If you are not part of the precipitate, you are part of the solution… Chapter 15 Solutions If you are not part of the precipitate, you are part of the solution…

solutions are everywhere! air, shampoo, soda, gasoline, cough syrup, etc… defined simply as a homogeneous mixture Solvent: substance in largest amount Solute: other substances in “aqueous solutions” (aq), water is the solvent

15.1 solubility when an ionic compound dissolves, it conducts electricity ionic compounds separate into their ions

as it dissolves, water molecules surround and rip away the ions

notice which end of the water butts up against the cations & anions

15.1 Dissolving a solid

15.1 water can also dissolve polar substances like ethanol

15.1 the H-bonding plays a big role here

sucrose dissolves well, why? H-bond attraction! (see all the OH’s)

not everything can dissolve in water if it doesn’t have the structure for H-bonding and it’s not polar, then water molecules don’t have a way to attach to it and dissolve it

15.1

15.1 like dissolves like Polar substances dissolve polar stuff Nonpolar substances dissolve nonpolar stuff The characteristics of the solvent must match the characteristics of the solute Polar H-bonding Nonpolar

15.2 solution concentration there is a limit to how much you can dissolve; like sugar in iced tea when the limit is reached, the solution is saturated The Temperature of the solvent plays a role (think of iced tea vs hot tea) if a solvent still hold more solute, it is unsaturated!

15.2 sometimes when a saturated solution is allowed to cool, it will remain saturated without crystals coming out It is supersaturated: a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances Supersaturated solutions are very unstable; a single crystal can cause the excess to crystallize out

15.2 Supersaturated Solution

concentrated and dilute concentrated a lot of solute dissolved in the solvent dilute a relatively small amount of solute dissolved. Super saturated the solvent holds more solute than it should Are those qualitative or quantitative terms? Qualitative

15.3 factors affecting the rate of dissolution There are three key factors on how fast a solute dissolves surface area stirring temperature

15.3 Dissolving occurs at the surface of a solid, so the more surface area exposed to a solvent, the faster it will dissolve (powdered sugar v. sugar cube) Stirring moves the particles of solute away from the solid mass and helps bring in fresh particles of solvent Increasing the temperature speeds up the particle motion. Also, most solvents can dissolve more solutes at higher temps

Solubility Curve How much silver nitrate will there be in a saturated solution at 10o C? How would you describe a solution of KBr that had 100 g at 60o C?

the opposite is true for gases dissolved in water 15.3 the opposite is true for gases dissolved in water higher T or more stirring means they don’t dissolve well Consider the effects of heating and stirring on a soda…

15.3 solutions of gas also depend on pressure lower the pressure and the gases are more easily released

15.3 higher pressure can “force” more gas into a liquid cans and glass bottles can keep gases under pressure

1986: Hundreds gassed in Cameroon lake disaster At least 1,200 people are feared dead and hundreds more hospitalized after a cloud of lethal gas escaped from a volcanic lake in West Africa Most of the victims died in their sleep. The gas killed all living things within a 15-mile radius of the lake, the area is still highly contaminated. It is not yet known what caused the gas to escape the lake Eyewitnesses described how the normally clear waters of the lake turned a reddish brown, while a sudden wind arose and whipped up huge waves.

15.3 Lake Nyos, Cameroon 1,700 people killed in 1986

15.3 Lake Nyos, Cameroon 1,700 people killed in 1986

Questions?

Quantitative Concentration Students know how to calculate the concentration of a solute in terms of grams per liter molarity parts per million percent composition.

Parts per million The Salton Sea contains 44 grams of salt per 1000 grams of water. What is the concentration in parts per million (ppm)? first, calculate how many parts there are per gram, then convert that to parts per million 44g/1000g = .044 x 1,000,000 = .044 44,000 ppm

Questions? 33

Molarity molarity: a measure of the concentration of a solute in a solution. molarity = moles solute/Liter solution M = mol / L e.g. a 2.7M soln means there are 2.7 mol of solute per liter of solution given that M = mol/L, you should be able to solve for any one variable if you know the other two

example 2.25 M = How many mols HCl are in 3.0 L of a 2.25 M soln? mol M = mol/L How many mols HCl are in 3.0 L of a 2.25 M soln? mol 3.0 L 2.25 M = 2.25 M • 3.0 L = mol 6.8 = mol

example 2.00 M = How many grams are in 2.50 L of a 2.00 M NaCl soln? 5.00 mol 58.5g 1 mol mol 2.50 L = 2.00 M = 293 g NaCl 2.00 M • 2.50 L = mol 5.00 = mol

the properties of solns: boiling point and freezing point Colligative properties are properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution, Colligative properties include: lowering of vapor pressure Elevation of boiling point Depression of freezing point Osmotic pressure.

the solute interferes with the formation of bubbles and raises the boiling point

Therefore it takes more energy to form the bubbles add more solute and the bp goes higher it doesn’t matter what the solute is, just how much

Salutes also interfere with freezing They get in the way of building a solid, lowering the freezing point

both the raising of bp and the lowering of fp is seen when we add antifreeze to water

Questions?