POGIL: Principles of Solubility

Transcription

1 NAME: DATE: AP Chemistry POGIL: Principles of Solubility Why? The previous POGIL discussed a few factors affecting how and why solutions form based on entropy and interparticle forces. Another factor affecting solution formation is the enthalpy of the solution, or the energy required for the solution to form. If there is not enough energy available, or if the enthalpy (thermal energy) conditions are not conducive to solution formation, then the solvent and solute will not mix. SUCCESS CRITERIA Determine if a solution will form based on the entropy, attractive forces, and enthalpy of the solution. Predict the enthalpy changes to form a solution. MODEL 1: Formation of an Ideal Solution For a solution to form, the solvent and solute particles must be attracted to each other like dissolves like. When a solution forms, the solute particles and solvent particles must partially separate from each other, so their forces of attraction must be overcome before they can mix. The following enthalpy diagram represents the hypothetical step-bystep formation of a solution between hexane (C 6 H 14 ) and pentane (C 5 H 12 ). HEXANE PENTANE Solvation Key Questions 1. What type(s) of intermolecular forces hold hexane molecules together? (Circle all that apply.) 2. What type(s) of intermolecular forces hold pentane molecules together? (Circle all that apply.) 3. What type(s) of intermolecular forces hold the mixture of hexane and pentane together? (Circle all that apply.)

2 P O G I L : P r i n c i p l e s o f S o l u b i l i t y Page 2 4. At stage 1 we have a sample of liquid hexane and a sample of liquid pentane. At stage 4 we have a solution of hexane and pentane. Explain what is occurring to intermolecular forces and enthalpy between the stages of: a. 1 and 2? b. 1 and 3? c. 2 & 3 when they are mixed to make 4? 5. What does the above enthalpy diagram suggest about the relative strengths of the intermolecular forces in the separated solvent and solute versus when they are combined? a. The sum of the intermolecular forces in liquid hexane and in liquid pentane is weaker than the intermolecular forces in a solution of hexane and pentane. (pure substances < mixture) b. The sum of the intermolecular forces in liquid hexane and in liquid pentane is the same as the intermolecular forces in a solution of hexane and pentane. (pure substances = mixture) c. The sum of the intermolecular forces in liquid hexane and in liquid pentane is stronger than the intermolecular forces in a solution of hexane and pentane. (pure substances > mixture) 6. Which of the following explains why hexane and pentane spontaneously form a solution? (Circle all that apply.) a. The solution is less disordered than the separated solvent and solute. b. The solution is more disordered than the separated solvent and solute. c. The heat of solution is endothermic. (The solution has a higher enthalpy than the pure substances.) d. The heat of solution is exothermic. (The solution has a lower enthalpy than the pure substances.) MODEL 2: The Formation of a Non-Ideal Solution Considering the formation of a solution between hexane and pentane, because the intermolecular forces before and after mixing have virtually the same strength, hexane and pentane mix together to form what is called an ideal solution. However, ideal solutions are very rare. An example of a non-ideal solution is that water and ethanol (CH 3 CH 2 OH). Consider the following enthalpy diagram for the formation of a solution between water and ethanol:

3 ΔH solvation H 2 O + CH 3 CH 2 OH P O G I L : P r i n c i p l e s o f S o l u b i l i t y Page 3 7. Explain what is occurring to intermolecular forces and enthalpy between the stages of: a. 1 2 b. 2 3 c The arrow for the heat of solution (ΔH SOLN ) is pointing downward from 1 4. What is the thermodynamic significance of this? (HINT: If the mixture is lower than the components, energy was released.) a. The mixture is more stable than the separated components and the solution process is endothermic. b. The mixture is less stable than the separated components and the solution process is endothermic. c. The mixture is more stable than the separated components and the solution process is exothermic. d. The mixture is less stable than the separated components and the solution process is exothermic.

4 P O G I L : P r i n c i p l e s o f S o l u b i l i t y Page 4 9. A process tends to be spontaneous if either: i. The final state is more disordered than the initial state. ii. Heat is transferred from the system to the surroundings. Why do water and ethanol mix to form a solution? (Circle all that apply.) a. The solution is less disordered than the separated solvent and solute. b. The solution is more disordered than the separated solvent and solute. c. The heat of solution is endothermic. d. The heat of solution is exothermic. 10. Draw the Lewis Structures for water and ethanol, with partial charges. (You can Google this.) WATER ETHANOL 11. What intermolecular forces hold water molecules together in liquid water? (Circle all that apply.) 12. What intermolecular forces hold ethanol molecules together in liquid ethanol? (Circle all that apply.) 13. What intermolecular forces hold the molecules together in an ethanol and water solution? (Circle all that apply.) 14. As you saw in the previous POGIL, oil and water don t mix. Oil is a hydrocarbon, like hexane, but with a longer chain of carbon atoms. Hexane and water don t mix for the same reasons oil and water don t mix. a. What IM forces hold hexane molecules together in liquid hexane? See Model 1 for the Lewis Structure. (Circle all that apply.) b. What IM forces would hold hexane and water together? (Circle all that apply.) c. The strength of dispersion forces is proportional to how much surface area two neighboring molecules electron clouds overlap. Compare the strength of dispersion forces between a molecule of water and a molecule of hexane. Circle which statement is most accurate. i. The sum of intermolecular forces in liquid hexane and in liquid water is weaker than the intermolecular forces in a solution of hexane and water. (pure substances < mixture) ii. The sum of intermolecular forces in liquid hexane and in liquid water is stronger than the intermolecular forces in a solution of hexane and water. (pure substances > mixture)

5 Enthalpy, H P O G I L : P r i n c i p l e s o f S o l u b i l i t y Page 5 d. Sketch an enthalpy diagram similar to Model 2 for the hypothetical formation of a solution of water and hexane. Include the following: i. Liquid Water ii. Liquid Hexane iii. Separated Water iv. Separated Hexane v. Mixed Water and Hexane e. Looking at your enthalpy diagram, why don t water and hexane mix? MODEL 3: Enthalpy of Solution When a solution forms between a solid and a liquid, or between two liquids, the following three things must happen: Solute particles must separate from each other. This is always an endothermic process, ΔH is positive! o Solute + Heat Separated Solute Particles ΔH solute > 0 kj/mol Solvent particles my separate from each other. This is always an endothermic process, ΔH is positive! o Solvent + Heat Separated Solvent Particles ΔH solvent > 0 kj/mol Solute and solvent particles mix. This is always an exothermic process, ΔH is negative! o Separated Solute + Separated Solvent Heat Solution ΔH solvation < 0 kj/mol The enthalpy for the entire process of forming a solution is calculated by: ΔH solution = ΔH solute + ΔH solvent + ΔH solvation VERY IMPORTANT: If enthalpy is negative, the process is EXOTHERMIC, heat is being removed. If enthalpy is positive, the process is ENDOTHERMIC, heat is being absorbed.

6 P O G I L : P r i n c i p l e s o f S o l u b i l i t y Page 6 Solvation 15. Refer to the enthalpy diagram for the following questions: a. Draw an arrow on the diagram that represents ΔH solution. b. The enthalpy of solution is (circle one): <0kJ/mol 0kJ/mol >0kJ/mol c. As the solution is mixed, the temperature will (circle one): increase decrease d. The enthalpy of solution is (circle one): exothermic endothermic e. What is the phase of the solute? How do you know? 16. When solid ammonium nitrate, NH 4 NO 3, dissolves in water the solution becomes extremely cold. (Ammonium nitrate is used in chemical cold packs.) a. The dissolving of NH 4 NO 3 is (circle one): exothermic endothermic b. The enthalpies for each stage of ammonium nitrate solution are listed. What is the enthalpy of solution? ΔH NH4NO3 = 646 kj/mol ΔH H2O = 239 kj/mol ΔH solvation = -381 kj/mol c. Given your answers for a and b, why does ammonium nitrate dissolve in water?