Ummm Solutions Solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent. Solutions The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles. How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them. For aqueous solutions, the interactions of solvation is called hydration. How Does a Solution Form If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal. Energy Changes in Solution Simply put, three processes affect the energetics of the process: Separation of solute particles Separation of solvent particles New interactions between solute and solvent 1
Energy Changes in Solution Therefore: ΔH soln = ΔH 1 + ΔH 2 + ΔH 3 The enthalpy change of the overall process depends on H for each of these steps. If established interactions release more energy than required to separate solute/solvent particles, the solvation process is exothermic If established interactions release less energy than required to separate solute/solvent particles, the hydration (aq sol) process is endothermic Some compounds form solutions in a given solvent and others do not; why? Polar compounds ΔH is high Factors Affecting Solubility Chemists use the axiom like dissolves like : Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar solvents. Non-polar compounds ΔH is low ΔH for polar-polar interactions is high ΔH for nonpolar-polar interactions is low ΔH for non-nonpolar interactions is low 2
Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another. Factors Affecting Solubility Example: Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water. So: Like Dissolves Like Solvent Solute Outcome Polar Polar Solution formed Polar Non-polar No Solution formed Non-polar Non-polar Solution formed Non-polar Polar No Solution formed Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered. Why Do Endothermic Processes Occur? Yet we know that in some processes, like the dissolution of NH 4 NO 3 in water, heat is absorbed, not released. Enthalpy Is Only Part of the Picture The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system. 3
Enthalpy Is Only Part of the Picture So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered. Guiding principle in the solution process: 1. Processes in which the energy content of a system decreases tend to occur spontaneously, without extra input of energy from outside the system 2. Processes in which the disorder of the system increases tend to occur spontaneously Types of Solutions Saturated Solvent holds as much solute as is possible at that temperature. Dissolved solute is in dynamic equilibrium with solid solute particles. Types of Solutions Unsaturated Less than the maximum amount of solute for that temperature is dissolved in the solvent. Types of Solutions Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can usually be stimulated by adding a seed crystal or scratching the side of the flask. Solubility is defined as the amount of a solute required to form a saturated solution in a given volume of solvent at a given temperature. (usually expressed in g/l) Saturated Vs. Unsaturated and Supersaturated 4
Beware! Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn t mean the substance dissolved. Dissolution is a physical change you can get back the original solute by evaporating the solvent. If you can t, the substance didn t dissolve, it reacted. Factors Affecting Solubility 1. Solute-Solvent Interactions 2. Temperature 3. Pressure Solute-Solvent Interactions Substances with similar intermolecular forces tend to be soluble in one another Like dissolves like Exception: Network-Covalent solids Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. Dropping the temperature of a solution is often used to precipitate a synthesized compound in organic chemistry. 5
The opposite is true of gases: Carbonated soft drinks are more bubbly if stored in the refrigerator. Warm lakes have less O 2 dissolved in them than cool lakes. Temperature Pressure Solubilities of a gas in any solvent is increased as the pressure over the solvent is increased. The solubilities of liquids and solids are not appreciably affected by pressure Gases in Solution In general, the solubility of gases in water increases with increasing mass. Larger molecules have stronger dispersion forces. William Henry (1803) Henry's Law = the amount of gas dissolved in a solution is directly proportional to the pressure above the solution. S g = K P g Where: (M, atm) S g = solubility of gas in solution (M) P g = partial pressure of the gas above solution K = Henry's Law constant (different for every solute-solvent pair) Henry s law is accurate for dilute solutions with no dissociation or reaction between gas and solvent. Also, it is subject to partial pressures for gas mixtures. 1. A soft drink is bottled so that a bottle at 25 o C contains carbon dioxide gas at a pressure of 5.0 atm over the liquid. Assuming the partial pressure of carbon dioxide in the atm of an open system is 4.0 x 10-4 atm, calculate the concentration of carbon dioxide gas in the soda both before and after the bottle is opened (Henry s Law constant is 4.48 x 10-5 mol/l atm at 25 o C) 6
Ways of Expressing Concentrations of Solutions Concentration of Solutions Qualitative 1. Concentrated = near saturation 2. Dilute = far from saturation Quantitative - Defines the amount of solute present mathematically. Molarity (M) M = mol of solute L of solution You will recall this concentration measure from Chapter 4. Because volume is temperature dependent, molarity can change with temperature. Concentration Units An IDEAL SOLUTION is one where the properties depend only on the concentration of solute. Need conc. units to tell us the number of solute particles per solvent particle. The unit molarity does not do this! Molality (m) Mass Percentage m = mol of solute kg of solvent Because both moles and mass do not change with temperature, molality (unlike molarity) is not temperature dependent. Mass % of A = mass of A in solution total mass of solution 100 7
Parts per Million and Parts per Billion Parts per Million (ppm) ppm = Parts per Billion (ppb) ppb = mass of A in solution total mass of solution 106 mass of A in solution total mass of solution 109 X A = Mole Fraction (X) moles of A total moles in solution In some applications, one needs the mole fraction of solvent, not solute make sure you find the quantity you need! 1. A solution is prepared by mixing 1.00 g ethanol with 100.0 g water to give a final volume of 101 ml. Calculate the molarity, mass percent, mole fraction and molality of the solution. 2. A 2.5 g sample of ground water was found to contain 5.4 micrograms of zinc (II) ions. What is the concentration of zinc in parts per million? Parts per billion? Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity, and vice versa. 3. The electrolyte in automobile lead storage batteries is a 3.75 molar sulfuric acid solution that has a density of 1.230 grams per milliliter. Calculate the mass percent and molality of the solution. 8