Chapter 15 Solutions Like Dissolves Like

Definitions l Solution - l Solution - homogeneous mixture Solvent Solvent - present in greater amount Solute Solute - substance being dissolved

Solvation l Solvation – the process of dissolving solute particles are separated and pulled into solution solute particles are surrounded by solvent particles

Solvation l Dissociation – separation of an ionic solid into aqueous ions – movie movie NaCl(s)  Na + (aq) + Cl – (aq)

Solvation l Molecular Solvation – molecules stay intact C 6 H 12 O 6 (s)  C 6 H 12 O 6 (aq)

Solvation l Ionization – breaking apart of some polar molecules into aqueous ions HNO 3 (aq) + H 2 O(l)  H 3 O + (aq) + NO 3 – (aq)

Solvation Strong Electrolyte Non- Electrolyte solute exists as ions only - + salt - + sugar solute exists as molecules only - + acetic acid Weak Electrolyte solute exists as ions and molecules

Making solutions l In order to dissolve, the solvent molecules must come in contact with the solute. l Stirring moves fresh solvent next to the solute. l The solvent touches the surface of the solute. l Smaller pieces increase the amount of surface area of the solute.

Solution Formation l Nature of the solute and the solvent – Whether a substance will dissolve – How much will dissolve l Factors determining rate of solution... – stirred or shaken (agitation) – particles are made smaller – temperature is increased l Why?

Temperature and Solutions l Higher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often. – Speeds up dissolving. l Usually increases the amount that will dissolve (exception is gases)

How Much? l Solubility- The maximum amount of substance that will dissolve at a specific temperature (g solute/100 g solvent) l Saturated solution- Contains the maximum amount of solute dissolved. movie movie l Unsaturated solution- Can still dissolve more solute l Supersaturated- solution that is holding more than it theoretically can; seed crystal will make it come out. moviemovie

Solubility SATURATED SOLUTION no more solute dissolves UNSATURATED SOLUTION more solute dissolves SUPERSATURATED SOLUTION becomes unstable, crystals form concentration

Cloud Seeding l Ever heard of seeding the clouds to make them produce rain? l Clouds- mass of air supersaturated with water vapor l Silver Iodide (AgI) crystals are dusted into the cloud l The AgI attracts the water, forming droplets to attract others

Liquids l Miscible means that two liquids can dissolve in each other – water and antifreeze, water and ethanol l Partially miscible- slightly dissolve l Immiscible can’t mix – oil and vinegar

Solvation NONPOLAR POLAR l Detergents – polar “head” with long nonpolar “tail” – can dissolve nonpolar grease in polar water “Like Dissolves Like”

Solubility? l For solids in liquids, as the temperature goes up-the solubility usually goes up. l For gases in a liquid, as the temperature goes up-the solubility goes down. l For gases in a liquid, as the pressure goes up-the solubility goes up.

Mentos and Diet Coke l Solubility can also be affected by other factors such as a reduction in water surface tension and the formation of nucleation sites. Nucleation sites are places where gas molecules gather to form a bubble. Mentos contains compounds that reduce surface tension and at the same time the rough surface of the dissolving tablet provides nucleation sites. The combination in rather dramatic as evidenced in the video clip.video

Solubility l Solubility – maximum grams of solute that will dissolve in 100 g of solvent at a given temperature – varies with temp – based on a saturated solution

Solubility l Solubility Curve – shows the dependence of solubility on temperature

Solubility l Solids are more soluble at... – high temperatures. l Gases are more soluble at... – low temperatures. – high pressures (Henry’s Law). – EX: nitrogen narcosis, the “bends,” soda

The Bends Shallow Water Blackout DVD Movie

Gases in liquids... l Henry’s Law - says the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid – think of a bottle of soda pop, removing the lid releases pres. l Equation: S 1 S 2 P 1 P 2 =

Henry’s Law Example

Concentration is... l a measure of the amount of solute dissolved in a given quantity of solvent l A concentrated solution has a large amount of solute l A dilute solution has a small amount of solute – thus, only qualitative descriptions l But, there are ways to express solution concentration quantitatively

Concentration l The amount of solute in a solution. l Describing Concentration – % by mass - medicated creams – % by volume - rubbing alcohol – ppm, ppb - water contaminants – molarity - used by chemists – molality - used by chemists

Molarity - Most Important l The number of moles of solute in 1 Liter of the solution. Note: not solvent. l M = moles/Liter; such as 6.0 molar l What is the molarity of a solution with 2.0 moles of NaCl in 250 mL of solution?

Dilution Adding water to a solution

Dilution l Preparation of a desired solution by adding water to a concentrate. l Moles of solute remain the same.

Dilution l The number of moles of solute doesn’t change if you add more solvent! l The # moles before = the # moles after l Since M x L = moles then l M 1 x V 1 = M 2 x V 2 l M 1 and V 1 are the starting concentration and volume. l M 2 and V 2 are the final concentration and volume. l Stock solutions are pre-made to a known Molarity

Dilution l What volume of 15.8M HNO 3 is required to make 250 mL of a 6.0M solution? GIVEN: M 1 = 15.8M V 1 = ? M 2 = 6.0M V 2 = 250 mL WORK: M 1 V 1 = M 2 V 2 (15.8M) V 1 = (6.0M)(250mL) V 1 = 95 mL of 15.8M HNO 3

Preparing Solutions l 250 mL of 6.0M HNO 3 by dilution – measure 95 mL of 15.8M HNO 3 95 mL of 15.8M HNO 3 water for safety 250 mL mark – combine with water until total volume is 250 mL – Safety: “Do as you oughtta, add the acid to the watta!”

Making Solutions l Pour in a small amount of solvent l Then add the solute (to dissolve it) l Carefully fill to final volume. l Also remember: M x L = moles of solute l How many moles of NaCl are needed to make 8.0 L of a 0.75 M NaCl solution?

Practice l 2.0 L of a 0.88 M solution are diluted to 3.8 L. What is the new molarity? l You have 150 mL of 6.0 M HCl. What volume of 1.3 M HCl can you make? l You need 450 mL of 0.15 M NaOH. All you have available is a 2.0 M stock solution of NaOH. How do you make the required solution?

Making Solutions l 10.3 g of NaCl are dissolved in a small amount of water, then diluted to 250 mL. What is the concentration? l How many grams of sugar are needed to make 125 mL of a 0.50 M C 6 H 12 O 6 solution?

Percent Solutions... l Percent means parts per 100, so l Percent by volume: = Volume of solute x 100 Volume of solution l indicated %(v/v) l What is the percent solution if 25 mL of CH3OH is diluted to 150 mL with water?

Percent Solutions l Percent by mass: = Mass of solute(g) x 100 Mass of solution(g) l Indicated %(m/m) l More commonly used l 4.8 g of NaCl are dissolved in 82 g of solution. What is the percent of the solution?

Colligative Properties Depend only on the number of dissolved particles Not on what kind of particle

Colligative Applications l Common Applications – salting icy roads – making ice cream – antifreeze cars (-64°C to 136°C) fish

Solute particles weaken IMF in the solvent. Boiling Point Elevation

l The vapor pressure determines the boiling point. l Lower vapor pressure = higher boiling point. l Salt water boils above 100ºC l The number of dissolved particles determines how much, as well as the solvent itself.

Depends on # of Pieces l Electrolytes form ions when they are dissolved = more pieces. NaCl  Na + + Cl - (= 2 pieces) l More pieces = bigger effect

Vapor Pressure Decreased l The bonds between molecules keep molecules from escaping. l In a solution, some of the solvent is busy keeping the solute dissolved. l Lowers the vapor pressure

Freezing Point Depression l Solids form when molecules make an orderly pattern. l The solute molecules break up the orderly pattern. l Makes the freezing point lower. l Salt water freezes below 0ºC l How much depends on the number of solute particles dissolved.

Freezing Point Depression

Properties of Water l Ice is less dense than water l movie movie

Molality mass of solvent only 1 kg water = 1 L water

Molality l a new unit for concentration l m = Moles of solute kilogram of solvent l m = Moles of solute 1000 g of solvent l What is the molality of a solution with 9.3 mole of NaCl in 450 g of water?

Molality l Find the molality of a solution containing 75 g of MgCl 2 in 250 mL of water. 75 g MgCl 2 1 mol MgCl g MgCl 2 = 3.2 m MgCl kg water

Molality l How many grams of NaCl are req’d to make a 1.54m solution using kg of water? kg water1.54 mol NaCl 1 kg water = 45.0 g NaCl g NaCl 1 mol NaCl

Preparing Solutions l 500 mL of 1.54M NaCl 500 mL water 45.0 g NaCl – mass 45.0 g of NaCl – add water until total volume is 500 mL – mass 45.0 g of NaCl – add kg of water 500 mL mark 500 mL volumetric flask l 1.54m NaCl in kg of water

Types of Property Change Boiling Point Elevation Boiling Point Elevation (  t b ) – b.p. of a solution is higher than b.p. of the pure solvent Freezing Point Depression Freezing Point Depression (  t f ) – f.p. of a solution is lower than f.p. of the pure solvent

Boiling Point Change? l The size of the change in boiling point is determined by the molality.  T b = K b x m x n  T b is the change in the boiling point l K b is a constant determined by the solvent. l m is the molality of the solution. l n is the number of pieces it falls into when it dissolves.

What About Freezing? l The size of the change in freezing point is also determined by molality.  T f = K f x m x n  T f is the change in freezing point l K f is a constant determined by the solvent. l m is the molality of the solution. l n is the number of pieces it falls into when it dissolves.

Calculations l n = the # of Particles – Nonelectrolytes (covalent) remain intact when dissolved 1 particle n=1 – Electrolytes (ionic) dissociate into ions when dissolved 2 or more particles n=2 or more

Boiling and Freezing Points and Constants

Boiling–Point Elevation and Freezing–Point Depression

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

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

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

Problems l What is the boiling point of a solution made by dissolving 1.20 moles of NaCl in 7.50e2 g of water? l What is the freezing point? l What is the boiling point of a solution made by dissolving 1.20 moles of CaCl 2 in 750. g of water? l What is the freezing point?

Mole Fraction l This is another way to express concentration l It is the ratio of moles of solute to total number of moles of solute + solvent n solute n solute + n solvent X =

Mole Fraction Problem l What is the mole fraction of solute in a 35.5 percent by mass aqueous solution of formic acid (HCOOH)?

Molar Mass l We can use changes in boiling and freezing to calculate the molar mass of a substance l Find: 1) molality 2) moles, and then 3) molar mass

MM Example Problem l An aqueous solution made using 500. mL of solvent has a freezing point of -5.00°C. If 245 g of covalent solid was used, what is the molar mass of the solute? l If the empirical formula of the solute above is CH 2 O, what is the molecular formula?