States of Matter Lesson 4.7 CHEMISTRY 2 HONORS Jeff Venables Northwestern High School

Properties of Solutions Jeff Venables Northwestern High School

The Solution Process A solution is a homogeneous mixture of solute (present in smallest amount) and solvent (present in largest amount). Solutes and solvent are components of the solution. In the process of making solutions with condensed phases, intermolecular forces become rearranged.

Consider NaCl (solute) dissolving in water (solvent): the water H-bonds have to be interrupted, NaCl dissociates into Na+ and Cl-, ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O. We say the ions are solvated by water. If water is the solvent, we say the ions are hydrated.

“Rule”: LIKE DISSOLVES LIKE!!! polar solvents dissolve polar solutes. Non-polar solvents dissolve non-polar solutes. Why? If Hsoln is too endothermic a solution will not form. NaCl in gasoline: the ion-dipole forces are weak because gasoline is non-polar. Therefore, the ion-dipole forces do not compensate for the separation of ions. Water in octane: water has strong H-bonds. There are no attractive forces between water and octane to compensate for the H-bonds.

Solution Formation and Chemical Reactions Example: a mixture of CCl4 and C6H14 is less ordered Consider: Ni(s) + 2HCl(aq)  NiCl2(aq) + H2(g). Note the chemical form of the substance being dissolved has changed (Ni  NiCl2). When all the water is removed from the solution, no Ni is found (only NiCl2·6H2O). Therefore, Ni dissolution in HCl is a chemical process.

NaCl(s) + H2O (l)  Na+(aq) + Cl-(aq). Example: NaCl(s) + H2O (l)  Na+(aq) + Cl-(aq). When the water is removed from the solution, NaCl is found. Therefore, NaCl dissolution is a physical process.

Saturated Solutions and Solubility Dissolve: solute + solvent  solution. Crystallization: solution  solute + solvent. Saturation: crystallization and dissolution are in equilibrium. Solubility: amount of solute required to form a saturated solution. Supersaturated: a solution formed when more solute is dissolved than in a saturated solution.

Factors Affecting Solubility 1. Solute-Solvent Interaction Polar substances tend to dissolve in polar solvents. Miscible liquids: mix in any proportions. Immiscible liquids: do not mix. Intermolecular forces are important: water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are re-established in the mixture. The number of carbon atoms in a chain affect solubility: the more C atoms the less soluble in water.

Solute-Solvent Interaction The number of -OH groups within a molecule increases solubility in water. Generalization: “like dissolves like!!!!”. The more polar bonds in the molecule, the better it dissolves in a polar solvent. The less polar the molecule the less it dissolves in a polar solvent and the better it dissolves in a non-polar solvent.

Solute-Solvent Interaction

Solute-Solvent Interaction

Solute-Solvent Interaction Network solids do not dissolve because the strong intermolecular forces in the solid are not re-established in any solution. 2. Pressure Effects Solubility of a gas in a liquid is a function of the pressure of the gas.

Pressure Effects

The higher the pressure, the more molecules of gas are close to the solvent and the greater the chance of a gas molecule striking the surface and entering the solution. Therefore, the higher the pressure, the greater the solubility. The lower the pressure, the fewer molecules of gas are close to the solvent and the lower the solubility. If Sg is the solubility of a gas, k is a constant, and Pg is the partial pressure of a gas, then Henry’s Law gives:

Pressure Effects Carbonated beverages are bottled with a partial pressure of CO2 > 1 atm. As the bottle is opened, the partial pressure of CO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

3. Temperature Effects Experience tells us that sugar dissolves better in warm water than cold. As temperature increases, solubility of solids generally increases. Sometimes, solubility decreases as temperature increases (e.g. Ce2(SO4)3).

Experience tells us that carbonated beverages go flat as they get warm. Therefore, gases get less soluble as temperature increases. Thermal pollution: if lakes get too warm, CO2 and O2 become less soluble and are not available for plants or animals.