Water and Aqueous Systems Chapter 15 Water and Aqueous Systems

Water and its Properties Water is a polar molecule Has hydrogen bonding This creates: High surface tension Low vapor pressure

Surface Tension The inward force, or pull, that tends to minimize the surface area of the liquid

Water Strider

Surfactant Any substance that interferes with the hydrogen bonding between water molecules Reduces surface tension Examples: soaps, detergents

Water and its Properties Vapor Pressure Is the result of molecules escaping from the surface of the liquid and entering the vapor phase

Vapor Pressure of Purell™

Water and its Properties Water in the Solid State Ice is less dense than water Structure of ice is a regular open framework

Homogeneous Aqueous Systems Aqueous Solution – water that contains dissolved substances Solvent – the dissolving medium (water) Salt is the solute (gets dissolved or dispersed in the water) NaCl (s) + H2O (l) → NaCl (aq) solute solvent solution

Figure: 13-01

Electrolytes A compound that conducts electric current when in aqueous solution or in the molten state All ionic compounds are electrolytes because they dissociate into ions Strong electrolytes – compounds dissociate completely as separate ions Weak electrolytes – conducts electricity poorly because ions/molecules do not dissociate completely

Figure: 13-09

Figure: 13-26

Nonelectrolyte A compound that does not conduct an electric current in wither aqueous solution or molten state Examples: molecular compounds (covalent bonds)

Figure: 13-31

Sample Exercise Which of the following substances dissolve appreciably in water? HCl NH3 CH4 NaI MgSO4 CaCO3

Sample Exercise Decide which compounds are more likely to dissolve in water and which are more likely to dissolve in gasoline CCl4 Na2SO4 Methane (CH4) KCl

Sample Exercise Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na2CO3  10H2O) Answer: 62.9%

Practice Problems What is the percent by mass of water in CuSO45H2O? Answer: 36%

Practice Problems Calculate the percent by mass of water in calcium chloride hexahydrate (CaCl26H2O)? Answer: 49.5%

Sample Exercise Write equations to show how these substances ionize or dissociate in water NH4Cl Cu(NO3)2 HC2H3O2 BaCl2

Chapter 16 Solutions

Solution Formation How quickly the solute dissolves in a solvent Rate can be increased by increasing: Agitation (stirring) Temperature Surface Area

Agitation Mixing the solute increases the chance for the solvent to react and form a solution

2. Temperature At high temperatures, the kinetic energy of molecules is greater than at lower temperatures The more rapid motion of molecules leads to an increase in frequency and force in collisions between molecules

3. Surface Area Increasing the surface area of a solute increases the chances for a reaction

Solubility Is the amount of substance that dissolves in a given quantity of a solvent at a given temperature to produce a saturated solution Solute + solvent  solution Ex: NaCl(s)(solute) + water(l)(solvent)  NaCl(aq) (solution) Saturated solution – contains the maximum amount of solvent at constant temperature Unsaturated – a solution that contains less solute than a saturated solution

Miscible – two liquids that dissolve in each other Immiscible – liquids that are insoluble in one another

Factors Affecting Solubility Temperature and Pressure Temperature Affects the solubility of solids, liquids, and gaseous solutes in a solvent Pressure Affects only the solubility of gaseous solutes

1. Temperature Table G – Solubility Curve The solubility of most solid substances increases as temperature increases (proportional) Gases are more soluble in liquids when temperatures are low (inverse) Supersaturated solution – contains more solute that it can theoretically hold at a given temperature

Sample Exercise At what temperature will the solubility of KNO3 be 50 g/per 100g of H2O? At 100 g of KNO3? How much KClO3 is needed to saturate 50 g of H2O at 90C? How many grams of NaNO3 will precipitate if a saturated solution of NaNO3 in 200g H2O at 50°C is cooled to 20°C?

Sample Exercise The solubility of KCl in water is 34.0 g KCl/100g H2O at 20°C. A warm solution containing 50.0 g KCl in 100 g H2O is cooled from 75°C to 20°C. How many grams of KCl remain dissolved? How many grams came out of solution?

Sample Exercise A solution contains 70 grams/100 g H2O at 60°C. Determine if the solution is supersaturated, saturated, or unsaturated for: KI, NaNO3, HCl, KNO3 Which solution has the highest solubility in 100 g of H2O at 40°C?

Sample Exercise A solution contains 14 g of KCl in 100 g of water at 40°C. What is the minimum amount of KCl that must be added to make this a saturated solution? How many grams of the compound KCl must be dissolved in 200 g of water to make a saturated solution at 60°C

Sample Exercise Which amount of the compound dissolved in 100 g of water at the stated temperature represents a solution that is saturated? 20 g KClO3 at 80°C 40 g KNO3 at 25°C 40 g KCl at 60°C 60 g NaNO3 at 40°C

2. Pressure Increasing pressure prevents vaporization Increasing pressure makes gaseous molecules more soluble in liquid form

Concentrations of Solutions Molarity (M) - The number of moles of a solute dissolved in 1L of solution Also known as molar concentration number of moles of solute Molarity (M) = number of liters of solution Table T

Dilute Solution – one that contains a small amount of solute Concentrated Solution – contains a large amount of solute

Sample Problem Household laundry bleach is a dilute aqueous solution of sodium hypochlorite (NaClO). How many moles of solute are present in 1.5 L of 0.70 M NaClO? Answer: 1.1 mol NaClO

Practice Problems How many moles of ammonium nitrate are in 335 mL of 0.425 M NH4NO3? Answer: 0.142 moles How many moles of solute are in 250 mL of 2.0 M CaCl2? How many grams of CaCl2 is this? Answer: 55 grams

Practice Problems A saline solution contains 0.90g of NaCl per 100.0 mL of solution. What is its molarity? Answer: 0.155 mol/L How many grams of solute are present in 1.5 L of 0.20M Na2SO4? Answer: 42.6 grams

Making Dilutions Making a solution less concentrated by diluting it with more solvent M1 x V1 = M2 x V2 Volumes can be in liters or in milliliters as long as the same units are used for both V1 and V2

Sample Problem How many milliliters of aqueous 2.00 M MgSO4 solution must be diluted with water to prepare 100.0 mL of aqueous 0.400 M MgSO4? Answer: 20.0 mL

Sample Exercise How many milliliters of a solution of 4.00 M KI are needed to prepare 250.0 mL of 0.760 M KI? Answer: 47.5 mL How could you prepare 250 mL of 0.20 M NaCl using only a solution of 1.0 M NaCl and water? Answer: 50 mL

Parts per Million Ratio between mass of solute and total mass of solution Useful for very dilute solutions Table T This method of reporting concentrations is useful for extremely dilute solutions when molarity and percent mass would be difficult to interpret. For example, chlorine is used as a disinfectant in swimming pools. Only about 2g of chlorine per 1,000,000g of water is necessary to keep the pool sanitized. Finding molarity and percent mass would result in numbers too small to be useful. Parts per million is often used to report a measured amount of air or water pollutants.

Sample Exercises Carbon dioxide gas has a solubility of 0.972g in 100.0g of H2O at 40°C, express this in parts per million. ppm = grams of solute x 1,000,000 grams of solution Approximately 0.0043g of oxygen can be dissolved in 100.0g of water at 20oC. Express this in terms of parts per million. 0.972g x 1,000,000 = 100.972g 9626 9630ppm CO2 0.0043g x 1,000,000 = 100.0043g 42.99 43ppm O2

Colligative Properties of Solutions Colligative property - A property that depends on the number of solute particles, not their identity Three important examples in solutions: Vapor-pressure lowering Freezing-point depression Boiling-point elevation

Vapor-Pressure Lowering a measure of the force exerted by a gas above a liquid Solutions with nonvolatile (not easily vaporized) solutes have lower vapor pressures than pure solvents E.x. Salt water has a lower vapor pressure than pure water The vapor pressure of a solution of a nonvolatile solute is less than the vapor pressure of a pure solvent. In a pure solvent, equilibrium is established between the liquid and the vapor. B. In a solution, solute particles reduce the number o free solvent particles able to escape the liquid. Equilibrium is established at a lower vapor pressure.

Vapor-Pressure Lowering Water forms a “shell” around dissolved particles This leaves fewer water molecules with enough KE to escape as vapor The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in a solution E.x. NaCl is 2x as effective as glucose, CaCl2 is 3x as effective as glucose Particle concentrations differ for dissolved covalent and ionic compounds in water. Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. B. Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. Three moles of calcium chloride dissolved in water produce 9 mil of particles because each formula unit of CaCl2 dissociates into three ions.

Freezing-Point Depression difference in temperature between the freezing point of a solution and the freezing point of the pure solvent Solutions have lower freezing-points than pure solvents

Freezing-Point Depression The presence of a solute disrupts the formation of the orderly pattern found in a solid Therefore, more KE must be removed before the solution can freeze The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent Why would CaCl2 be better than NaCl for melting ice? NaCl produces 2 ions CaCl2 produces 3 ions Ethylene glycol, antifreeze, is added to the water in automobile cooling systems to depress the freezing point of water below 0oC. Automobiles can thus withstand subfreezing temperatures without freezing up.

Boiling-Point Elevation Boiling point – temperature at which the vapor pressure of liquid equals the atmospheric pressure Boiling-point elevation – the difference in temperature between the boiling point of a solution and the boiling point of the pure solvent

Boiling-Point Elevation Solutions have higher boiling-points than pure solvents Recall… nonvolatile solids lower the vapor pressure Therefore, it takes more KE for the solvent particles to overcome the attractive forces that keep them in the liquid The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent Elevating the boiling point of a solution has some familiar applications. For example, to make fudge, a lot of sugar and some flavorings are mixed with water and the solution is boiled. As the water slowly boils away, the concentration of sugar in the solution increases. As the concentration increases, the boiling point steadily rises. As you watch the temperature on a candy thermometer rise, you know that the solution is becoming more concentrated.

Sample Exercises An equal number of moles of KI and MgF2 are dissolved in equal volumes of water. Which solution has the higher: Boiling point - Vapor pressure - Freezing point - Why is salt, NaCl, put on icy roads and sidewalks in the winter? MgF2 solution KI solution KI solution The addition of NaCl lowers the temperature at which water freezes.

Assume equal aqueous concentrations of each of the following substances. Which has the lowest freezing point? C2H22O6 CH3OH C12H22O11 NaOH NaCl produces the most particles upon dissolving. Freezing point depression is a colligative property that depends on the number of solute particles in a solvent.

Which solution has the highest boiling point? seawater 1.0M KNO3 or 0.100M KCl or How does the freezing point of a 1.5M solution of potassium nitrate compare to a 1.5M solution of sodium nitrate? or distilled water 1.5M KNO3 0.100M MgCl2 Both solutions are effectively the same since they both produce 2 ions; there is no difference in their freezing points.

Vapor Pressure & Boiling Liquids are held together by weak forces Some particles at the surface of the liquid have sufficient energy to escape & enter the gaseous phase They vaporize! Vapor pressure = the pressure that these gaseous particles exert on the liquid below What will happen to the vapor pressure as the temperature is increased? As more particles gain more KE, they can escape the liquid, became a gas & exert pressure It increases!

As temperature ↑, vapor pressure ↑ At some point the vapor pressure = atmospheric pressure Gases vaporize not only at the surface, but also within the liquid (bubbles) This is the boiling point - When the vapor pressure is equal to the external pressure

Of the substances shown on the graph, propanone exerts the most pressure, about 93 kPa at a temperature of 50oC. It can be inferred that propanone has the weakest intermolecular forces holding it in the liquid phase, while ethanoic acid has the greatest, exerting only about 8 kPa of pressure. The normal boiling point of water is 100oC. At this temperature, the vapor pressure of water is 101.3 kPa. The line representing 101.3 kPa on Table H shows the normal boiling point of ethanol to be 78oC. When the pressure is less than 101.3 kPa, the boiling point will be less than the normal value. Water will boil at about 70oC when the pressure is about 30 kPa. If the pressure is greater than normal, liquids will boil at temperatures above their normal boiling points. When atmospheric pressure is about 145 kPa, water boils at 110oC.

Sample Exercises What is the vapor pressure of water at 105°C? What is the vapor pressure for a sample of ethanoic acid at 100°C? Which of the substances on Table H has the greatest intermolecular forces of attraction between molecules? Which of the substances on Table H has the weakest intermolecular forces of attraction? 120 kPa 58 kPa Ethanoic acid Propanone

What is the normal boiling point for water? What is the vapor pressure of water at its normal boiling point? At what temperature will water boil at a pressure of 30 kPa? At what temperature will water boil at a pressure of 145 kPa? As the pressure on a liquid is changed from 100 kPa to 120 kPa, the temperature at which the liquid will boil will be? 100oC 101.3 kPa 70oC 110oC Increased