chapter 11: properties of solutions. i.solution composition a.units of solution concentration...

Chapter 11: Properties of Solutions

I. Solution Composition

A. Units of solution concentration1. Molarity:

2. Mole fraction:

3. Percent by mass:

B. Converting between units1. If 1.275 g of glucose is dissolved in

65.39 mL of solution, what is the molarity of the resulting solution? What is the percent by mass of the solution? D = 1.00 g/mL for the solution.

2. What is the mole fraction of urea, (NH2)2CO, in an aqueous solution that is 3.42 M? The density of the solution is 1.045 g/mL.

II. The Solution Process

A. When you add an ionic compound to water, the ions separate and the crystal lattice breaks up

1. Some ions will find each other and reform the solid

2. Eventually a dynamic equilibrium is achieved: NaCl(s) Na+(aq) + Cl-(aq)

3. Solubility: mass of a solute that will dissolve in a given amount of solvent

4. Saturated: a solution that contains the max amount of solute per given amount of solvent (system is at equilibrium)

5. Unsaturated: a solution that contains less than the max amount of solute

6. Supersaturated: a solution that contains more dissolved solute than a saturated solution does

Supersaturated solutions

B. Will a solute dissolve in a solvent?1. “Like dissolves like”—if the solute has the

same type of IMFs as the solvent then it is likely to dissolve

2. Ionic solids and polar compounds will only dissolve in a polar solvent

3. Nonpolar compounds will only dissolve in a nonpolar solvent

4. Miscible: liquids that mix in all proportions

C.The dissolution of a solid in a liquid takes place in 3 steps:

1. Expanding the solvent (separating the particles)—requires energy (∆H1)

2. Expanding the solute (separating the particles)—requires energy (∆H2)

3. Allowing the solute + solvent to interact & form a solution—releases energy (∆H3)

D. Enthalpy of Solution: the energy change when 1 mol of a solid is dissolved in water

1. To calculate

2. If E required is greater than E released then is positive and the dissolution is endothermic

3. If E required is less than E released then is negative and the dissolution is exothermic

Comparing Endothermic & Exothermic

E. For an ionic solute, ∆H1 and ∆H3 are often combined

1. Hydration energy: the amount of energy released when 1 mol of gaseous ions is hydrated ( ) K+(g) + F-(g) KF(aq)

2. Ionic compounds with large lattice energies are often insoluble

3. Ions with large charges often form insoluble compounds, smaller charges are often soluble

III.Factors Affecting Solubility

A. Henry’s law: the pressure of a gas above a solution is proportional to the concentration of the gas in solution

B. Solubility depends on temperature

1. Substances with a positive enthalpy of solution (endothermic) have an increase in solubility when temp goes up

a. ∆Hsol’n + LiF(s) Li+(aq) + F-(aq)

b. This is true for MOST ionic compounds

2. Substances with a negative enthalpy of solution (exothermic) have a decrease in solubility as temp goes up

a. O2(g) O2(aq) + ∆Hsol’n

b. This is true for ALL gases and a few ionic compounds

IV.The Vapor Pressure of Solutions

A. The vp of a solution (with a nonvolatile solute) is lower than the vp of a pure solvent

1. The vp of a solution is directly proportional to the mole fraction of the solvent

2. Thus a solution that is 50% (by moles) of each component will have half the vp of the pure solvent

B. This occurs because the solute decreases the # of solvent molecules in a given volume, thus decreasing the rate of evaporation

V. Boiling Point Elevation and Freezing Point Depression

A. Boiling point elevation1. Remember bp is temp when vp = atm P

2. Adding solute lowers vp at a given temp

3. So solution must be heated to a higher temp to get vp equal to atm P

4. Thus bp increases when you add a solute; more solute added = higher bp

B. Freezing point depression

1. Adding a solute lowers the freezing point of a solution

2. More solute added = lower fp

C. What if the solute is ionic?1. When an ionic solid dissolves in water, it

dissociates, and it is the # of ions that form which determines the value of the vp lowering, bp elevation or fp depression

2. More ions = greater effect of any of these properties

3. Ex: Which 1 M solution has the lowest vapor pressure?

a. NaCl b. CuCl2 c. C12H22O11 d. Mg3N2

V. Connecting IMFs & Biology

A. Proteins are composed of amino acids

1. The sequence of the amino acids determines the 3D structure of the protein

2. All amino acids have the same basic structure—C bonded to an amino group (-NH2), a carboxyl group (-COOH), H, and a unique side chain (R group)

3. Some of the side chains are polar due to the presence of an OH or NH bond

a. Amino acids with polar side chains interact well with water and are called hydrophilic

b. These amino acids tend to be on the surface of proteins, making the protein water-soluble

c. Examples: aspartate, arginine

4. Some of the side chains are nonpolar (containing only C atoms bonded to Hs)

a. Amino acids with nonpolar side chains are not water soluble and are called hydrophobic

b. They form the water insoluble core of the protein

c. Examples: valine, leucine

B. The 3D structure of the protein is determined by the interactions btw. hydrophilic and hydrophobic ends

1. The secondary structure is stabilized by hydrogen bonds

2. The tertiary structure is stabilized by hydrophobic interactions between nonpolar side chains

C.Some vitamins are water-soluble (so hydrophilic) & some are fat-soluble (so hydrophobic)

1. Vitamin A is virtually nonpolar so dissolves in body fat

a. This means it can be stored—so the body can tolerate a diet deficient in vitamin A for a period of time

b. Consumption of excess amounts can cause hypervitaminosis

2. Vitamin C is polar and water soluble

a. This means that it is excreted by the body and must be consumed regularly

b. Lack of vitamin C causes scurvy—as noted by British navy, so sailors started taking limes with them to eat on board