Factors Affecting solubility Mr Shields Regents Chemistry U

Factors Affecting solubility Mr. Shields Regents Chemistry U 12 L 06 1

We learned there are 3 things that affect the RATE solids dissolve in liquids. 1) Temperature 2) Surface area 3) Stirring It turns out that there also 3 things that can Affect how much solute will dissolve in a solvent 1) The Nature of solute & solvent 2) The Temperature 3)The Pressure (when dealing w/gases) 2

The Nature of Things When we talk about the NATURE of solute & solvent we are referring to their similarities in Polarity In other words are the solute and solvent… a) molecules (if so are they non-polar or polar) b) ionic For example, how would you classify the following: H 2 0 CH 3 CH 2 OH Ca(OH)2 CCL 4 Br 2 KI CHCl 3 NH 4 NO 3 3

The Nature of Things Compounds that are of similar polarity will in general Be soluble in one another. So… - polar solvents will dissolve polar solutes - non-polar solvents will dissolve non-polar solutes BUT - In general ONLY water will dissolve Ionic compounds In other words … the principal of “LIKE DISSOLVES LIKE” 4

Solubility Table Hey! What’s with the Ethanol? This molecule is Polar but it dissolves Non-polar solutes! Polar Non. Polar Ethanol Polar S NS S Non. Polar NS S S Ionic S NS NS Since it’s polar why don’t we just classify it under Polar? Well, it has something to do with the ends of this molecule. 5

Ethanol as a solvent has Unique properties. It behaves Like BOTH a polar and Non-polar solvent. How does it do this? Ethanol, and solvents like it, have both polar and non-polar regions within the molecule CH 3 CH 2 -OH Non-polar end Polar end Due to this duality the molecule is able to dissolve both Polar and non-polar covalent solutes BUT NOT IONIC 6 SALTS

Temperature When we consider the effect of temperature on Solubility we need to ask two questions: 1) Are we dissolving a solid in a liquid 2) Are we dissolving a gas in a liquid? This is important because … The solubility of gases in liquids dec. with inc. temp. and … The solubility of solids in liquids (in general) inc. with inc. temp 7

Gases & Temperature Let’s consider gases in liquids first… It is important to recognize that for most gases their Solubility in WATER is typically very low. - At RT only 8 mg of O 2 will dissolve in 1000 g H 20 Consider a glass of soda You pour it while sitting outside on a warm sunny day. The soda is at first very bubbly and refreshing but after awhile it becomes “flat”. Why? Because the solubility of CO 2 (g) decreases as the soda Warms up, and the excess gas leaves the liquid (as bubbles). 8

This relationship is true for all gases – As temperature goes up solubility goes down RT ICE/WATER Inc - solubility of O 2 at 0 deg C = 15 mg /1000 g H 20 - solubility of O 2 at 25 deg C = 8 mg /1000 g H 20 9

Gas Solubility Some gases like CO 2 have a higher solubility than other gases Because they can interact with water in a reversible reaction CO 2 + H 2 O H 2 CO 3 Room Temp Non-polar 10

SO… gas solubility decreases with increasing temperature But why is this? It has to do with the KE of the dissolved gas molecule As temperature increases so does KE and thus so does Molecular Velocity KE = ½ mv 2 Molecules of gas moving at higher Velocities can escape the intermolecular Force of Attraction between them & the Liquid more easily As KE inc. Molecular Velocity Increases 11

Total gas dissolved One last point. The total amount of dissolved gas is simply the sum of the Solubility of all The gases present. Which means… the Solubility of each Gas Is independent Of the presence of Other gases. + 12

Solubility Of Solids Table G in your reference table shows what the solubility Of several solids and gases is as a function of temperature. These represent “saturation” curves What does the x and Y axis Tell us? What is the solubility of KNO 3 At 50 deg C? 83 g/100 g H 2 O What is the solubility of KCl in 200 g of water at 50 deg. C? 84 g 13

Solubility Curves Using Table G … What gases are represented? SO 2, NH 3, HCl Which solute is least affected By temperature change? Na. Cl Which solute is most affected By temperature change? KNO 3 Which compound is least soluble at 20 deg. C? KCl. O 3 Which compound is most soluble at 20 deg. C? KI 14

Problem: If 100 g of water at 100 deg C can dissolve 165 g of Na. NO 3, how many grams of Na. NO 3 precipitate from solution (i. e. crystallize) as the solution is chilled to 20 deg. C? 15

Predicting Solubility Table F in the reference tables handbook will help you to Predict the solubility of an ionic compound. Notice the table is divided into two sections: - Ions that form Soluble Compounds - Ions that form Insoluble Compounds IMPORTANT: Each section lists EXCEPTIONS to the rule 16

Predicting Solubility Table F in the reference tables handbook will help you to Predict the solubility of an ionic compound. 17

Solubility Problem Predict the solubility of the following compounds: Ca(NO 3)2 Rb 2 S Ni(OH)2 Ag. Cl Al 2(SO 4)3 Ba. SO 4 Li. OH (NH 4)2 Cr. O 4 Ca. HCO 3 Sol Insol. Sol. 18

Predicting Solubility Though you can refer to Table F you might want to Remember a few rules such as the following: ALL COMPOUNDS OF GROUP 1 METALS ARE SOLUBLE Li+ Na+ K+ Rb+ Cs+ Fr+ ALL AMMONIUM COMPOUNDS and ALL NITRATES ARE SOLUABLE NH 4 + NO 3 - 19

Effect of Pressure 2 rules regarding pressure… 1) Pressure HAS NO EFFECT on the solubility of solids in liquids or liquids in liquids AND … 2) Pressure has a BIG EFFECT on the solubility of a gas in a liquid. For example … Consider what happens when you open a bottle of warm soda 20

Why does soda FIZZ Or the Can “POP” when opened? Pressure allows more gas to dissolve in the liquid. Popping the top releases The pressure … So what happens to the Solubility of the gas in The liquid? 21

Henry’s Law The amount of a gas dissolved in a solution (C) is directly proportional to the pressure of the gas (P) above the solution. C 1 = C 2 P 1 P 2 In other words, if P doubles then the concentration of the Gas doubles 22

Solubility of a gas Problem; Henry’s Law at work: If the initial pressure of a gas over a liquid is 2 atm. and the Solubility of the gas is 9. 5 mg per 100 g of water what is the Solubility of the gas if the pressure of the gas is increased To 8 atm. C 1 = C 2 P 1 P 2 C 2 = 38 g/100 ml 9. 5 = C 2 2 8 C 1 = 9. 5 P 1 P 2 23