SOLUTIONS 2011

Components of Solutions

Defining Solutions Solute- the substance that is dissolved Solvent- the substance that does the dissolving Solution- a homogeneous mixture of a solute dissolved in a solvent The parts of a solution are indistinguishable from one another Solutions may exist as homogeneous mixtures of solids, liquids, gases, or combinations of two states

Solvation Water molecules are in constant fluid motion When a solid is added, the water molecules bump into the solid crystals There is an attraction between the poles of the water molecule and the ions or molecules, that is greater than the attraction between the ions or molecules themselves The water molecules surround the ions, those ions move away from the crystals, exposing the ions underneath

Solution of Ions in Water Na+ Cl-

Solution of Molecules in Water

Describing Solutions Qualitatively

Saturated Solutions Solutions that contain the maximum amount of dissolved solute or a given amount of solvent at a specific temperature Any excess solvent that is added will fall to the bottom of the container Saturated solutions exist in a dynamic equilibrium- the rate of solvation is equal to the rate of crystallization

Unsaturated Solutions Solutions that contain less dissolved solute for a given temperature Any excess solvent added will dissolve

Supersaturated Solutions Solutions that contain more dissolved solute than a saturated solution at the same temperature Supersaturated solutions are formed by heating the solvent to a temperature where it will hold more solute then cooling it slowly Supersaturated solutions are very unstable- adding a “seed” crystal or disturbing the solution will often cause the solute to “fall out” of solution

Describing Solutions Quantitatively

Transmittance and Absorbance Percent Transmittance- the amount of light that is able to pass through a solution Percent Absorbance- the amount of light absorbed by a solution Measured using an instrument called a spectrophotometer

Molarity Molarity is a unit of concentration defined as moles of solute per liter of solution Formula for Molarity: M= mol L

Calculating Molarity A 100.5mL intravenous solution contains 5.10g of glucose (C6H12O6). What is the molarity of this solution?

Preparing a Molar Soution How many grams of CaCl2 would be dissolved in 1.0L of a 0.10M solution of CaCl2?

Dilution Dilution is the process of making a solution less concentrated Dilution Formula: M1V1=M2V2

Dilution What volume in milliliters of a 2.00M calcium chloride (CaCl2) stock solution would you use to make 0.50L of a 0.300M calcium chloride solution?

Molaility Molality is a unit of concentration defined as moles of solute per kilogram of solvent Formula for Molality: m= mol kg

Calculating Molality In the lab, a student adds 4.5g of sodium chloride (NaCl) to 100.0g of water. Calculate the molality of a solution.

Factors that Affect Solubility

Agitation Agitation is the process of increasing the movement of the solute and solvent molecules In general as agitation increases, solubility increases

Temperature Adding temperature adds kinetic energy to the molecules In general as temperature increases, solubility increases Gases are an exception, as the temperature of a liquid-gas solution is increased, the solubility of the gas decreases

Surface area Surface area is the amount of solute that touches the solvent In general as surface area increase, solubility increases

Type of Solvent The general rule is like dissolves like Polar solvents dissolve ionic compounds and polar covalnet compounds Nonpolar solvents dissolve only nonpolar solutes

Type of Solvent Solbule- when a substance completely dissolves in another solvent Insoluble- when a substance is not able to dissolve in a solvent Miscible- when two liquids are able to form a solution Imiscible- when two liquids are not able to form a solution

Amount of Solute The amount of solute that is able to dissolve is dependent on all the other factors Eventually, the solution becomes saturated and no more solute can dissolve Dynamic Equilibrium- state in which the rate of solvation is equal to the rate of crystalization

Reading Solubility Curves

Reading Solubility Curves Each line represents a solution that is saturated solution Saturated solutions exist in a dynamic equilibrium where solvation and crystallization are occurring at the same rate Any point above the line where all the solute is dissolved at a certain temperature represents a supersaturated solution Any point below the line where all the solute is dissolved at a certain temperature represents an unsaturated solution Solutes whose curves move upward with increased temperature are typically solids because the solubility of solids generally increases with temperature Solutes whose curves move downward are typically gases because the solubility of gases generally decreases with increasing temperature

Electrolytes

Electrolytes Electrolytes are ionic compounds that dissociate in water to form solutions that conduct electrical current Dissociation is the process of an ionic compound breaking apart into ions Dissociation of NaCl:

Colligative Properties

Colligative Properties Colligative properties are properties of solutiosn that are affected by the number of particles but not the identity of the solute

Boiling Point Elevation: A Colligative Property Boiling point elevation is the temperature difference between a solution and pure solvent The value of the boiling point elevation is directly proportional to molality, meaning the greater the number of solute particles, the greater the elevation

Formula for Boiling Point Elevation: Kb= molal boiling point elevation constant The boiling point of pure water= 100°C Kb for water= 0.512 °C/m m-= molality (mole solute/kg of solution)

Freezing Point Depression: A Colligative Property Freezing point depression is the difference in temperature between a solution and a pure solvent The value of freezing point depression is directly proportional to molality, meaning the greater the number of solute particles

Formula for Freezing Point Depression: Kf= molal freezing point depression constant The freezing point of pure water= 0°C Kffor water= 1.86°C/m m= molality

Example- Changes in Boiling and Freezing Points What are the boiling and freezing points of a 0.029m aqueous solution of sodium chloride (NaCl)?

Reaction Equilibrium and le Chatlier’s Principle

Chemical Equilibrium Many chemical reaction exist in a dynamic equilibrium meaning the reactions are reversible and products and reactants are made at the same rate

Le Chatlier’s Principle: Le Chatlier’s Principle: If stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress In general … We say a reaction shifts to the right when it is moving from reactants to products We say a reaction shifts to the left when it is moving from products to reactants

Changes in Concentration of Reactants and Products Adding reactants: shits equilibrium right Adding products: shifts equilibrium left Removing reactants: shifts equilibrium left Removing products: shifts equilibrium right

Changes in Pressure and Volume When pressure of a gas is increased, volume is decreased When pressure of a gas is decreased, volume is increased

Increasing pressure of a container in which two or more gases are reacting Decreases the volume, moving particles closer together Increases collision of particles Increase reaction rate, shifting equilibrium right

Decreasing the pressure of a container in which two or more gases are reacting Increases the volume Particles move apart, decreasing collisions Decreases reaction rate, shifting equilibrium left

Changes in Temperature

Exothermic Reactions Reactions that release energy in the form of heat Represented by the symbol ΔH° which for an exothermic reaction will have a negative value (heat is treated as a product) Increasing the temperature of an exothermic reaction shifts equilibrium left Decreasing the temperature of an exothermic reaction shifts equilibrium right

Endothermic Reaction Reactions that require energy in the form of heat Represented by the symbol ΔH° , which for an endothermic reaction will have a positive value (heat is treated as a reactant) Increase the temperature of an endothermic reaction shifts the equilibrium right Decreasing the temperature of an endothermic reaction shifts equilibrium left

Examples: Use Le Chatlier’s principle to predict how each of these changes would affect the ammonia equilibrium system: N2 + 3H2  2NH3 Removing hydrogen Adding ammonia Adding hydrogen

Examples: Predict how this equilibrium would respond to increasing temperature: CO + Cl2  COCl2 ΔH°= -220kJ