Chemistry Solutions Overview

Solutions

• A solution is a homogenous mixture of two or more substances, where the substance present in the largest amount is the solvent and the other substances are considered solutes.
• Solutes dissolve in solvents.
• For example, water is a solvent in a solution with salt and a solute in a solution with sugar.
• An exception to the rule is concentrated sulfuric acid, in which water is the solvent, even though it makes up only 4% of the solution.
• A concentrated solution contains a larger proportion of solute to solvent, while a diluted solution contains a smaller proportion.
• A saturated solution is a solution where the solvent can dissolve no more solute at a particular temperature. The amount of solute that reaches saturation is defined as the solutes solubility.
• An unsaturated solution contains less solute than required for saturation.
• For example, at 25°C, 35.7 g of NaCl per 100 mL of water will result in a saturated solution while 20 g of NaCl per 100 mL of water will be an unsaturated solution.
• If we have a 20 g/100 mL solution of NaCl, we need to add 15.7 g to make the solution saturated at 25°C.
• It is important to note that saturated solutions aren't always concentrated and vice versa.
• For example, a saturated solution of AgCl is 0.000089 g AgCl/100 mL, which is considered very dilute.
• However, a saturated solution of Lithium Chlorate is 500 g/100 mL, which is considered concentrated.
• A supersaturated solution is unstable, it contains more solute than ordinarily required for saturation. It will reach the point of saturation if:
  • A crystal (seed) of solid is added, extra solute crystallizes on the seed crystal.
  • The solution is further cooled.

Electrolytes and Nonelectrolytes

• Nonelectrolytes are molecular substances that lack the tendency to dissociate in water. Their solutions do not conduct electricity. For example, sugar and ethyl alcohol.
• Electrolytes dissociate in water, producing ions that conduct electricity.
  • Strong electrolytes completely dissociate (ionize) in aqueous solutions. They conduct electricity well. For example, NaCl and HCl (strong acids and bases).
  • Weak electrolytes dissociate to a small extent in aqueous solutions. They are weakly conductive to electric current. For example, acetic acid (weak acids and bases).
    • Acetic acid ionizes to only 0.42% w/v.
  • Other examples of weak electrolytes are NH4OH (ammonium hydroxide) and HCN (hydrogen cyanide).
  • Even water is a very weak electrolyte, only ionizing to 10-7 M.

Acids and Bases

• Acids donate protons (H+) upon ionization (Arrhenius theory) and donate protons upon ionization (Bronsted-Lowry theory).

• Acids can also accept a lone-pair of electrons from a base (Lewis theory).

• When an acid ionizes, the conjugate base is formed.

  • For example: HCl ionizes to Cl- (chloride).

• Oxoacids are acids that contain Oxygen, such as HNO3, H2SO4, and H3PO4.

• Monoprotic acids give away one H+ (example: HCl, HNO3).

• Dioprotic acids give away two H+ (example: H2SO4).

• Triprotic acids give away three H+ (example: H3PO4).

• Acidic anhydrides are acids without water, such as CO2 and SO2.

• Acid salts form when a base reacts with a polyprotic acid. For example, salts of H2SO4 (NaHSO4,Na2SO4) and salts of H3PO4 (Na2HPO4, NaH2PO4, Na3PO4).

• Bases produce OH- upon ionization (Arrhenius Theory) and accept protons (H+) upon ionization (Bronsted-Lowry theory).

• Bases can also donate a lone pair of electrons to form a coordination bond (Lewis Theory).

• For example, NaOH, KOH, NH4OH (NH3).

• Basic anhydrides are metal oxides, such as Na2O and CaO.

Amphoteric Substances

• Amphoteric substances act as both acids and bases. Examples are some metal hydroxides.
• For example, Zn(OH)2:
  • Can react with NaOH, producing [Zn(OH)4]2- (tetrahydroxozincate complex).
  • Can also dissolve in strong acids, producing Zn2+ and water.
• Another example is Cr(OH)3:
  • Can react with OH-, producing [Cr(OH)4]-
  • Can react with H+, producing Cr3+ and water.
• The Isoelectric point refers to the point where acidic ionization and basic ionization are equal.
• Amphoteric substances have analytical applications, such as separating Al3+ from Fe3+ by adding excess NaOH, where Al3+ is precipitated as Al(OH)3, then dissolves in NaOH to form soluble sodium aluminate.
• Similarly, Zn2+ can be separated from Mn2+ using the same principle.