Solubility Rules and Dissolving Process

• Chapter 11 Lecture I discusses the properties of solutions.
• The key topics covered include types of solutions, factors affecting solubility, energies of solution formation, solution composition which is basically the concentration unit, and colligative properties.
• Colligative properties include Vapor Pressure Lowering, Boiling Point Elevation, Freezing Point Depression and Osmotic Pressure.

Definitions Associated with Solutions

• A solution is a homogeneous mixture.
• A solute is dissolved in a solvent.
• The solute is the substance being dissolved.
• The solvent is the liquid in which the solute is dissolved.
• An aqueous solution has water as the solvent.
• Dissolution is a physical change so the original solute can be retrieved by evaporating the solvent.
• If the substance doesn't dissolve, it reacted.

Solubility Explained

• Solubility quantifies how much of a substance, the solute, can dissolve in a given amount of another substance, the solvent.
• Solutions need to form a homogeneous mixture under specific temperature and pressure conditions.
• Solubility is expressed as the maximum amount of solute that can dissolve in a given quantity of solvent.
• Solubility is measured in units of grams of solute per 100 mL of solvent, shown as g/100mL.
• Sparingly soluble substances have solubility of less than 0.1 g/100 mL, while slightly soluble are between 0.1 - 1 g/100 mL, and soluble substances are above 1 g/100 mL.
• Table salt (NaCl) has a solubility of 35.7g/100 mL water at 25 °C.
• Table sugar (sucrose) has a solubility of 211g/100 mL water at 25 °C.
• Aspirin (acetylsalicylic acid) has a solubility of 0.33g/100 mL water at 25 °C.
• CO₂ has a solubility of 0.145 g/100 mL water at 25 °C.

Types of Solutions

• Solution types are determined by the states of the solute and solvent.
• Air is a gas solute dissolved in a gas solvent.
• Vodka is a liquid solute dissolved in a liquid solvent.
• Brass is a solid solute dissolved in a solid solvent.
• Carbonated water is a gas solute dissolved in a liquid solvent.
• Seawater is a solid solute dissolved in a liquid solvent.
• Hydrogen in platinum is a gas solute dissolved in a solid solvent.
• Saturated solutions contain the maximum amount of a solute that can dissolve in a given solvent at a specific temperature.
• Saturated solutions are at equilibrium, where the rate of dissolving equals the rate of crystallization.
• Salt continues to dissolve and crystallize at the same rate, which causes no change.
• Unsaturated solutions contain less solute than the solvent can dissolve at a specific temperature.
• Supersaturated solutions of a chemical compound in a liquid are formed when the temperature of a saturated solution changes.
• Usually, solubility decreases when temperature decreases, causing excess solute to rapidly separate as crystals or an amorphous powder.
• Some solutions, like sodium acetate (CH3COONa), do not quickly form crystals, as the solution remains supersaturated after cooling due to thermodynamic barriers.
• Recrystallization of the excess dissolved solute in a supersaturated solution can be started by adding a tiny crystal of solute called a seed crystal.
• The seed crystal provides a site for excess dissolved crystals to grow.
• Miscible and immiscible describe a liquid's ability to form a solution in a given solvent.
• Alcohol is miscible in water, while oil is immiscible in water.
• Soluble and insoluble describe a solid's ability to form a solution in a given solvent
• Salt is highly soluble in water, while chalk is insoluble in water.

Energies of Solution Formation

• Like dissolves like, is a rule for solubility.
• A polar solvent is used to dissolve a polar or ionic solute.
• A non-polar solvent is used to dissolve a nonpolar solute.
• The formation of a liquid solution is considered to occur in three steps.
• The three steps in solution formation are separating the solute into its individual components by expanding the solute, and the solvent by overcoming intermolecular forces to make room for the solute, and allowing interaction to form the actual solution.
• Enthalpy measures heat energy in a chemical reaction at constant pressure with the symbol H.
• Units for Enthalpy are joules (J) or kilojoules (kJ).
• Change in Enthalpy (ΔΗ) represents the energy change as heat is during the reaction.
• When a reaction releases heat, the value of ΔΗ is negative and the reaction is exothermic.
• When a reaction absorbs heat, the value of ΔΗ is positive and the reaction is endothermic.
• Steps 1 (ΔH₁) and 2 (ΔH₂) require energy because forces must be overcome to expand the solute and solvent.
• Step 3 (ΔH₃) usually releases energy.
• Steps 1 and 2 are endothermic, while step 3 is often exothermic.
• Separation of solute and solvent requires energy and is endothermic, causing positive + ΔH value.
• Interaction of solute and solvent releases energy and is exothermic, which gives a negative -ΔH value
• ΔHsoln = ΔH₁ + ΔH₂ + ΔH₃

Intermolecular Forces

• Two substances with similar intermolecular forces are likely to be soluble in each other.
• The stronger the intermolecular attractions are between solute and the solvent, the more likely it is that the solute will dissolve successfully.
• Intermolecular forces include Hydrogen bonds, dipole-dipole interactions, and dispersion forces.
• Ions in water also have ion-dipole forces.

Factors That Affect Solubility

• In nature of solute and solvent, "like dissolves like".
• Solubility of solid solutes in liquid solvents increases with increasing temperature, but solubility of gases decreases.
• The solubility of liquids and solids do not change appreciably with pressure and is directly proportional in gases.
• According to Henry's Law, the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution.
• C = kP, where C is the concentration, M, of the dissolved gas,P is the pressure of the gas over the solution, and k is a constant (mol/L atm) that is only dependent on temperature.
• The amount of solute gas dissolved in solution is directly proportional to the amount of pressure above the solution, hence the formula.
• Increased pressure means increased solubility as more pressure above will make the gas dissolve more.
• Divers who use compressed gases must be concerned about the gases' solubility in their blood.
• At depth, the blood contains higher concentrations of dissolved gases.
• According to the "bends", Ascension can cause blood to fizz.
• This decompression sickness affects nerve impulses and is painful, sometimes fatal.

This quiz covers solubility rules, factors affecting the dissolving process and enthalpy changes. It tests knowledge of soluble and insoluble compounds, the role of enthalpy, and the effect of solute-solvent interactions.