Strong Electrolytes - substance that dissolves almost completely in water to produce many ions to conduct electricity

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1 I. What are Solutions A. Solution - homogeneous mixture made up of individual molecules, atoms or ions. B. Solute - the substance being C. Solvent - the substance D. Soluble - substance that in a solvent E. Insoluble - substance that in a solvent F. Immiscible liquids that are in each other G. Miscible two liquids that in each other H. Types of solutions 1. Aqueous solution - is the solvent 2. Tincture - is the solvent 3. Alloys - solid solution of two or more metals - - an alloy in which one metal is Hg II. Electrolytes vs. Nonelectroyles A. Electrolytes: any substance that, when it is dissolved in a solution, will conduct an electric current by means of movement of ions NaCl (aq) 6M HCl (aq) 1M HCl (aq) Glacial Acetic Acid (aq) 6M HC 2 H 3 O 2(aq) 1M HC 2 H 3 O 2(aq) NaOH(aq) tap H 2 O distilled H 2 O Strong Electrolytes - substance that dissolves almost completely in water to produce many ions to conduct electricity 1. NaCl (s) 2. HCl (g) 3. NaOH (s) 1

2 Weak (or non) electrolyte - poor conductor of electricity 1. HC 2 H 3 O 2 (aq) 2. H 2 O (l) III. Heterogeneous Mixtures 1. Suspension (NOT A SOLUTION) a mixture containing particles that remain while thoroughly mixed, but then settles out when left undisturbed. Can you think of examples of a suspension? 2. Colloids (NOT A SOLUTION) Colloids or colloidal dispersions are suspensions in which the suspended particles are larger than molecules but too small to separate out of the suspension due to gravity. Particle size: 10 to 2000 Å. Can you think of examples colloids? 3. Tyndall Effect Dispersed colloid particles that are large enough to scatter light. The path of a beam of light projected through a colloidal suspension can be seen through the suspension. Watch and observe the demo? Hydrophilic and Hydrophobic Colloids Water-loving colloids: hydrophilic. Water-hating colloids: hydrophobic. In the human body, large biological molecules such as proteins are kept in suspension by association with surrounding water molecules. These macromolecules fold up so that hydrophobic groups are away from the water (inside the folded molecule). Hydrophilic groups are on the surface of these molecules and interact with solvent (water) molecules. Typical hydrophilic groups are polar (containing C O, O H, N H bonds) or charged. 2

3 Consider a small drop of oil in water. They will not mix unless soaps act on the oil and water. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. Most dirt stains on people and clothing are oil-based. Biological application of this principle: The gallbladder excretes a fluid called bile. Bile contains substances (bile salts) that form an emulsion with fats in our small intestine. Emulsifying agents help form an emulsion. Emulsification of dietary fats and fat-soluble vitamins is important in their absorption and digestion by the body. IV. The Solutions Process Can you draw how you think a salt crystal dissolves? What do the ions of NaCl and molecules of water look like? How do they interact? Consider NaCl (solute) dissolving in water (solvent). Water molecules orient themselves on the NaCl crystals. H-bonds between the water molecules have to be broken. NaCl dissociates into Na + and Cl. Ion-dipole forces form between the Na + and the negative end of the water dipole. Similar ion-dipole interactions form between the Cl and the positive end of the water dipole.. The process by which the charged particles in an ionic solid separate from one another is called Dissociation Equation: NaCl (s) Solvation the process of surrounding solute particles with solvent particles to form a solution. If water is the solvent, the interaction is called hydration. Factors that affect rate of solvation must increase collisions between solute and solvent

4 Solubility measure the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature and pressure. usually expressed a quantity of solute per definite amount of solvent at a particular temperature (ex. 36g NaCl/100g H 2 O at 20 C) Unsaturated Saturated Supersaturated the amount of solute dissolved is less than the maximum that could be dissolved solution holds the maximum amount of solute per amount of the solution under the given conditions solutions contain more solute than the usual maximum amount and are unstable They cannot permanently hold the excess solute in solution and may release it suddenly Solution Equilibrium is the physical state in which the opposing processes of dissolving and crystallization of a solute occur at equal rates. Graphs and solubility tables are useful planning tools for making solutions. Can you draw the curve of a graph as a salt increases in solubility as temperature increases? Factors affecting solubility The tendency of a substance to dissolve in another depends on: 1. Nature of the solute and solvent example: at room temp, 1g of PbCl 2 /100g H 2 O and 200g ZnCl 2 /100g H 2 O Which solute is more soluble and why, can you explain? Intermolecular forces are an important factor in determining solubility of a solute in a solvent. The stronger the attraction between solute and solvent molecules, the greater the solubility. For example, polar liquids tend to dissolve in polar solvents. Favorable dipole-dipole interactions exist (solute-solute, solvent-solvent, and solutesolvent). 4

5 Water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are re-established in the mixture. However, not all alcohols are miscible with water. Why? The number of carbon atoms in a chain affects solubility. The greater the number of carbons in the chain, the more the molecule behaves like a hydrocarbon. Thus, the more C atoms in the alcohol, the lower its solubility in water. Increasing the number of OH groups within a molecule increases its solubility in water. The greater the number of OH groups along the chain, the more solute-water H-bonding is possible. Generalization: Like dissolves like. Substances with similar intermolecular attractive forces tend to be soluble in one another. The more polar bonds in the molecule, the better it dissolves in a polar solvent. The less polar the molecule the less likely it is to dissolve in a polar solvent and the more likely it is to dissolve in a nonpolar solvent. Network solids do not dissolve because the strong intermolecular forces in the solid are not reestablished in any solution. 2. Pressure The solubility of a gas in a liquid is a function of the pressure of the gas over the solution. a. solid in liquid? 5

6 b. gas in liquid? What happen to when the pressure goes up? c. Henry s Law At a given temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid. S = P = 3. Temperature temp, kinetic energy, randomness a. solid in liquid? what if cools back down? b. gas in liquid? What happen to a gas as the liquid warms up? Heat of Solution: overall energy change that occurs during the solution process Energy Changes and Solution Formation There are three steps involving energy in the formation of a solution: Separation of solute molecules (ΔH 1 ), Separation of solvent molecules (ΔH 2 ), and Formation of solute-solvent interactions (ΔH 3 ). We define the enthalpy change in the solution process as: ΔH soln = ΔH 1 + ΔH 2 +ΔH 3 To determine whether ΔH soln is positive or negative, we consider the strengths of all solute-solute, solventsolvent and solute-solvent interactions: Breaking attractive intermolecular forces is always endothermic. ΔH 1 and ΔH 2 are both positive. Forming attractive intermolecular forces is always exothermic. ΔH 3 is always negative. 6

7 It is possible to have either ΔH 3 > (ΔH 1 + ΔH 2 ) or ΔH 3 < (ΔH 1 + ΔH 2 ). Examples: MgSO 4 added to water has ΔH soln = 91.2 kj/mol. NH 4 NO 3 added to water has ΔH soln = kj/mol. MgSO 4 is often used in instant heat packs and NH 4 NO 3 is often used in instant cold packs. How can we predict if a solution will form? In general, solutions form if the ΔH soln is negative. If ΔH soln is too endothermic a solution will not form. Rule of thumb : Polar solvents dissolve polar solutes. Nonpolar solvents dissolve nonpolar solutes. Consider the process of mixing NaCl in gasoline. Only weak interactions are possible because gasoline is nonpolar. These interactions do not compensate for the separation of ions from one another. Result: NaCl doesn't dissolve to any great extent in gasoline. Consider the process of mixing water in octane (C 8 H 18 ). Water has strong H-bonds. The energy required to break these H-bonds is not compensated for by interactions between water and octane. Result: Water and octane do not mix. Factors affecting rate of solution 1. size of particle a. dissolving only occurs at surface b. smaller pieces, surface area, rate of solution 2. stirring a. brings fresh parts of solvent in contact with solute, so rate of sol 3. amount of solute already dissolved a. the more solute dissolved, rate of solution 4. temperature a. solids: temp, rate of solution b. gases: temp, rate of solution 7

8 Ways of Expressing Concentration All methods involve quantifying the amount of solute per amount of solvent (or solution). Concentration may be expressed qualitatively or quantitatively. The terms dilute and concentrated are qualitative ways to describe concentration. A dilute solution has a relatively small concentration of solute. A concentrated solution has a relatively high concentration of solute. Quantitative expressions of concentration require specific information regarding such quantities as masses, moles, or liters of the solute, solvent, or solution. Solution Concentrations what does concentration mean? Mass percentage, ppm, and ppb Mass percentage is one of the simplest ways to express concentration. mass of component in soln Mass % of component 100 total mass of solution By definition: Similarly, parts per million (ppm) can be expressed as the number of mg of solute per kilogram of solution. Parts per million (ppm) of component mass of component in soln total mass of solution If the density of the solution is 1g/ml, then 1 ppm = 1 mg solute per liter of solution. We can extend this again!

9 Parts per billion (ppb) can be expressed as the number of µg of solute per kilogram of solution. Parts per billion (ppb) of component mass of component in soln solution total mass of If the density of the solution is 1g/ml, then 1 ppb = 1 µg solute per liter of solution Mole Fraction, Molarity, and Molality Common expressions of concentration are based on the number of moles of one or more components. Recall that mass can be converted to moles using the molar mass. Recall: Mole fraction of component, X Note: Mole fraction has no units. Note: Mole fractions range from 0 to 1. Recall: M (molarity) = moles of component total moles of all components # of moles of solute 1 Liter of solution example: What is the molarity of a solution which contains 4.9 g of H 2 SO 4 in 250 ml of solution? example #2: How many moles of KNO 3 are contained in 300 ml of a 0.50 M solution? Note: Molarity will change with a change in temperature (as the solution volume increases or decreases). We can define molality (m), yet another concentration unit: Molality, m Molality does not vary with temperature. Note: converting between molarity (M) and molality (m) requires density. moles of solute kilograms of solvent Colligative Properties: Those people who live in the colder climates of the world add antifreeze to the water in their automobiles radiators. From the name of the product, it is obvious that its purpose is to prevent the water from freezing. How does the antifreeze prevent freezing? It is not that the antifreeze itself has such a low freezing point. The best protection is afforded by a mixture that is roughly 50% antifreeze and 50% water. Next we will investigate the behavior of solutions in such situations as freezing 9

10 and boiling. We will look at those properties of solvents that are changed by solutes. Properties which are determined by the number of particles in solution rather than by the type of particle in solution, are colligative properties. Freezing point depression and boiling point elevation A. Vapor Pressure Lowering: 1. VP: the pressure exerted in a closed container by liquid particles that have escaped the liquid s surface and have entered the gaseous state. 2. Vapor pressure for water shown in first beaker, a solute lowers the vapor pressure in the second beaker. 3. There are fewer solvent particles on the surface of the second beaker to enter the gaseous state and the vapor pressure is lowered. 4. Remember Boiling Point is defined as: B. Boiling point elevation and freezing point depressions depend on: 1. molal concentration - number of moles of solute in a kilogram of solvent 2. nature of the solvent Can you think of examples of a BP elevation or FP depression? Example: #2 What would lower the freezing point better on the roads, sugar (C 12 H 22 O 11 ), table salt (NaCl), another salt CaCl 2, and why? C. Molal freezing and boiling point constants 1. A 1 molal solution will lower the freezing point of water 1.86 C. Water could freeze at C instead of 0 C. This value is called the molal freezing point constant for water (1.86 C/molal). T f = change in freezing point Tf = kfm k f = constant for water m = molal concentration of solution 2. A 1 molal solution will raise the boiling Tb = Kbm point of water 0.52 C. sugar are what will freezing The Examples: If 85.0 grams of dissolve in 392 grams of water, be the boiling point and points of the resulting solution? molecular formula of sugar is C 12 H 22 O 11. (answer =

11 C, C) Lowering the Vapor Pressure Nonvolatile solutes (with no measurable vapor pressure) reduce the ability of the surface solvent molecules to escape the liquid. Therefore, vapor pressure is lowered. The amount of vapor pressure lowering depends on the amount of solute. Osmosis Semipermeable membranes permit passage of some components of a solution. Often they permit passage of water but not larger molecules or ions. Examples of semipermeable membranes are cell membranes and cellophane. Osmosis is the net movement of a solvent from an area of low solute concentration to an area of high solute concentration. Consider a U-shaped tube with a two liquids separated by a semipermeable membrane. One arm of the tube contains pure solvent. The other arm contains a solution. There is movement of solvent in both directions across a semipermeable membrane. The rate of movement of solvent from the pure solvent to the solution is faster than the rate of movement in the opposite direction. As solvent moves across the membrane, the fluid levels in the arms become uneven. The vapor pressure of solvent is higher in the arm with pure solvent. Eventually the pressure difference due to the difference in height of liquid in the arms stops osmosis. Osmotic pressure,, is the pressure required to prevent osmosis. Everyday examples of osmosis include: If a cucumber is placed in NaCl solution, it will lose water to shrivel up and become a pickle. A limp carrot placed in water becomes firm because water enters via osmosis. Eating large quantities of salty food causes retention of water and swelling of tissues (edema). Water moves into plants, to a great extent, through osmosis. Salt may be added to meat (or sugar added to fruit) as a preservative. 11

12 Salt prevents bacterial infection: A bacterium placed on the salt will lose water through osmosis and die. 12