CHM1046 Exam 2 Study Guide Fall 2012 12 4 Solution Equilibrium and Factors Affecting Solubility When solutions are mixed the rate of dissolution forming of a solution of the solute increases rapidly However eventually the concentration of dissolved solute increases causing the rate of deposition to increase o When the rate of deposition and dissolution are the same in a solution this is called the dynamic equilibrium This means that there are an equal amount of molecules disassociating with the solution as are becoming associated with it A solution in dynamic equilibrium is written like this NaCl s Na aq Cl aq H20 When a solution is in dynamic equilibrium it is said to be saturated The opposite is said to be unsaturated o If any solutes are added to a saturated solution they will not dissolve In special circumstances a solution containing more than the equilibrium amount of solute can form This is called a supersaturated solution o Usually the excess solution precipitates out until the solution is at equilibrium again Factors Affecting the Solubility of Gases in Water Solutions with gases dissolved in water are common There are a few factors that affect how soluble these gases are 1 Temperature a As temperature increases the solubility of gases in liquids decreases i This can be observed in a pan of boiling water When the water is first heated a few bubbles form as oxygen leaves the water As the water rises in temperature the bubbles form more vigorously and more oxygen leaves the water a The higher the pressure of a gas above a liquid the more soluble the gas is in 2 Pressure that liquid i This is observed when a can of soda is opened The pressure of CO2 in a can maintains the level of dissolved CO2 in the soda However when the can is opened the pressure is released causing the soda to bubble b The solubility of gases in a solution can be quantified with Henry s Law SGas kHPGas Where SGas is solubility of the gas KH is a constant of proportionality Henry s Law Constant that depends on the specific solute solvent and temperature and PGas is the partial pressure of the gas 12 6 Vapor Pressure of Solutions As we discussed in the last chapter vapor pressure of a solution is the pressure of the gas that is above a liquid when the two are in dynamic equilibrium when the rate of vaporization is the same as the rate of condensation SO WHAT HAPPENS TO THE VAPOR PRESSURE WHEN A NON VOLATILE SOLUTE IS ADDED Simply the vapor pressure of the solution is lower than the vapor pressure of the pure solvent The added solute particles affect the solvent s rate of vaporization by occupying some of its surface area o This change in rate of vaporization then causes the rate of condensation to be greater than the rate of evaporation Eventually the rate of condensation and evaporation return to equilibrium but the concentration of gas molecules is decreased The result of this is a lower vapor pressure of solution than the solvent Vapor pressure of a solution is quantified with Raoult s Law Psolution XSolventP Solvent Where Psolution is the pressure of the solution Xsolvent is moles solvent moles solute moles solvent P Solvent is the vapor pressure of the pure solvent at the same temperature Example 102 moles of Ca NO3 2 is dissolved in 927 moles of H2O The vapor pressure of H2O is 118 1 torr at 55 C What is the vapor pressure of the solution 1 First solve for Xsolvent X solvent moles solvent moles solvent of dissolved ions molves solute a b 927 mol 927 3 102mol c 7518 NOTE The three multiplied with 102 moles comes from the ratio of different ions in Ca NO3 2 There are two moles of NO3 and one mole of Ca thus giving us three moles 2 Now solve for the rest of the equation 7518 118 1 torr a b 88 8 torr A solution that follows Raoult s Law at all concentrations for both the solute and the solvent is called an ideal solution This concept is similar to that of an ideal gas o In an ideal solution the solute solvent interactions are equal to that of the solute solute and solvent solvent interactions o In a non ideal solution the solute solvent interactions are NOT equal to that of the solute solute and solvent solvent interactions 12 7 Freezing Point Depression Boiling Point Elevation and Osmosis The Colligative Properties of Solutions In winter salt is often added to roads to melt the ice Salt actually lowers the melting point of ice by creating an ice salt solution allowing it to melt even if the temperature is below freezing o The net effect is a solution with a lower melting point and a higher boiling point than just the pure solvent in this case ice o These effects are referred to as freezing point depression and boiling point elevation Freezing Point Depression The amount that a freezing point is lowered for solutions is given by the equation Where Tf im x Kf Tf is the change in temperature of the freezing point in degrees Celsius i is the van t Hoff factor moles of particles in solution moles of formula units dissolved m is the molality of the solution in moles solute per kilogram solvent mol kg Kf is the freezing point depression constant for the solvent usually given Boiling Point Elevation The amount that a boiling point is raised for solutions is given by the equation Tb im x Kb Tb is the change in temperature of the boiling point in degrees Celsius i is the van t Hoff factor moles of particles in solution moles of formula units dissolved m is the molality of the solution in moles solute per kilogram solvent mol kg Kb is the boiling point elevation constant for the solvent usually given Where Osmosis Formally defined as the flow of a solvent from a solution of lower of lower solute concentration to an area of higher solute concentration Osmosis is a common process in cells which are semipermeable membranes meaning that some substances can pass through while others cannot Osmotic pressure the pressure needed to stop osmotic flow is given by the following equation Where is the osmotic pressure M is the molarity of the solution R is the ideal gas constant 08206 L x atm mol x K and T is the temperature in Kelvin MRT 12 8 Colloids When water and soap are mixed together the resulting mixture has a distinctive has Soapy water is hazy because soap and water do not form a true solution but instead a colloidal dispersion or simply a colloid o Definition a mixture in which a dispersed substance which is solute like is finely divided in a dispersing medium which is solvent like o Some other examples of
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