Solutions and Their Properties McMurray and Fay ch. 11

Vocabulary: Molar enthalpy of solution OR Heat of solution (DHsoln) Total energy absorbed or released when a solute dissolves in a solvent

Breaking bonds takes energy Forming bonds releases energy Solution thermodynamics: Why is energy released or absorbed when a solute dissolves? Breaking bonds takes energy Forming bonds releases energy

When something dissolves… What bonds are being broken When something dissolves… What bonds are being broken? What bonds are being formed? O H O H Cl- Na+ O H O H Cl- Na+ O H O H Cl- O H Na+ O H O H O H

When something dissolves… What bonds are being broken When something dissolves… What bonds are being broken? What bonds are being formed? O H O H O H Cl- Na+ O H Cl- Na+ O H O H Cl- O H Na+ O H O H O H

1. IMFs between solvent molecules are broken H Cl- O H O H Na+ O H Cl- Na+ O H Cl- O H Na+ O H O H O H O H

2. IMFs or bonds between solute molecules are broken H O H O H Cl- Na+ O H Cl- Na+ O H O H Cl- O H Na+ O H O H O H

3. IMFs are formed between solute and solvent H O H O H Cl- Na+ Na+ O H Cl- Na+ O H O H O H Cl- O H O H O H

Heat of Solution: A Hess’s Law view Energy Add energy to separate solvent particles Energy released as solute and solvent form IMFs Add energy to separate solute particles Energy difference between initial & final energies = heat of solution Separate solute + solvent

Heat of Solution Bonds broken: Bonds formed: Solvent-solvent Solvent-solute Solute-solute

Is formation of a solution exothermic or endothermic? Compare: Bonds broken: Bonds formed: Solvent-solvent Solvent-solute Solute-solute

Is formation of a solution exothermic or endothermic? Compare: Bonds broken: Bonds formed: Solvent-solvent Solvent-solute Solute-solute If there are strong bonds between solvent molecules or if there are strong bonds between solute molecules a lot of energy is required to break these bonds If there are strong bonds between solvent & solute molecules a lot of energy is released when forming these bonds If there are weak bonds between solvent molecules or if there are weak bonds between solute molecules very little energy is required to break these bonds If there are weak bonds between solvent & solute molecules very little energy is released when forming these bonds

Is formation of a solution exothermic or endothermic? Compare: Bonds broken: Bonds formed: Solvent-solvent Solvent-solute Solute-solute Breaking strong bonds + forming weak bonds = endothermic reaction Breaking weak bonds + forming strong bonds = exothermic reaction

NaCl dissolving in hexane

Bonds broken? C H Cl- Na+ Cl- Cl- Na+ C H Cl- Na+ Na+

Bonds formed? C H Cl- Na+ Cl- Na+ Cl- C H Cl- Na+ Na+

The bonds broken are so much stronger than the bonds formed that it is very hard to dissolve NaCl in hexane at room temperature C H Cl- Na+ C H Cl- Cl- Na+ Cl- Na+ Na+

When IMFs of very different strengths are present in a solute and a solvent, the solute is less likely to dissolve.

Solubility: ‘Like dissolves like’ Polar solvents dissolve polar solutes Non-polar solvents dissolve non-polar solutes

POLAR SOLVENTS (substances that form dipole-dipole forces or hydrogen bonds) like to mix with other polar substances

NON-POLAR SOLVENTS (substances that only have London dispersion forces) like to mix with other non-polar substances

In which of the following will NaCl be the most soluble? Benzene, C6H6 Carbon tetrachloride, CCl4 Hexane, C6H14 Methanol, CH3OH

In which of the following will Vitamin D will be the most soluble? Hexane, C6H14 Chloromethane, CH3Cl Methanol, CH3OH Water, H2O

Liquids dissolving in Liquids

Hexane and water: will they mix? C H O H O H O H C H O H O H O H O H

What bonds would be broken? C H O H O H O H C H O H O H O H O H

What bonds would be formed? C H O H O H C H O H O H O H O H

Keeping the water and hexane separate is a more stable (lower energy) state C H O H O H O H C H O H O H O H O H

Gases dissolving in liquid

CO2 in water C O C O C O O H O H O H O H O H O H O H O H O H O H

CO2 in water Bonds broken? Bonds formed? O H C O C O O H O H O H O H O

Henry’s Law: Effect of Pressure on Gas Solubility

Cgas = kHPgas Henry’s Law Henry’s Law constant Concentration of gas in solution Pressure of gas above solution

The Henry’s Law constant for N2 at 25 oC is 6. 8 x 10-4 mol/L atm The Henry’s Law constant for N2 at 25 oC is 6.8 x 10-4 mol/L atm. What is the concentration of N2 in water when the pressure of nitrogen is 0.8 atm? Cgas = kHPgas

Calculate the concentration change for CO2 when a sealed soft drink can (PCO2= 4.0 atm) is opened (PCO2 = 0.00039 atm). kH for CO2 at 25oC is 3.1 x 10-2 mol/L atm. Cgas = kHPgas

If you don’t know kH: Cgas = kHPgas so kH = Cgas/Pgas C1 C2 P1 P2 =

The solubility of methane, the chief component of natural gas, in water at 20o C and 1.0 atm pressure is 0.025 g L-1. What is its solubility in water at 1.5 atm and 20.0oC? C1 C2 P1 P2 =

Concentration Units for Solutions

Units of concentration Molarity (M): moles per liter Mole fraction (XA) Mass percent(also called w/w) ppm ppb Molality (m): moles per kilogram of solvent

Mole Fraction nA XA = nA + nB + nC + …. Tells what fraction of the total number of moles a given component constitutes # of moles of component A nA nA + nB + nC + …. XA = Mole fraction of component A Total # of moles (add up # of moles of each component)

Mole Fraction (XA) nA XA = nA + nB + nC + …. Example: A solution is made of 10 moles of ethanol and 20 moles of water. Ethanol comprises what mole fraction? nA nA + nB + nC + …. XA =

Mole Fraction (XA) nA XA = nA + nB + nC + …. Example: A solution is made of 10 moles of water, 15 moles of acetone, and 25 moles of toluene. Acetone comprises what mole fraction? nA nA + nB + nC + …. XA =

Mass Percent (w/w) mass of solute Percentage by mass = x 100% (or any component of a thing) Percentage by mass = x 100% mass of solute mass of solution (the total mass)

39.55 g salt is dissolved in 1000. g of water. What is the percentage by mass of salt in the solution? Mass percent = x 100% mass of solute mass of solution

Problem solving strategies for harder concentration problems Identify which measure of concentration you need to calculate 2. Use the definition for that measure of concentration to determine what you need to know 3. Based on information given, create and carry out a plan to find what you need to know. If necessary, start plan by assuming 100. 4. Do calculations for measure of concentration

What is the molarity of a 3% (w/w) H2O2 solution? The density of this solution is 1.00 g/mL. Mass percent = x 100% mass of solute mass of solution

Identify which measure of concentration you need to calculate What is the molarity of a 3% (w/w) H2O2 solution? The density of this solution is 1.00 g/mL.

What is the molarity of a 3% (w/w) H2O2 solution? 2. Use the definition for that measure of concentration to determine what you need to know What is the molarity of a 3% (w/w) H2O2 solution? The density of this solution is 1.00 g/mL. molarity = moles of solute L of solution Need to find: moles of solute L of solution

3. Based on information given, create and carry out a plan to find what you need to know. If necessary, start plan by assuming 100. Calculate: moles of H2O2; L of solution 100.0 g of solution 100.0 mL of solution a. Calculate g of solution a. Calculate g of H2O2 b. Calculate g of H2O2 b. Calculate moles of H2O2 c. Calculate moles of H2O2 c. Calculate L of solution d. Do molarity calculation d. Do molarity calculation

What is the molarity of a 3% (w/w) H2O2 solution? The density of this solution is 1.00 g/mL. Mass percent = x 100% mass of solute mass of solution 100.0 g of solution 100.0 mL of solution a. Calculate g of H2O2 a. Calculate g of solution b. Calculate moles of H2O2 b. Calculate g of H2O2 c. Calculate L of solution c. Calculate moles of H2O2 d. Do molarity calculation d. Do molarity calculation

Very small percentages by mass ppm (parts per million) 1 ppm =1/1,000,000 of the mass of a solution For example: 1 mg in 1000 g of solution in aqueous solution: 1 ppm =1 mg/L ppb (parts per billion): 1 ppb =1/1,000,000,000 of the mass of a solution

Molality moles of solute Molality (m)= kg solvent Densities of liquids can change with temperature Molality used when temperature is a consideration

moles of solute Molality (m)= kg solvent What is the molality of a solution created by adding 20.7 g of KBr to 250. g of water?

What is the molality of a solution created by adding 122 g of sucrose (C12H22O11) to 587 g of water?

A 6. 79 M solution of HCl has a density of 1. 15 g/cm3 A 6.79 M solution of HCl has a density of 1.15 g/cm3. What is the molality of the solution? Identify measure of concentration to calculate 2. Use definition for that measure of concentration to determine what you need to know 3. Based on information given, create & carry out a plan to find what you need to know. Start plan by assuming 100. 4. Do calculations for measure of concentration

Definition: Colligative Properties Properties of solutions that change depending on concentrations of solute in the mixture. Vapor pressure of a solution (Raoult’s Law) Boiling point elevation Freezing point depression Osmotic pressure

Raoult’s Law (part 1) Psoln = Xsolvent P0solvent Explains the relationship between vapor pressure and concentration Psoln = Xsolvent P0solvent Vapor pressure of the solution Vapor pressure of the pure solvent (without any solute) Mole fraction of the solvent

The vapor pressure of pure water is 23. 8 mm Hg The vapor pressure of pure water is 23.8 mm Hg. If 103 g of seawater contains 3.5 g NaCl and 99.5 g water, what is the vapor pressure of seawater? Psoln = Xsolvent Posolvent

Raoult’s Law for solutions with two or more volatile components Psoln = XA P0A + XB P0B+ …

What is the vapor pressure of a solution containing 18 What is the vapor pressure of a solution containing 18.6 moles water (P0= 23.8 torr) and 25.8 moles hydrogen peroxide (P0 = 1.97 torr)? Psoln = XA PoA + XB PoB

van’t Hoff factor Changes in colligative properties may be determined by the number of particles in solution Ionic compounds dissociate in water 1 M NaCl = 2 M ions (1 M Na+, 1 M Cl-) so i = 2* 1 M CaCl2 = 3 M ions (1 M Ca2+, 2 M Cl-) so i = 3* 1 M glucose = 1 M particles so i = 1 * In an ideal world…

Boiling point elevation by solutes Addition of a solute changes the vapor pressure of a liquid and therefore also changes the boiling point DTb = i Kbm Molality of the solution Change in boiling point van’t Hoff factor Molal boiling-point-depression constant

A chef, wanting to cook pasta, increases the boiling temperature of water by adding 1 T of salt (~15 g of NaCl) to 2.00 kg of water. How much does he change the boiling temperature? (Kb for water = 0.512 oC/m) DTb = iKbm

Acetic acid has a normal boiling point of 118. 5 oC. If 56 Acetic acid has a normal boiling point of 118.5 oC. If 56.0 g sucrose (C12H22O11) is added to 1000. g of acetic acid, what will the boiling point of the mixture be? (Kb = 3.07 oC/m) DTb = iKbm

Freezing point depression by solutes Solutes interfere with the formation of intermolecular forces that turn liquids into solids DTf = i Kfm Molality of the solution Change in freezing point van’t Hoff factor Molal freezing-point-depression constant

Artificial seawater can be created by adding 35. 5 g of NaCl to 1000 Artificial seawater can be created by adding 35.5 g of NaCl to 1000. g water. At what temperature will artificial seawater freeze? (Kf for water is 1.86 oC/m) DTf = iKfm

Definition: Osmosis Net shift of solvent (usually water) through a membrane that only allows passage of solvent molecules

Membranes and Permeability Membrane: separator that keeps mixtures separated Permeability: ability of a membrane to allow substances to pass through permeable inpermeable semi-permeable

Cell membranes are semi-permeable e.g. aquaporins only let water through

Why does osmosis happen? Unequal concentrations on either side of the membrane System tries to reach equilibrium by making concentrations similar Only solvent can pass through membrane

Osmotic Pressure A. Solution B separated from pure water, A, by osmotic membrane. (No osmosis yet) B. Osmosis has occurred; volume of fluid in tube higher. C. Pressure needed to prevent osmosis = osmotic pressure.

Calculating Osmotic Pressure PV = i nRT P = i MRT

An aqueous solution of a sugar was prepared at a concentration of 0 An aqueous solution of a sugar was prepared at a concentration of 0.42 g/L at 25 oC. Its osmotic pressure was 2.5 torr. Calculate the molecular mass of the compound. P = i MRT