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`Chemistry 121 Chapter 4 Study material for this Chapter: Textbook sections 4.1 through 4.6 AQUEOUS REACTIONS AND SOLUTION STOICHIOMETRY 4.1 General Properties of Aqueous Solutions Terms: Solvent: the medium in which the dissolution process takes place. o Solute: the substance that dissolves in the solvent. o Solution: the combined situation after the solute has dissolved in the solvent. Many different solvents exist: o Liquid solvents: water, ethanol, methanol, hexane, liquid ammonia, gasoline, etc. o Gaseous solvents: oxygen, hydrogen, carbon dioxide, etc. o Solids: platinum, iron, etc. Aqueous solution – any solution with liquid water as the solvent. Water is a highly polar molecule. Electrolytic properties All aqueous solutions can be classified in terms of whether or not they conduct electricity. If a substance forms ions in solution, then the substance is an electrolyte and the solution conducts electricity. An example is NaCl. It a substance does not form ions in solution, then the substance is a non- electrolyte and the solution does not conduct electricity. An example is sucrose. Ionic Compounds in Water The ionic solids dissociates into its constituent ions. Note that these ions do not change in any way. They stay as they are. 1
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In the solution these ions are surrounded by water molecules because of electrostatic attraction. The individual ions “float” in the water. The dissolved ions are indicated by (aq), showing they are surrounded by water molecules. Note there is no chemical reaction between the dissolved ions and the water molecules. The dissolution process is a physical change. Molecular Compounds in Solution The molecular solute can be solid, liquid or gaseous. The solute molecules are now surrounded by the water molecules. Note that these molecules do not change in any way. They stay as the are. The solute molecules can be polar or non-polar. They do not have formal electrical charges like ions. The extent of the electrostatic attraction between the solute molecules and water molecules depends on the polarity of the solute molecules. Some molecules can react with water to yield ions. Examples are acids and bases. o For example, NH 3 (g) reacts with water to form NH 4 + (aq) and OH - (aq). o For example, HCl(g) in water ionizes to form H + (aq) and Cl - (aq) Strong and Weak Electrolytes Electricity is conducted through a solution by the movement of ions. A solution without any ions does not conduct electricity. A substance of which the aqueous solution can conduct electricity is called an electrolyte. The solution is called an electrolytic solution. 2
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