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Lecture 11

Solutions

Shaun Williams, PhD

The Composition of Solutions

Making a Solution

A photograph showing table salt, sodium chloride, being poured into a beaker of water.

Various Solutions

Photograph of adding sodium chloride solid to water, bubbling hydrogen chloride gas into water, and adding solid sodium hydroxide to water.

Solubility

Solubility Rules

# Ions Rules
1 \( \chem{Na^+,\, K^+,\, NH_4^+} \) Most salts of sodium, potassium, and ammonium ions are soluble.
2 \( \chem{NO_3^-} \) All nitrates are soluble.
3 \( \chem{SO_4^{2-}} \) Most sulfates are soluble. Exceptions: \(\chem{BaSO_4}\), \(\chem{SrSO_4}\), \(\chem{PbSO_4}\), \(\chem{CaSO_4}\), \(\chem{Hg_2SO_4}\), & \(\chem{Ag_2SO_4}\)
4 \( \chem{Cl^-,\, Br^-,\, I^-} \) Most chlorides, bromides, and iodides are soluble. Exceptions: \(\chem{AgX}\), \(\chem{Hg_2X}\), \(\chem{PbX_2}\), & \(\chem{HgI_2}\) (\(\chem{X = Cl,\, Br,\, or\, I)}\)
5 \( \chem{Ag^+} \) Silver salts are insoluble.
6 \( \chem{O^{2-},\, OH^-} \) Oxides and hydroxides are insoluble. Exceptions: \(\chem{Ba(OH)_2}\) & \(\chem{Ca(OH)_2}\) (somewhat soluble)
7 \( \chem{S^{2-}} \) Sulfides are insoluble. Exceptions: Salts of alkaline metals and alkaline earth metal ions
8 \( \chem{CrO_4^{2-}} \) Most chromates are insoluble. Exceptions: Salts of \(\chem{Mg^{2+}}\), \(\chem{Ca^{2+}}\), \(\chem{Al^{3+}}\), and \(\chem{Ni^{2+}}\)
9 \( \chem{CO_3^{2-},\, PO_4^{2-},\, SO_3^{2-},\, SiO_3^{2-}} \) Most carbonates, phosphates, sulfites, and silicates are insoluble.

Electrolytes

Examples of Electrolytes

Adding nitric acid, sodium chloride, sodium hydroxide, or hydrochloric acid to water allows the solution to conduct electricity well.

Like Dissolves Like

Solute Solvent
Polar Nonpolar
Ionic Soluble Insoluble
Polar Soluble Insoluble
Nonpolar Insoluble Soluble

The Solution Process

When table salt is added to water, the water pulls apart the sodium cations and the chloride anions.

Solution Process

  • Ion-dipole force
    • An attraction between an ion and a polar molecule
  • When an ionic compound dissolves in water:
    • Ionic bonds in the solute break.
    • Hydrogen bonds between water molecules break.
    • Ion-dipole forces form between ions and water molecules.
Images of a sodium cation surrounded by six water molecules pointing the negative end of their dipole at the ion and a chloride anion surrounded by six water molecules pointing the positive end of their dipoles are the anion.

The Energetics of the Solution Process

It takes every to break up the molecules of the solid and liquid and you get energy when they are then mixed.

Heat of Solution

Endothermic Solution Process

It takes more energy to break up the solute and the solvent than you get out when the solute and solvent are mixed. Therefore the process is endothermic.

Solution Process for Nonpolar-Nonpolar Interactions

Purple crystals of solid iodine are mixed with clear liquid carbon tetrachloride to make a purple solution.

Entropy

Entropy in Dissolving

Pure water and crystalling table salt are quite order and therefore have low entropy. When the salt dissolves in the water, the solution has a very high entropy.

Factors that Affect Solubility

  • Three factors affect solubility:
    • Structure
    • Temperature
    • Pressure
Carbon tetrachloride and hexane are both nonpolar so when they are poured together they dissolved into a solution.

Structure

  • For a solid to be soluble in a given solvent, the energy released by the solvation process must compensate for all of the energy required to break up the forces at work both within the solute and the solvent.
    • Entropy is also a factor
  • The solubility (or miscibility) of liquids in other liquids depends largely on the polarity of the solute and solvent molecules.
Long polar molecules do not dissolved in water while small polar molecules do dissolve in water.

Temperature

Plots showing that as the temperature of the solution increases, the amount of solids that can dissolve in the liquid increase while the amount of gases that can dissolve in the hotter liquid decreases.

Pressure

Pressure Effect in Action

A cylinder contains a liquid and gas. When the piston is compressed, thereby conpressing the gas, the amount of the gas that can dissolve in the liquid increases.

Measuring Concentrations of Solutions

Saturated Solution

  • A solution that contains the maximum possible amount of solute
  • When this type of solution forms, a dynamic equilibrium is established.
  • Undissolved solute continues to dissolve, but at the same time, previously dissolved solute is deposited from solution.
A beaker containing a clear liquid and a yellow solid, lead(II) iodide.

Supersaturated Solution

A clear supersaturated solution has a small seed crystal dropped into this. This causes all of the excess solute to rapidly precipiate out of the solution.

Concentration Units

Unit Definition
Percent by mass \( \frac{\text{grams of solute}}{\text{grams of solution}} \times 100\% \)
Percent by volume \( \frac{\text{volume of solute}}{\text{volume of solution}} \times 100\% \)
Mass/volume percent \( \frac{\text{grams of solute}}{\text{volume of solution}} \times 100\% \)
Parts per million \( \frac{\text{grams of solute}}{\text{grams of solution}} \times 10^6 \)
Parts per billion \( \frac{\text{grams of solute}}{\text{grams of solution}} \times 10^9 \)
Molarity (\( M\)) \( \frac{\text{moles of solute}}{\text{liters of solution}} \)
Molality (\( m \)) \( \frac{\text{moles of solute}}{\text{kilograms of solvent}} \)

Percent by Mass

Percent by Volume

Mass/Volume Percent

Parts per Million and Billion

Molarity

Molality

Quantities for Reactions That Occur in Aqueous Solution

Precipitation Reactions

Acid-Base Titrations

Acid-Base Titration Observation

When a clear acid is added to a clear base, nothing changes even though the reaction is occuring. The addition of the indicator cause the color to change from clear to purple when the moles of acid and moles of base are equal.

Colligative Properties

Osmotic Pressure

  • Osmosis
    • A process in which solvent molecules diffuse through a barrier that does not allow the passage of solute particles
  • The barrier is called a semipermeable membrane.
    • A membrane that allows the passage of some substances but not others
  • Osmotic Pressure
    • Pressure that can be exerted on the solution to prevent osmosis
A membrane with holes seperating an aqueous solution from pure water. Water flows from the pure water side to the solution side.

Measuring Osmotic Pressure

A semipermeable membrane at the bottom of a glass tube bent into a U shape. Osmosis forces one toward the solution side thereby making the water level higher on that side.

Vapor Pressure Lowering

Sealed containers of pure water and a solution of a nonvolatile solute. The amount of water vapor above the pure water is higher than the amount of water vapor above the pure water.

Effect of Vapor Pressure Lowering

Two beakers, one with pure water and the other a sugar solution, are placed in a sealed container. Over time, the volume of pure water drops while the volume of the sugar solution increases.

Phase Diagram

A plot of pressure versus temperature for water showing the regions where solid, liquid, and gases exist. When a solute is added, the triple point is reduced thereby shifting the melting line to lower temperatures and shifting the boiling line to higher temperatures.

Boiling Point Elevation

Freezing Point Depression

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