Chapter 12 Solutions Types of Mixtures 1. Solutions – AKA Homogeneous Mixtures/can only be separated by evaporation – Atoms, molecules, or ions are thoroughly mixed, resulting in a mixture that has the same composition and properties throughout – Made by having one substance dissolved into another • Soluble: capable of being dissolved • Insoluble: unable of being dissolved • Components of solutions – Solvent: the dissolving medium • Water is the universal solvent (aqueous solutions or hydrated solutions) • Alcohol is also used frequently as solvent (tinctures) • Solvent is greatest quantity – Solute: the substance that is dissolved • Usually designated as the component that is of lesser quantity • Once dissolved, the solute particles can no longer be seen (usually less than 1 nm in diameter) • As long as conditions do not change, solutes will remain in solvent indefinitely • Types of solutions Gas-gas: air Gas-liquid: carbon dioxide and water (pop) Liquid-liquid: fruit juice Liquid-solid: dental amalgam Solid-liquid: sugar water Solid-solid: alloys 2. Suspensions • • • • Occur when particles are too large to remain in the solvent unless constantly agitated Particles are usually over 1000nm in diameter Would be identified as heterogeneous mixture Can be separated by a filter 3. Colloid – Particles are intermediate of solutes and suspensions – Particles are between 1-1000nm – Particles become dispersed in a dispersing medium (think fog) – See table 2 page 404 for ex. – Can not be filtered, but will scatter light (tyndall effect) – Also see brownian motion (dust particles in air) Electrolytes versus nonelectrolytes • An electrolyte is a substance that will conduct an electric current when in an aq. Soln. – Ionic compounds and polar covalent molecules in water generally are electrolytes • A nonelectrolyte is a substance that will not conduct an electric current when in an aq. Soln. – Nonpolar covalent molecules generally are nonelectrolytes Factors affecting the rate of dissolution • Increase the surface area of the solute – Crushing/grinding will increase the amount of surface area a solvent affects • Agitate the solution – Think KMT, faster moving particles have more collisions, more collisions increase the amount of solvent affecting the solute • Heating the solvent – KMT Solubility • Soln equilibrium: the physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates – When in equilibrium in a closed system, the solution is saturated (holds the max. amount of solute at given conditions) – When a soln has less than max, it is said to be unsaturated – When holds more solute than a saturated soln has under the same conditions, called supersaturated • Requires heating and allowing soln to cool to saturated point • As long as undisturbed, solute will remain dissolved. Disruptions will cause excess to precipitate (rock candy) • Solubility of a substance is the amount of the substance required to form a saturated soln with a specific amount of solvent at a specified temperature (if gas, pressure must also be stated) • The rate of dissolution is not related to the solubility (at that temperature) Max amount of solute that dissolves to reach equilibrium is always the same under same conditions • Solubility is related to polarity. – “like dissolves like” (polar dissolves polar, nonpolar dissolves nonpolar, nonpolar doesn’t dissolve polar) • Liquid solvents and solutes – If the liquids are not soluble in each other, use term immiscible instead of insoluble – If the liquids are soluble in each other, use term miscible instead of soluble Effects of Pressure on Solubility • Changes in pressure have little effect on the solubilities of liquids or solids • Pressure increases the ability of gas to dissolve in liquid (increase P, increase solubility) Henry’s Law • chemical law stating that the amount of a gas that dissolves in a liquid is proportional to the partial pressure of the gas over the liquid, provided no chemical reaction takes place between the liquid and the gas. • It is named after William Henry (1774–1836), the English chemist who first reported the relationship • An everyday example of Henry's law is given by carbonated soft drinks such as colas (and also by beers and sparkling wines). • Before the bottle or can is opened, the gas above the drink is almost pure carbon dioxide at a pressure slightly higher than atmospheric pressure. • The drink itself contains dissolved carbon dioxide. When the bottle or can is opened, some of this gas escapes, giving the characteristic hiss • Because the pressure above the liquid is now lower, some of the dissolved carbon dioxide comes out of solution as bubbles----this is called effervescence (think Mentos and diet pop) • If a glass of the drink is left in the open, the concentration of carbon dioxide in solution will come into equilibrium with the carbon dioxide in the air, and the drink will go "flat" • Henry's law can be put into mathematical terms (at constant temperature) as – • where p is the partial pressure of the solute, c is the concentration of the solute and kH is a constant with the dimensions of pressure divided by concentration. • The constant, known as the Henry's law constant, depends on the solute, the solvent and the temperature. • Pressure-Solubility Law (Henry's Law) The concentration of a gas in a liquid at any given temperature is directly proportional to the partial pressure of the gas on the solution. • • Cg = kgPg where Cg = the concentration of the gas, kg is the proportionally constant for that gas and Pg is the partial pressure of the gas above the solution. Constant temperature is assumed. This equation is true only at relatively low concentrations and pressure and for gases that do not do react with the solvent. • A more useful expression of Henry's Law is • C1= C2 P1 P2 • • At 20oC the solubility of N2 in water is 0.0150 g/L when the partial pressure of nitrogen is 580 torr. What will be the solubility of N2 in water at 20oC when its partial pressure is 800 torr? • C1 = 0.0150 g/L P1 = 580 torr C2 = ? P2 = 800 torr C2 = C1 X P2 P1 = 0.0150 g/L X 800 torr 580 torr = 0.0207 g/L The solubility under the higher pressure is 0.0207 g/L Enthalpy of solution • The enthalpy of solution (or enthalpy of dissolution) is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure. • We will expand on this later Solution Composition • Perhaps the most important property of a solution is its concentration • In order to quantify the concentrations of solutions, chemists have devised many different units of concentration each of which is useful for different purposes. • Weight percent = wt of solute wt of solution x 100% • What is the weight percent of glucose in a solution made by dissolving 4.6 g of glucose in 145.2 g of water? • Determine total weight of solution:4.6 g + 145.2 g 149.8 glucose-water solution • Calculate percent: • Weight % glucose = 4.6 g glucose x 100 = 3.1%glucose 149.8 g solution • How would you prepare 400. g of a 2.50% solution of sodium chloride? • Analysis: We need to find out how much salt is needed and how much water is needed. • Determine weight of salt: 400. g x 2.50% salt = 10.0 g salt 400. g solution x 2.50 g salt 100 g solution = 10.0 g salt • Determine weight of water: 400. g total - 10. g salt 390. g water Answer: Dissolve 10.0 g salt in 390. g water. Molarity • The term concentration is used to indicate the amount of solute dissolved in a given quantity of solvent or solution. • The most widely used way of quantifying concentration in chemistry is molarity. • The molarity (symbol M) of a solution is defined as the number of moles of solute in a liter volume of solution: Molarity • abbreviated M. • For example, calculate the volume of a 1.5 M solution of HCl necessary to completely react with 0.32 moles of NaOH: • • • What is the molarity of a solution made by dissolving 20 grams of NaCl in 100 mls of water? • M solution • Calculate the number of moles of CaCl2 in 0.78 liters of a 3.5 M solution: • How many liters of a 2.0 M solution of HNO3 do we need to have 5 moles of HNO3? • Dilution alters the molarity (i.e. concentration) of the solution but not the total number of moles of molecules in the solution (in other words, dilution does not create or destroy molecules). • One of the standard equations for determining the effects of dilution upon a sample is to set up an equation comparing (concentration)*(volume) before and after dilution • Since (concentration)*(volume) gives us the total number of moles in the sample, and since this does not change, this value before and after dilution are equal: • (concentration)*(volume) = (concentration)*(volume) • (moles/liter)*(liter) = (moles/liter)*(liter) • moles = moles • How much of a 5 M stock solution of NaCl will you need to make up 250 mls of a 1.5 M solution? • X liters = 0.075 liters (or 75 mls) • What is the concentration of water? – Molecular weight of H2O = 18.0g/mole – Density of H2O = 1g/ml or 1000g/L – Pure water is 55.6M H2O Molality • An alternative unit of concentration to molarity is molality. • The molality of a solute is the number of moles of that solute divided by the weight of the solvent in kilograms. • For water solutions, 1 kg of water has a volume close to that of 1 liter, so molality and molarity are similar in dilute aqueous solutions. • molality (M) = moles solute/kg of solution • To convert a volume and a molality of a solution to moles of solute, simply solve the above equation for moles of solute: – moles solute = molality * kg of solution • If you have 10.0 grams of Br2 and dissolve it in 1.00 L of cyclohexane, what is the molality of the solution? The density of cyclohexane is 0.779 kg/l at room temperature. – 10 g / (159.8 g/mole) = 0.063 moles BR2 – 1.00 L * 0.779 kg/l = 0.779 kg (convert the volume of solvent to the weight of solvent using the density) – 0.063 moles Br2/ 0.779 kg cyclohexane = 0.080 molal Normality • When you need to compare solutions on the basis of concentration of specific ions or the amount of charge that the ions have, a different measure of concentration can be very useful. It is called normality • Normality, the number of molar equivalents of solute per liter of solution, has the units equivalents / L which are abbreviated N. • The normality of a solution is simply a multiple of the molarity of the solution. Generally, the normality of a solution is just one, two or three times the molarity. In rare cases it can be four, five, six or even seven times as much.