DATE: POGIL: Colligative Properties Part 1

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Norma Bishop
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1 NAME: AP Chemistry DATE: POGIL: Colligative Properties Part 1 Why? There is a general misconception that adding sodium chloride to cooking water for pasta increases the temperature of the boiling water so that it cooks the pasta faster. Adding sodium chloride to boiling water does increase the temperature by a degree or so, but not enough to greatly speed up the cooking of your pasta. (You are adding flavor to the pasta.) But it does raise the temperature of the boiling water slightly, and the reason why has to do with the forces acting inside the water. Success Criteria Explain how solutes affect the intermolecular forces of a solvent and vapor pressure. Calculate the boiling point elevation for a solvent after the addition of a molecular or an ionic solute. Phenomenon What is vapor pressure? Bell Jar Demonstration - Observations: Possible Explanation: Mason Jar Demonstration - Observations: Possible Explanation:

2 Model 1: Vapor Pressure Lowering Suppose a pure liquid is placed in an otherwise evacuated container. Flask A The container is sealed. Flask B An equilibrium is established between the liquid and its vapor. Curve 1 The vapor pressure is measured. P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 2 Flask C A nonvolatile solute is dissolved in the same liquid as Flask A. Flask D An equilibrium is established between the solution and the vapor of the solvent. Curve 2 The vapor pressure is measured. Key Questions 1. The liquid in Flask A is volatile, therefore, it absorbs heat form the surroundings and its molecules gain enough energy to enter the gas phase. What does in mean for a substance to be nonvolatile? 2. There are two pressures measured, which one is P, and which one is P? 3. What happens to the vapor pressure of the liquid when a nonvolatile solute is dissolved into it? 4. The pressure change is directly related to the mole fraction of solvent to solution through Raoult s Law. P A = X A P A What does X A represent? How is it calculated?

3 P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 3 5. If you have only two components in a solution, solvent A and solute B, Dalton s Law of Partial Pressures states: P solution = P A + P B a. If solute B is nonvolatile, what is its vapor pressure contribution? PB = b. Rewrite Dalton s Law including Raoult s Law for solvent A and solute B s pressure contribution. Psolution = c. Which of the following is true for the mole fraction for solvent A? X A = i) X A <1 ii) X A = 1 iii) X A > 1 d. Referring to your answers for 5b and 5c, which of the following is true? i) P soln < P A ii) P soln = P A iii) P soln > P A mol A (mol A+mol B) PRACTICE A. Suppose you dissolved 10.0 g of sucrose (C 12 H 22 O 11 ) in 50.0 g of water. What will be the vapor pressure at 100C? 1) Calculate the number of moles of sucrose. 2) Calculate the number of moles of water. 3) What is the mole fraction (X) of water? 4) The vapor pressure of pure water at 100C is 1 atm. What is the vapor pressure of the sugar solution?

P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 4 Model 2: Phase Diagrams Phase diagrams show

Label the following on the phase diagram for water: a. Solid Phase b. Liquid Phase c. Gas Phase d.

Will the sugar solution in Practice A boil at 100C and 1 atm of pressure? Why/why not? 8.

4 P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 4 Model 2: Phase Diagrams Phase diagrams show the states and transitions between states for a substance at different pressure and temperatures. Key Questions 6. Label the following on the phase diagram for water: a. Solid Phase b. Liquid Phase c. Gas Phase d. Melting, Freezing e. Boiling, Condensing f. Sublimating, Deposition 7. Will the sugar solution in Practice A boil at 100C and 1 atm of pressure? Why/why not? 8. Below is a phase diagram for water after the addition of a nonvolatile solute. Describe what has happened to the different phases of the solution, and the normal boiling point and normal freezing point.

5 P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 5 Model 3: Molality and Boiling Point Elevation The presence of solute particles increases the boiling point of the solvent because the solute particles interfere with the escape of the solvent molecules from the liquid phase to the gas phase. The solvent molecules require higher kinetic energy to overcome the intermolecular forces in order to vaporize. The extent to which the boiling point is elevated, ΔT B, is directly dependent on the concentration of the solute. Since molarity (M) can change with a change in temperature, we use molality (m) for these calculations. molality (m) = ΔT B = i K B m mol solute kg solvent i = van t Hoff Factor the number of particles of solute in solution o Molecules = 1 o Ionic Compounds = # ions It s a little more complicated, but this will do at this stage. K B is the boiling point constant, which is specific to each substance. o Ex: K B water = C/m Key Questions 9. Explain how the molarity (M) of a solution is dependent on the temperature of the solution. 10. Plug in the units for i, K b, and m and show how they cancel. 11. Is the boiling point of 0.01 m KF higher or lower than that of 0.01 m glucose (C 6 H 12 O 6 )? Explain your answer.

6 P O G I L : C o l l i g a t i v e P r o p e r t i e s P a r t 1 Page 6 PRACTICE B. Ethylene glycol (HOCH 2 CH 2 OH) is a molecular substance that is added to that water put in an automobile s radiator. One of the advantages antifreeze offers is that the water will boil at higher temperature than its normal boiling point, helping to prevent engine overheating. What is the boiling point of a 60.0% (w/w) ethylene glycol solution? 1) Using the percentage, determine the grams of ethylene glycol and water in the solution. 2) What is the molality of the solution? 3) What is the boiling point elevation (ΔT B )? 4) What is the boiling point of the solution? C. Two beakers were placed in a closed container. One beaker contained water, and the other a concentrated aqueous sugar solution. With time, the solution volume increased and the water volume decreased. Explain what is happening on a molecular level. Consider the vapor pressure of the water versus that of the sugar solution.

Soluble: A solute that dissolves in a specific solvent. Insoluble: A solute that will not dissolve in a specific solvent. "Like Dissolves Like"

Soluble: A solute that dissolves in a specific solvent. Insoluble: A solute that will not dissolve in a specific solvent. Like Dissolves Like Solutions Homogeneous Mixtures Solutions: Mixtures that contain two or more substances called the solute and the solvent where the solute dissolves in the solvent so the solute and solvent are not distinguishable