a
suspension system. Suspension is not a type of the colloidal systems. The suspension systems
are not stable and separate into phases (for example: sand in water).
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