Solutions Chapter 15

What Are Solutions? Section 15.1

What Are Solutions? Solution: homogeneous mixture of 2 or more substances Solid, liquid, or gas Solvent: dissolving medium Solute: substance that dissolves When in solution, you cannot distinguish solvent and solute

What is a Solution? Soluble – a substance that can dissolve in a given solvent Miscible: two liquids that can dissolve in each other Example: water and antifreeze Insoluble – substance cannot dissolve Immiscible: two liquids that cannot dissolve in each other Example: oil & water

Why Do Some Substances Dissolve and not Others? To dissolve, solute particles must dissociate from each other and mix with solvent particles Attractive forces between solute and solvent must be greater than attractive forces within the solute Process of surrounding solute particles with solvent particles is called SOLVATION In water, it is also called HYDRATION

Aqueous Solutions of Ionic Compounds Remember: Water molecules are polar (+ and – ends) Water molecules are in constant motion When you put salt in water, water molecules collide with surface of crystal Charged ends of water attract ions of salt Dipole interaction (water/salt) is stronger than ions in crystal, so it pulls them away

Solvation

Aqueous Solutions of Molecular Compounds Water is also a good solvent for many molecular compounds (Example: sugar) Sugar has many O-H bonds (polar) When water is added, the O-H bond becomes a site for hydrogen bonding with water Water’s hydrogen bonds pulls the sugar molecules apart Oil is not a good solute because it has many C-H bonds (not polar) and few or no O-H (polar) bonds

Factors that Affect Solvation Rate Increase Solvation Rate (Dissolve Faster) by: Agitation (stirring) Increase surface area (make particles smaller) Temperature (make it hotter) All these increase the number of collision between water and the solute

Heat of Solution During Solvation it takes energy to make the solute particles come apart. Solvent particles must also move apart This energy requirements is called “Heat of Solution”

Solubility Has Anyone ever made rock candy? How much water does it take to dissolve the sugar at room temperature? What happens when we raised the temperature? Only a limited amount of solute can dissolve in a given amount of solvent Every solute is unique for the solvent This is ‘Solubility’ – the amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure

Solubility Continued Solubility can also be understood at the particle level: As particles collide, some particles are deposited back to the solute Some particles are removed from the solute. When the rate of deposit equals the rate of solvation, then the solution is SATURATED Saturated Solution – no more solute can be dissolved in the solvent at this temperature and pressure Unsaturated Solution – there is still room for more solute to be dissoved

Factors that Affect Solubility Most substances are MORE soluble at high temperature than at low If you dissolve a substance until saturated at high temperature and then reduce the temperature, the solution becomes “supersaturated” Supersaturated solutions are unstable A small change makes the solute reappear Rock candy worked that way. How?

Factors that Affect Solubility Pressure affects solubility of gaseous solutes Carbonated beverages Henry’s Law At a given temperature solubility (S) of a gas in a liquid is directly proportional to pressure (P) S2 = S1P1 P2 S1 P1 = S2 P2 OR

Solution Concentration Section 15.2

Expressing Concentration Concentration is a measure of how much solute is dissolved in a specific amount of solvent. Concentration can be qualitative or quantitative Qualitative: strong, weak, etc. Quantitative: percent by mass, percent by volume, molarity, molality

Using Percent to Express Concentration Percent by mass = Mass of solute Mass of solution X 100 Percent by volume = Volume of solute Volume of solution X 100

Molarity Molarity is the most common method of expressing concentration in Chemistry Molarity is moles of solute in 1 liter of solution. You make it by taking 1 mole of a solute and filling up with solvent to the 1 liter level. Molarity (M) = Moles of solute Liters of solution

Molarity - Example SOLVE for Mole/Liter: 5.10g glucose X 1 mol glucose An IV solution contains 5.10 g of glucose (C6H12O6) in 100.5ml of water. What is the molarity of this solution? Known: Mass of solute = 5.10 g glucose Molar mass of glucose = 180.0 g/mol Volume of solvent = 100.5 ml SOLVE for Mole/Liter: 5.10g glucose X 1 mol glucose 180 g/mol glucose = 0.028 mol glucose Convert ml to liters: 100.5 ml X 1 L 1000 ml = 0.1005 L 0.028 mol 0.1005 L Moles Liter = 0.28 M Molarity = =

Preparing Molar Solutions How do you prepare a 1.5M solution of sucrose (C12H22O11) ? 1.5 moles sucrose x 342g/mol = 513 g sucrose 1.5 Molar would be 513g in 1 L of water Measure out 513 g sucrose Put it in a 1L graduated cylinder Add distilled water to make 1L total solution To make 100ml, use 1/10th of each 51.3g sucrose in 100ml of water

Making Dilute Solutions Concentrated HydroChloric Acid is 12M. How would I make ½ the concentration, or 6 M ? Use M1V1 = M2V2 12moles x 1L = 6moles x ?L = 12moles x 1L/6 moles = 2 L So put in twice the solvent and you have ½ the concentration.

Molality and Mole Fraction Molality is Moles of Solute per 1 kg of Solvent Abbreviated m and is read as molal. This is because volume increases with temperature and this changes the molarity.

Calculating Molality If a student adds 4.5 g of sodium chloride to 100.0g of water, what is the molality? Known: Mass of water is 100.0 g Mass of sodium Chloride is 4.5g Unknown: m or mol/kg 4.5g NaCl X 1 mol NaCl 58.5 g NaCl = 0.077 mol NaCl 100.0 g water X 1 kg H2O 1000 g H20 = 0.1000 kg H2O m = Moles of solute Kg solvent = 0.077 mol NaCl 0.1000 kg H2O = 0.77 mol/kg

Mole Fraction Mole fraction = moles solute divided by total moles of solute + solvent Mole Fraction (X) = ___nA__ nA + nB Example: What is the mole fraction of hydrochloric acid if for every 100 g of solution, 37.5 g is HCl 37.5 g HCl x 1 Mol HCl 36.5 g HCl = 1.03 mol HCl 62.5 g H2O x 1 mole H2O 18.0 g H2O = 3.47 mol H2O XHCl = ___nHCL___ nHCl + nH2O = _____1.03 mol HCl_______ 1.03 mol HCl + 3.47 mol H2O = 0.229

Colligative Properties of Solutions Chapter 15.3

Electrolytes and Colligative Properties When solutions are made, the physical properties of the solutions are affected by the number of particle dissolved Colligative: depending on the collection

Colligative Properties Electrolytes vs non-electrolytes Ionic compounds ARE electrolytes because they form ions in solution that conduct electricity Molecular compounds ARE NOT electrolytes because they do not conduct electricity Vapor Pressure Lowering Adding a non-volatile solute lowers the vapor pressure of the solution (vs. solvent) More solute  more vapor pressure lowering

Colligative Properties Boiling Point Elevation Because vapor pressure is lowered, it takes more energy to make it boil Boiling point temperature is raised Boiling point elevation is directly proportional to solution molality What is the benefit of adding salt to boiling water for pasta? Freezing Point Depression Freezing point temperature is lowered Solute particles interfere with attractive forces of solvent Freezing point of a solution is always lower than the freezing point of a pure solvent FP Depression is directly proportional to molality

Colligative Properties Osmosis and Osmotic Pressure Diffusion: mixing of gasses or liquids through random motions Osmosis is diffusion of solvent through a semi permeable membrane from high solvent concentration to lower solvent concentration Living cells use this to get materials in/out of cells

Colligative Properties Example: Salt/water During Osmosis, Water molecules move both directions through membrane But only water can move through the membrane So pure water builds up on one side of the membrane Water/salt builds up on the other. Higher concentration of water on one side creates: Osmotic pressure A pressure or push to equalize the water/salt concentrations Pressure depends on concentration of solute

Colligative Properties - Antifreeze

Freezing Point Depression B. Types Freezing Point Depression View Flash animation.

Boiling Point Elevation B. Types Boiling Point Elevation Solute particles reduce vapor pressure of solvent, requiring more energy for vapor pressure to reach atmospheric pressure (boiling), thus raising the boiling point temperature.

B. Types Applications salting icy roads making ice cream antifreeze cars (-64°C to 136°C) fish & insects

C. Calculations t: change in temperature (°C) t = k · m · n t: change in temperature (°C) k: constant based on the solvent (°C·kg/mol) m: molality (m) n: # of particles

C. Calculations # of Particles Nonelectrolytes (covalent) remain intact when dissolved 1 particle Electrolytes (ionic) dissociate into ions when dissolved 2 or more particles

C. Calculations GIVEN: WORK: b.p. = ? m = 0.73mol ÷ 0.225kg tb = ? At what temperature will a solution that is composed of 0.73 moles of glucose in 225 g of phenol boil? GIVEN: b.p. = ? tb = ? kb = 3.60°C·kg/mol WORK: m = 0.73mol ÷ 0.225kg tb = (3.60°C·kg/mol)(3.2m)(1) tb = 12°C b.p. = 181.8°C + 12°C b.p. = 194°C m = 3.2m n = 1 tb = kb · m · n

C. Calculations GIVEN: WORK: f.p. = ? m = 0.48mol ÷ 0.100kg tf = ? Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL water. GIVEN: f.p. = ? tf = ? kf = 1.86°C·kg/mol WORK: m = 0.48mol ÷ 0.100kg tf = (1.86°C·kg/mol)(4.8m)(2) tf = 18°C f.p. = 0.00°C - 18°C f.p. = -18°C m = 4.8m n = 2 tf = kf · m · n

Heterogeneous Mixtures Chapter 15.4

Types of Heterogeneous Mixtures Look like a solution, but are really mixtures Mixtures of substances that exist in 2 different phases 2 Types: Suspensions Colloids Solutions: Particles of solute are atomic sized compared to solvent

Suspensions Particle Size Suspended particles are large compared to solvent Larger than 1000 nm for solvated particles CAN be filtered When stirred, solid-like state begins to flow like a liquid Called Thixotropic Examples housepaint

Colloids Particle Size Cannot be Separated by filtration or settling Particles of solute are intermediate sized (between atomic and large suspension sized) compared to solvent Between 1 nm and 1000 nm diameter Cannot be Separated by filtration or settling Example: milk, butter, cheese,

Colloids Types of Colloids Solid Sol: Solid in solid (gemstones) Sol: Solid in Liquid (Blood, gelatin) Solid emulsion: Liquid in solid (butter, cheese) Emulsion: liquid/liquid (milk, mayonaise) Solid foam: gas/solid (marshmallows, soap that floats) Foam: gas/liquid (whipped cream, beaten egg whites) Aerosol: solid/gas (smoke, dust in air) Aerosol: liquid/gas (clouds, spray deodorant

Brownian Motion Liquid colloid under a microscope shows random, jerky motions of dispersed particles Brownian motion From collisions of particles of dispersion medium with dispersed particles Collisions prevent particles from settling out Due to polar or charged atomic particles Link for display http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=24

Tyndall Effect Dilute Colloids sometimes appear as clear solutions (concentrated colloids do not) Because particles are too small to be seen with naked eye But: dispersed colloid particles are large enough to scatter light Tyndall effect Solutions do not scatter light (particles are too small)

Tyndall Effect Colloid Solution

Chapter Summary Chapter 15 Test on Friday (5/16) Chapter 15 Vocabulary Due Friday (5/16) Chapter 15 HO’s (remaining) Due Friday (5/16)

Chapter Test Covers: Characteristics of Solutions Solvation – ionic vs molecular Factors Affecting Solubility Solution Concentrations: Calculate Percent by mass Percent by volume Molarity Molality Colligative Properties of Solutions Suspensions and Colloids