Solutions Entry Task: Nov 19 th Monday Question: What are the 2 components that make up a solution? You have 5 minutes.

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Entry Task: Nov 19th Monday

Question:

What are the 2 components that make up a solution?

You have 5 minutes

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Agenda:

• Sign off discuss Ch. 13 sec. 1-3 reading• In-class solubility curve• HW: Pre-Lab Solutions

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I can…

• Explain the components and physical/chemical processes of a solution

• Describe and interpret solubility curves of a substance.

• State the factors that affect the solubility of a solute in solution.

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BREAK OUT AP EQUATION SHEET

These formulas are rarely or not at all on the AP Exam

Density of gas

Root mean Speed of gas

Kinetic energy of gas molecules and moles of gas

Grahams Law

Osmotic pressure and Beers Law

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Chapter 13Properties of Solutions

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• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

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The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

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How Does a Solution Form?

As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

© 2012 Pearson Education, Inc.

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Energy Changes in Solution• Three processes affect the energetics of the process:

Separation of solute particles

Separation of solvent particles

New interactions between solute and solvent

The enthalpy change of the overall process depends on H for each of these steps.

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Energy Changes in Solution• Three processes affect

the energetics of the process: Separation of solute

particles Separation of solvent

particles New interactions

between solute and solvent

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Why Do Endothermic Processes Occur?

• Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.

• Yet we know that in some processes, like the dissolution of NH4NO3 in water, heat is absorbed, not released.

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Enthalpy Is Only Part of the Picture

• The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.

• So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.

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Student, Beware!

Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved. It may have reacted.

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Student, Beware!

• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.

• If you can’t get it back, the substance didn’t dissolve, it reacted.

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Types of Solutions• Saturated

Solvent holds as much solute as is possible at that temperature.

Dissolved solute is in dynamic equilibrium with solid solute particles.

• UnsaturatedLess than the maximum

amount of solute for that temperature is dissolved in the solvent.

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Types of Solutions

SupersaturatedSolvent holds more solute than is normally possible at

that temperature.These solutions are unstable; crystallization can

usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

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Factors Affecting Solubility

• Chemists use the axiom “like dissolves like”:

Polar substances tend to dissolve in polar solvents.Nonpolar substances tend to dissolve in nonpolar

solvents.

• The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

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Factors Affecting Solubility

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Factors Affecting SolubilityGlucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.

• Vitamin A is soluble in nonpolar compounds (like fats).

• Vitamin C is soluble in water.

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Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

• Larger molecules have stronger dispersion forces.

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Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But the solubility of a gas in a liquid is directly proportional to its pressure.

© 2012 Pearson Education, Inc.

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Gases in Solution – Henry’s Law• The solubility of liquids and solids

does not change appreciably with pressure.

• The solubility of a gas in a liquid is directly proportional to its pressure.

Cg = kPg

whereCg is the solubility of the gas;

k is the Henry’s law constant for that gas in that solvent;

Pg is the partial pressure of the gas above the liquid.

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Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

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Temperature

• The opposite is true of gases:Carbonated soft

drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

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Solubility Chart

What is the solubility of KNO3 at 30ºC?

~45 grams of KNO3

What temperature will 50 grams of KCl dissolve?

~75°C

If I add 30 grams of K2Cr2O7 at 60°C, what type of solution do we have?

Unsaturated

How much more solute can we add to make it saturated?

If the temperature increased from 60ºC to 80ºC, how much more K2Cr2O7 can be added?

~less than 10 grams

58-38 = 20 more grams

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4. Rank the following solutions, KCl in water, CH2Cl2 in benzene, C6H6, methanol, CH3OH, in water, in terms of increasing magnitude of solvent-solute interaction, and indicate the principal type of interaction in each case.

• The benzene has dispersion forces and the interactions with CH2Cl.

• Methanol has hydrogen bonding and would dissolve in water.

• KCl being ionic would have a ion-dipole attraction and would readily dissolve in water

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5. The dissolution of ammonium nitrate in water is an endothermic process. Describe the solution process on terms of three distinct components, and show how the

sum of these can be a net positive overall.

• ∆H1 is solute to solute (NH4+ NO3

-) <0

• ∆H2 is solvent to solvent (H2O H2O ) <0

• ∆H3 is solvent to solute (NH4+ NO3

- H2O ) >0

• When the overall process requires MORE energy into the system then the dissolution process then its endothermic

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8. The enthalpy of solution of KBr in water is about +19.8 kJ/mol. The process, then, is endothermic. Nevertheless, the solubility of KBr in water is relatively high. Why does the solution process, although endothermic, proceed?

• There must be a stronger attraction between the water and potassium and water and bromide than for each other and a greater need for disorder. So energy is required for this process- endothermic.

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13.9 The solubility of Cr(NO3)3 •9H2O in water is 208g per 100g of water at 15°C. A solution was prepared by dissolving 324 g per 100g of water at 35°C. When this solution slowly

cooled to 15°C , no precipitate forms. A) What term describes this solution? B) What action might you take to cause

crystallization from the solution?

• As you heat the solution, you increase the solubility making it super saturated.

• If you seed (add a crystal of solute), the solution will crystalize and precipitate out.

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13.16 Which of the following in each pair is likely to dissolve in hexane, C6H14: a) C6H12 or C6H12O6; b) CH3CH2COOH or CH3CH2COONa; c) HCl or CH3CH2Cl. Explain each case.

• a) C6H12 or C6H12O6; because its nonpolar – like hexane

• b) CH3CH2COOH or CH3CH2COONa; The propionic acid does have H-bonding but its weaker than the ion-ion IMF

• c) HCl or CH3CH2Cl. The hydrocarbon group at the end has dispersion forces

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In-class Solubility Chart

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PRACTICE:

1. What is the solubility of K2Cr2O7 at 30°C?_______

2. What temperature will 20 grams of KCl dissolve?_____

3. If I add 40 grams of Pb(NO3)2 at 40°C, what type of solution do we have?

4. How much more solute can we add to make it saturated?

5. If the temperature increased from 60ºC to 80ºC, how much more NaCl can be added?

15 g

There is none

Unsaturated

35 grams

~2-3 grams

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YOUR TURN:

6. What is the minimum temperature needed to dissolve 35 grams of potassium chloride in 100 grams of water?

 7. At what temperature do potassium chloride and potassium nitrate have the same solubility?

 8. If 250 grams of potassium dichromate are mixed with 100 grams of water at 85°C, how much will not dissolve?

9. If 15 grams of potassium nitrate are added to 100 grams of water at 30 °C, how much more must be added to saturate the solution?

30°C

~22°C

63-77 = 14 g

15-75 = 60 grams

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10. A 100 grams of water at 20 °C are saturated with lead II nitrate. If this solution is heated to 40 °C; how much more can be dissolved?  

11. A 100 grams of water at 90 °C are saturated with potassium chlorate. If this solution is cooled to 35°C, how much of the solid will precipitate? 

12. How much lead II nitrate will dissolve in 50 grams of water at 40°C? 

YOUR TURN:

55g-75g = 20g

45g – 13g = 32g

76/2 = 38 g

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13. How much sodium nitrate will dissolve in 10 grams of water at 10 °C? 

14. If 50 grams of water are saturated at 90 °C with potassium chlorate and then cooled to 40°C, how much will precipitate?  

15. What temperature is needed to dissolve twice as much potassium chloride as can be dissolved at 0 °C in 100 grams of water?

YOUR TURN:

34g x 10% = 3.4g

46-13g/2 = 16.5g

At 0ºC 27 grams x 2 = 54g at 85ºC

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HW: Ch. 13 sec 4-5 notes