Unit 9 -- Solutions

Water – the most common solvent Water is a liquid with many unique properties. Oxygen is covalently bonded to 2 hydrogen atoms resulting in a “bent” molecule. The shape is important!!

Polar Covalent Bond (Intramolecular bond-between elements) Slightly (- ) @ more electronegative element Slightly +

Water Water is a fluid because it can flow and take the shape of its container. Properties include a high surface tension, a low vapor pressure and high boiling point. These properties are due to hydrogen Bonding.

Surface Tension Surface tension is the force that tends to pull adjacent parts of a liquid’s surface together thereby minimizing the surface area of a liquid.

Water Vapor pressure  or equilibrium vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. Water has a very low vapor pressure, because it takes a lot of energy to break the hydrogen bonds and push a water molecule into the gas phase.

Water Water’s boiling point is higher than expected due to hydrogen bonds. It takes significantly more energy to boil water than it does for molecules of a similar molar mass.

Hydrogen Bonds Hydrogen bonds (intermolecular force) are a bond between molecules NOT ATOMS. They form when Hydrogen bonds to Fluorine, Oxygen or Nitrogen. (DNA matches up genes correctly due to Hydrogen bonding.) Because H2 is so small and has a partial positive charge, it can get very close to the partial negative side of a different electronegative atom. (N, O, F)

Hydrogen Bonds The dotted lines indicate the attraction between electronegative oxygen atoms and electropositive hydrogen atoms of neighboring molecules. This is an intermolecular bond. Both molecules must be polar.

Solution Definitions Solution: A solute plus a solvent. Solutions are stable homogeneous mixtures. Solute: The substance being dissolved. Solvent: The substance doing the dissolving. Usually a liquid. Soluble: Capable of being dissolved.

Solution Definitions Concentrated: Amount of solute is high compared to solvent. Dilute: More solvent than solute.

Solutions -- Electrolytes An electrolyte is a solution that conducts electricity. Ionic compounds form electrolytes, covalent compounds are non-electrolytes

Suspensions Suspension: the mixture will separate upon sitting so it is not a solution.

Solubility Miscible: When 2 liquids can mix in any proportion. ex. antifreeze and water. Immiscible: When 2 liquids cannot mix. eg. Oil and water Solvents are not always liquids. A solid dissolved in a solid is an Alloy. Gases always dissolve in each other.

Solubility Soluble: The solute dissolves in the solvent with nothing left on the bottom. Insoluble: 2 substances will not dissolve. Remember: Like dissolves Like! Polar and ionic solutes will dissolve in polar solvents and non-polar solutes will dissolve in non-polar solvents.

Emulsifiers or Surfactants An emulsifier is a molecule that has oboth polar and non-polar aspects. An emulsifier allows oil or grease and water to mix! Emulsifiers have a polar and a non-polar end. The polar end bonds to water (polar) and the non-polar end bonds to the oil (non-polar) Remember like dissolves like. An emulsion is a mixture between 2 substances that do not usually mix, like oil and water. Examples of common food emulsifiers are lecithin, glycerides, polysorbates and xanthan gum. Soaps and detergents are also emulsifiers.

Emulsions

Soap as an Emulsifier -- Micelles

Ways to Express Concentration There are numerous ways to express the concentration of a solution: Dilute or concentrated Saturated, Unsaturated or Supersaturated Molarity of the solution % concentration (Part over whole X 100)

Saturation & Solutions Unsaturated: The amount of solute is much less than solvent. (it can hold more solute and the solute will dissolve) Saturated: The amount of dissolved solute is equal to the solubility for that temp. (if there is excess solute it does not dissolve.) Supersaturated: A solution that contains more than the usual solubility but there is no excess undissolved solute. Requires manipulating the solution.

Super-Saturated Solution

Rock Candy

Solubility Solubility: The maximum amount of solute that will dissolve in a solvent at a specific temperature. Solubility is always given with a temperature. Generally speaking: As temperature increases, solubility also increases. For most gases, the opposite is true. Ex., warm ponds have less oxygen, potentially killing fish

٭ Any point on the line is a Saturated solution. ٭ Any point below the line is an Unsaturated solution ٭ Any point above the line is a Supersaturated solution, if no solute is on the bottom. *Any point above line is saturated if solute is on the bottom.

Supersaturated (if NO undissolved solute) saturated Unsaturated

Practice WS #2 Solubility Curves

All single bonds mean a saturated fat. One double bond means a mono-unsaturated fat. Multiple double bonds mean a polyunsaturated fat.

Solubility & Double Replacement Reactions Remember, double replacement reactions involve ionic compounds in aqueous solution. Not all double replacement reactions occur. Only those reactions that produce a precipitate (insoluble product), will occur. To determine if a double replacement reaction will occur, you need to learn the solubility rules for ionic compounds.

Solubility of Salts in Water Write these in your notes!!! Note: slightly and marginally soluble means some precipitate will form!!!

Concentration There are multiple terms used to express concentration Concentrated or dilute Saturated, unsaturated or supersaturated Molarity

Solute Concentration The concentration of a solution is a measure of how much solute is dissolved in a specific amount of solvent. There are many ways of measuring concentration Percent by mass Percent by volume Molarity Molality Mole fraction We will learn the first three.

Mass Percent (Percent by Mass)

Mass Percent Example What is the mass percent of 12 g of NaCl in 50 mL of water? Determine the mass of the solute (12 g NaCl) and solution (50 mL water = 50 g H20). Determine the total mass (12 g + 50 g = 62 g) Divide the mass of what you are looking for (solute NaCl) by the total mass (62 g). Multiply the answer by 100%

Percent by Volume

Percent by Volume Example A solution is made by adding 25 mL of benzene to 80 mL of toluene. What is the percent by volume of benzene?\

Molarity (M)

Molarity Example What is the molarity of a solution in which 8 mol of HCl are dissolved in 2.5 L of water?

Practice WS #3 Solution Calculations Practice

Phase Diagrams A phase diagram is a graph of pressure vs. temperature that shows the conditions under which the phases of a substance exist. It also reveals how the states of a system change with changing temperature or pressure.

Changes of State

Phase Diagrams The triple point of a substance indicates the temperature and pressure conditions at which the solid, liquid and vapor of the substance can coexist at equilibrium. The critical point of a substance indicates the critical temperature and critical pressure. Beyond the critical point, there is no difference between a liquid and a gas. This is called a supercritical fluid.

Phase Diagram for CO2

Phase Diagram for Water

Practice WS #4 Phase Diagrams

Hard Water Hard water contains divalent cations such as Mg+2, Fe+2 & Ca+2 (main one). Na ions carry a +1 charge and Ca and Mg carry a +2 charge. The negatively charged portion of the polar head of a soap or detergent molecule is more attracted to the +2 charge. Soap or detergent molecules therefore give up their Na ions to bind to Ca or Mg ions.

Hard Water Soap or detergent molecules bound to Ca+2 or Mg+2 ions tend to be insoluble in water. As they come out of solution, they form a scum that can appear as a ring around the tub, (in sink or shower). Because the soap molecules are tied up with Ca+2 and Mg+2 ions, more of the cleanser must be added to maintain cleaning effectiveness (to lather) which can lead to dry skin. Soft water contains no divalent cations. Water softeners exchange monovalent cations, ions with a single positive charge like Na+1, for divalent cations, ions with a +2 charge like Ca+2 or Mg+2 ions.

Water Softening Unit In a water softening unit hard water is passed through a large tank filled with tiny beads of a water-insoluble resin known as an ion-exchange resin. The surface of the resin contains many negatively charged ions bound to positively charged Na+1 ions. As the hard water with its Ca+2 and Mg+2 ions pass over the resin, the ions displace the sodium ions and thereby become bound to the resin. The water that exits the unit is free of Ca+2 and Mg+2 ions but does contain Na+1 ions.

Water Softeners