Ask a chemist, they always have Ask a chemist, they always have Solutions

Definitions Mixture: several pure substances mixed together in an indefinite ratio Homogeneous Heterogeneous Solution: a homogenous mixture that form when one or more substances dissolve into another. Suspensions: cloudy mixtures that form when two or more substances mix but do not dissolve. Emulsions: suspension of 2 liquids

Solutes and Solvents Solution: a homogenous mixture Solution: a homogenous mixture Solute: thing that dissolves Solvent: thing that does the dissolving (found in the largest amounts) If the solvent is water, then it is called an aqueous solution

Solubility Example: iced tea Solute sugar tea Solvent water

Solubility: Ionic Compounds Ions form, separate (dissociate) and move throughout the solution The forces that hold the ions together are overcome by the ions’ attractions to polar water. Ion- dipole interaction Because ions are present, ionic solutions can conduct a current Current is just movement of electrons

Figure 15.1: Dissolving of solid sodium chloride. Solvation animation Animation with Audio

Figure 15.2: Polar water molecules interacting with positive and negative ions of a salt.

Solubility: Polar Compounds “Like dissolves like” Typically, hydrogen bonding occurs between the substance being dissolved and the polar water molecules Example: Sugar in water Ethanol in water

Figure 15.3: The polar water molecule interacts strongly with the polar O—H bond in ethanol.

Figure 15.4: Structure of common table sugar. Get interactions between water molecules the polar regions on the sugar (the Os) , and some hydrogen bonding at the -OH groups

“They go together like oil and water “They go together like oil and water.” (things that don’t dissolve or mix) Anything nonpolar will not mix well with anything polar Examples: Oil spill Salad dressing Can mix when shaken (LDF) and then may separate out (other forces)

How much is too much? There is a limit to the amount of a substance dissolved Saturated: the solution holds as much solute as possible at that temperature. Unsaturated: solution has not reached the limit

Can you have too much? YES! Can you have too much? YES! Supersaturated: have as much solute dissolved as possible, then cooled and all the solute stays dissolved. In other words…the solution contains more dissolved solid than a saturated solution created at the same temperature. These can begin crystallization of the solute at the slightest change

Energy changes and Solvation Any chemical change (including solvation) requires a change in energy Energy removed from or added to the reactants from the surroundings NaOH(s) Na+ (aq)+ OH- (aq) ΔH= -44.5 kJ/mol (that’s 44.5kJ released, so exothermic, per mole of NaOH) Because you are breaking the ionic bond, energy must be either released when breaking the bond, or consumed when making the new ions ALL changes in formula indicate a change in energy. However, sometimes the energy change is so small, you can’t tell that a change has occurred

Why some coffees “Put hair on your chest.” “Strong” coffee has more coffee dissolved in a given amount (say 1 pot) than “weak” coffee. Strong coffee = concentrated Weak coffee = dilute Concentration: the amount of solute in a given amount of solvent (or solution).

Molarity (M) M = moles of solute Most common way to express concentration Molarity is the number of moles of solute dissolved in each liter of solution Formula M = moles of solute liters of solution Dependent on temperature The higher the molarity the stronger the concentration

Molality (M ) M = moles solute . Another way to calculate concentration Formula M = moles solute . kilograms of solvent Not dependent on temperature The higher the molality the stronger the concentration

Colligative properties Colligative properties In the winter, why do we throw salt when it snows? Why does Emeril add salt to boiling water when cooking pasta?

Freezing point depression By adding salt (or other solutes) to water, the temperature of freezing drops it freezes at a lower temperature Because H bonding is disturbed Dependent on how much solute is added

Freezing Point Depression Calcs Antifreeze protects cars from freezing and overheating. Calculate the freezing point depression of a solution of 100. g of ethylene glycol (C2H6O2) antifreeze in 0.500 kg of water. Kf water = 1.86 oC/m Formula: Tf = Kfm i Kf : Molal Freezing Point depression constant (oC/m) i= Pieces that the material dissociates into (for ionic compounds only) (Keep I at 1 (one) for covalent compounds)

Freezing Point Depression and Boiling Point Elevation Freezing Point Depression and Boiling Point Elevation Solvent Formula Melting Point (°C) Boiling Point (°C) Kf (°C/m) Kb Water H2O 0.000 100.000 1.858 0.521 Acetic acid HC2H3O2 16.60 118.5 3.59 3.08 Benzene C6H6 5.455 80.2 5.065 2.61 Camphor C10H16O 179.5 ... 40 Carbon disulfide CS2 46.3 2.40 Cyclohexane C6H12 6.55 80.74 20.0 2.79 Ethanol C2H5OH 78.3 1.07

Boiling point elevation By adding salt (or other compounds) to water, the temperature of boiling goes up it boils at a higher temperature Interrupts H bonding Need more vapor molecules and greater pressure to get bubbles to form Takes more time to get vapors to add to bubbles The molecules that do get into the bubbles need more energy Dependent on how much solute is added

Boiling Point Elevation Calculations Water with salt added boils at a higher temperature than pure water. By how much will the boiling point change if 100.g of salt is added to 500. g of water? Kbwater = 0.52 oC/m Formula: Tb = Kbm i Kb : Molal Boiling Point elevation constant (oC/m) i= = Pieces that the material dissociates into (for ionic compounds only) (Keep I at 1 (one) for covalent compounds)

Freezing Point Depression and Boiling Point Elevation Solvent Formula Melting Point (°C) Boiling Point (°C) Kf(°C/m) ( Kb(°C/m) Water H2O 0.000 100.000 1.858 0.521 Acetic acid HC2H3O2 16.60 118.5 3.59 3.08 Benzene C6H6 5.455 80.2 5.065 2.61 Camphor C10H16O 179.5 ... 40 Carbon disulfide CS2 46.3 2.40 Cyclohexane C6H12 6.55 80.74 20.0 2.79 Ethanol C2H5OH 78.3 1.07

Figure 15.10: Pure water.

Figure 15.9: A bubble in the interior of liquid water surrounded by solute particles and water molecules.

Figure 15.10: Solution (contains solute). Figure 15.10: Solution (contains solute).

Vapor Pressure Reduction Vapor pressure changes as IMFs change For the same reasons boiling point is disturbed What would evaporate faster: Salt water Distilled water WHY?

Colligative properties interactive