Chapter 15 Solutions

solutions are everywhere! air, brass, shampoo, Diet Coke, gasoline, cough syrup, etc. defined simply as a homogeneous mixture (chm a) substance in largest amt = solvent other substances = solute in aqueous solns water is solvent

15.1 solubility when an ionic cmpd dissolves, it conducts electricity therefore ionic cmpds break up into their ions

15.1 as it dissolves, waters surround and rip away the helpless ions

15.1 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 here; why?

15.1 the H-bonding plays a big role here the more you look like water the more likely it will dissolve you

15.1 why do you think sucrose here dissolves so well? H-bond attraction! (see all the OH’s?)

not everything can dissolve in water if you have no polar handle on you, water doesn’t recognize you like the long nonpolar hydrocarbon above

15.1

15.1 so to dissolve in water you have to be able to break the net, to interrupt the H-bonding; you have to fit in polar molecules and ionic cmpds can do that; nonpolar cannot like dissolves like is a good general phrase nonpolar solutes can dissolve into nonpolar solvents, too!

15.1 sugar sugar in np solvent sugar in water

15.1 grease grease in np solvent grease in water

15.2 solution concentration there is a limit to how much you can dissolve; like sugar in iced tea when the limit is reached = saturated depends on T (think: iced T vs hot tea) can it still hold more solute? unsaturated! add more it will dissolve…

15.2 sometimes when a saturated soln is allowed to cool, it will remain saturated without crystals coming out!!! = supersaturated very unstable; a single crystal will cause the xs to crystallize out to saturation

15.2 Supersaturated Solution

15.2 chemists also use concentrated and dilute - both really qualitative terms concentrated basically means there’s a lot of stuff dissolved in there dilute means not so much but neither tells us about how much (quantitative)

15.3 factors affecting the rate of dissolution Three big factors here on how fast something dissolves surface area stirring temperature

15.3 dissolving occurs at the surface of a solid, so… the more surface exposed to a solvent, the faster it will dissolve like powdered sugar vs rock candy fine salt vs rock salt

15.3 stirring will increase the rate, too in slow motion the solute is dissolved in the solvent, but instead of hanging around and getting in the way, stirring gets them out of the way this exposes new solute to new solvent

15.3 raising the T means speeding up the solvent molecules this lets them get their job done faster not only does it happen faster, but most solutes dissolve more completely at higher T’s…

15.3

15.3 but! the opposite is true for gases dissolved in water higher T or more stirring means they don’t dissolve well that’s why we don’t heat up soda or aquariums, and why we don’t stir (shake) soda

15.3 why do carbonated beverages do this? sol of gas also depends on pressure release the P and bye bye gases

15.3 can “force” more gas into liq w/ higher pres cans and glass bottles can keep gases under pres; not plastic

15.3 Lake Nyos, Cameroon overturned 2000 people killed in 1986

15.5 molarity concentration with numbers! most commonly used unit is molarity molarity (M) = moles solute/L soln e.g. a 2.7M soln means there are 2.7 mol of crud per litre of soln given that M = mol/l, you shd be able to solve for any one variable if I give you other two…

example 2.25 M = How many mols HCl are in 1.45 L of a 2.25 M soln? mol M = mol/L How many mols HCl are in 1.45 L of a 2.25 M soln? mol 1.45 L 2.25 M = 2.25 M • 1.45 L = mol 3.26 = mol

example 6.5 M = How many mols HCl are in 3.5 L of a 6.5 M soln? mol M = mol/L How many mols HCl are in 3.5 L of a 6.5 M soln? mol 3.5 L 6.5 M = 6.5 M • 3.5 L = mol 23 = mol

example 1.5 M = How many mols NaCl are in 2.5 L of a 1.5 M soln? mol M = mol/L How many mols NaCl are in 2.5 L of a 1.5 M soln? mol 2.5 L 1.5 M = 1.5 M • 2.5 L = mol 3.8 = mol

example 1.34 M = How many grams are in 2.50 L of a 1.34 M NaCl soln? 3.35 mol 58.5g 1 mol mol 2.50 L = 1.34 M = 196 g NaCl 1.34 M • 2.50 L = mol 3.35 = mol

15.10 the properties of solns: boiling point and freezing point when you add solute to water it boils at a higher T than 100˚C! and freezes at less than 0˚C! why does solute have this effect; raising bp and lowering fp [BTW, it happens with all the solutes in all liquid solvents]

15.10 those darn solute guys make it tough for the water to get into the bubble and exert vp!

15.10

15.10

15.10 so we have to give them more E to maintain the vp to fight the atm above add more the bp goes higher notice it doesn’t matter what the solute is, just how many = colligative property

15.10 also interferes with freezing they get in the way of building a solid this means lowering the fp like salt on ice!

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