StudyHub+ 1 StudyHub: AP Chemistry Solution Composition and Energies, Boiling Point, Freezing Point, and Vapor Pressure
StudyHub+ 2 Solution Composition: Mole Fraction: Formula: Mole Fraction of Component A = (Moles of Component) / (Total Moles in Solution). Mole Fraction is the ratio of the moles of solute to the moles of the solution. It is most commonly used for calculating partial pressures. Molarity: Formula: Molarity = (Moles of Solute) / (Liters of Solution). Molarity is a calculation of the concentration of solute for every liter of solution. Molality: Formula: Molality = (Moles of Solute) / (Kilograms of Solution) Unlike Molarity, Molality can calculate the concentration of a solution while including the masses of both the solute and the solvent whereas in Molarity, only the volume of the solute and solvent is used in calculating the concentration. Mass Percent: Formula: Mass Percent = 100 * ((Mass of Element) / (Mass of Solution)) Mass Percent represents how much (in percentage) of the solution is made up of a specific element. The Energies of Solution Formation Like Dissolves Like Ionic Compounds and Polar Molecules tend to dissolve in polar solvents, and Nonpolar Molecules dissolve in Nonpolar Compounds. Like Intermolecular Forces dissolve other Like Intermolecular Forces. Example: Which molecule will more likely dissolve in water: CH 3 OH or CH 3 CH 3? Because Water has Hydrogen Bonds, CH 3 OH is more likely to dissolve in Water because CH 3 OH also has Hydrogen Bonds.
StudyHub+ 3 Steps of Solution Formation 1. Solute is broken down into different parts and components. 2. Intermolecular forces in the solvent are overcome, this creates more space for the solute. 3. Allowing the solute and solvent to interact to form the solution. Enthalpy of the Solution = H(solution) = H 1 + H 2 + H 3 H 1 (Process 1): Solute breaks apart the Intermolecular Forces that are holding the solute in place, causing the solute molecules to separate from each other. This is an endothermic process because it requires energy in order for the reaction to occur. Thus, H 1 > 0. H 2 (Process 2): This process is very similar to the first process. The solvent also needs to overcome the Intermolecular Forces that hold it together. The molecules separate, and since this reaction also requires energy, it is an endothermic reaction as well. Thus, H 2 > 0. H 3 (Process 3): This final process is a process that mixes the first substance and the second substance produced from the previous processes. When these substances join together, energy is given off as the two substances bond together. This makes process three an exothermic reaction because energy is released. Thus, H 3 < 0. In simple terms, H and H 1 2 represents the solution separating into solute and solvent -> H 1 and 2 are positive. H 3 represents the mixing of water and ions -> H 3 is negative. Energy Diagram for Endothermic Dissolving Process:
StudyHub+ 4 Energy Diagram for Exothermic Dissolving Process: Energy Diagram for Ideal Dissolving Process Factors Affecting Solubility: Structure Effects: Hydrophilic and polar molecules dissolve in polar solvents. Example: Water Soluble Vitamins Hydrophobic and nonpolar molecules dissolve in nonpolar solvents. Example: Fat Soluble Vitamins Pressure Effects: Henry s Law:
StudyHub+ 5 The amount of gas that is dissolved in a solution is directly proportional to the pressure of the gas above the solution. P = kc P = Partial pressure of gaseous solute above the solution. C = Concentration of dissolved gas. k = Constant of the given solution. Temperature Effects: Solids: As temperature increases, the dissolving occurs more rapidly. As temperature increases, the solubility usually increases as well. Gases: As temperature increases, the solubility decreases. Soluble salts: Attraction between ions and water > Attraction between ions Insoluble salts: Attraction between ions > attractions between ions and water Boiling Point Elevation and Freezing Point Depression: Vapor Pressure The rate at which a liquid turns into gas. Vapor pressure is indirectly proportional to freezing point, boiling point, intermolecular forces, etc. Colligative Properties Properties are dependent on the number of solute particles but not on their identity Boiling-point elevation Freezing-point depression Osmotic Pressure Boiling Point Elevation Boiling Point, by definition, is when the vapor pressure of the solution equals the atmospheric pressure. Nonvolatile solutes elevate the boiling point of the solvent When a solute is dissolved in a solution, it increases the boiling point. This is because when a solute is dissolved in a solution, vapor pressure decreases due to the fact that on the surface, the solutes are in the way from the
StudyHub+ 6 solvents from turning into vapor. By increasing the temperature, the vapor pressure will increase. Formula : T = K * m b solute T represents the boiling point elevation. K b represents the boiling point elevation constant of the solvent. m solute represents the molality of the SOLUTE in the solution. Freezing Point Depression Freezing Point, by definition, is the temperature at which a liquid turns to a solid. Nonvolatile solutes depress the freezing point of the solvent. When a solute is dissolved in a solution, the freezing point of the total solution decreases. This is due to the fact that when a substance freezes, it must shed itself of all impurities before it can completely freeze. A solution with both a solute and a solvent tends to have more entropy (disorder) than just a pure solvent. The increased entropy means that the freezing point of the solution will need to be even lower than that of the solvent. An example of this is when salt is utilized on icy roads. The salt mixes with a small amount of water which prevents melting ice from refreezing because the salt-melted ice solution causes the freezing point to decrease. Formula : T = K * m f solute T represents the freezing point depression. K f represents the molal freezing point depression constant of the solvent. m solute represents the molality of the solute in the solution. Example: Which compound, when dissolved in water, will have a lower freezing point? NaCl -> Na + + Cl - CaCl 2 -> Ca 2+ + 2Cl - Answer: CaCl 2 because the more particles are present,the more disruption water will have. As previously stated, freezing point depression is a colligative property of matter, which means that this is primarily dependent on the number of particles in the solution. CaCl 2, when dissolved, will dissociate into 3 particles whereas NaCl will dissociate into 2 particles.
StudyHub+ 7 Colligative Properties of Electrolyte Solutions: van t Hoff Factor The relationship between the moles of solute dissolved and the moles of particles in the solution. Formula: i = (moles of particles in solution) / (moles of solute dissolved). The expected value is determined by noting the number of ions per formula unit. Example: K 2 SO 4 Answer: i = 3 because there are two K + 2- ions and one SO 4