HW #8: 8.34, 8.36, 8.44, 8.48, 8.50, 8.56, 8.60, 8.62, 8.74, 8.78, 8.88, 8.94, 8.108, 8.110, 8.114, 8.116

Transcription

1 Chemistry 121 Lecture 16: States of Matter and Phase Changes; Intermolecular Forces and Consequences: Liquids, Vapor Pressure, Boiling Point, & Relative Humidity Sections 8.1, 8.2, 8.12, 8.13 in McMurry, Castellion, et. al. 6 th edition HW #8: 8.34, 8.36, 8.44, 8.48, 8.50, 8.56, 8.60, 8.62, 8.74, 8.78, 8.88, 8.94, 8.108, 8.110, 8.114, Lesson Objectives: 1. Use the Gibbs-Helmholtz equation and values for H and S to determine melting points and boiling points of common substances 2. Describe intermolecular forces and rank them according to strength 3. Relate strength of intermolecular force to vapor pressure & boiling point, surface tension, and water solubility 4. Given a series of compounds, be able to rank them in terms of boiling point and water solubility 5. Rationalize why acid/base reactions within a molecule can greatly affect water solubility and boiling point 6. Define vapor and equilibrium vapor pressure 7. Relate vapor pressure to boiling point 8. Given a series of compounds, be able to rank them in terms of vapor pressure and surface tension 9. Define relative humidity and carry out straightforward relative humidity calculations 10. Relate relative humidity and temperature to the ability of humans to maintain temperature by evaporative cooling 1

2 8.1: States of Matter and Their Changes Phase changes are also known as changes of state and occur at the point where enthalpy changes between phases and entropy changes between phases balance since equilibrium is obtained no energetic drive towards reactants or products, between phases, really any physical system when G = 0 o Recall enthalpy reflects changes in bonding energy and intermolecular forces may be viewed as a type of bonding energy; as such, moving from solid to liquid is going to cost a little energy ( H > 0) and moving from liquid to gas is going to cost a lot of energy, depending on the forces of interaction holding the substance together o Conversely, moving from solid to liquid is a modest positive change in entropy while moving into the gas phase from either solid or liquid is a large increase in entropy; recall G = H - T S Let s take the obvious example of the melting and boiling of water For melting: H = 1.44 kcal/mol, S = 5.26 x 10-3 kcal/mol K G = 0 = H - T S For boiling: H = 9.72 kcal/mol, S = 26.1 x 10-3 kcal/mol K 1 G = 0 = H - T S 1 The text has a sign error in problem 8.1 2

3 8.2, 8.12, 8.13: Forces of Attraction between Molecules & Consequences: Surface Tension, Boiling Point, and Solubility When you see ice floating on water, you should think How odd it is that a solid is floating on its liquid counterpart When you see that water boils at 100 o C at atmospheric pressure, you should think How odd it is that a molecule of molecular weight 18 boils at such a high temperature Water is the gold standard for pure polar solvents There are 5 possible interactions between atoms, ions, and molecules. We will focus on the first 4 intermolecular interactions in order of increasing strength 2 : London or van der Waal s Forces Induced electrostatic interactions that are weak but can significantly add up for non-polar molecules. They are the only attractive forces for non-polar molecules: Name Formula Boiling Point ( o C) Methane CH4-162 Pentane CH3(CH2)3CH3 36 Decane CH3(CH2)8CH3 174 Lubricating oil CH3(CH2)18CH3 varies > While we have left out ion-ion interactions and melting point for the purposes of this discussion, note NaCl melts at 801 o C. 3

4 Dipole Interactions Dipole interactions are attractions between oppositely [partially] charged ends in molecules that possess a permanent dipole. Let s compare butane (bp = 0 o C) with acetone (bp = 58 o C) Hydrogen Bonding A special case of dipole-dipole interactions occurs when N or O has an available pair of electrons (hydrogen bond donor) which can partially bond to a H which is bonded to another N or O (hydrogen bond acceptor). This is why water has such a high boiling point for such a small molecular weight: Consider ethanol: bp = 78 o C, and dimethyl ether: bp = -24 o C. Oddly, these 2 have about the same solubility in water, unlike butane and acetone above: 4

5 Question: Which of the following molecules would have the greatest solubility in water? Question: Which of the following molecules would have the greatest solubility in water? Ion-Dipole Interactions: These are the interactions that give many salts/ionic compounds, such as NaCl and MgSO4 high water solubility; to occur the large number of ion-dipole interactions must release more energy than the breaking of the ion-ion interactions of the initial salt absorbs 3 3 Note CaCO3 as the main component of seashells 5

6 Ion-dipole interactions are very important interactions for explaining why some compounds that can act as acids and bases show different solubilities at different ph values; again, the stronger the electronic interaction, the stronger the intermolecular interaction. Question: Plants contain a wide variety of chemical compounds. Those that can be extracted into water by treatment with acid are known as plant alkaloids. Plant alkaloids are typically amines; that is, they have nitrogen bonded to one or more carbons. How does the nitrogen allow extraction into water with acid treatment? Question: Cocaine hydrochloride is treated with ammonia or baking soda to form crack cocaine or free base cocaine. What is the purpose of doing this? Surfactants/detergents, emulsions, and the water strider disappearing act: 6

7 The Relationship between Vapor Pressure and Intermolecular Forces Vapor defined: the gas phase portion of a gas-liquid equilibrium At a Given Temperature: For 2 compounds at the same temperature, the one with the stronger intermolecular forces will have the lower vapor pressure At a Given Vapor Pressure: For 2 compounds at the same vapor pressure, the one with the stronger intermolecular forces will require a higher temperature to achieve that vapor pressure Question: Which would you expect to exert a higher vapor pressure under standard conditions, acetone (CH3)2CO (MW = 58) or propanol CH3CH2CH2 (MW = 60)? Question: Which would you expect to exert a higher vapor pressure under standard conditions, hexane CH3(CH2)4CH3 (MW = 86) or decane CH3(CH2)8CH3 (MW = 142)? 7

8 Vapor Pressure and Dynamic Equilibrium In a closed, evacuated system at equilibrium, the rate of vaporization will equal the rate of condensation. This is the equilibrium vapor pressure and is characteristic of the secondary forces holding a given liquid together, as well as the temperature of the system Evaporation in an open system is not an equilibrium process, and results in the loss of heat from the liquid Question: Why does the temperature of a liquid remain constant after dynamic equilibrium has been reached? Answer: Question: Why is chloroethane (bp = 12.3 o C) useful as a topical anesthetic agent? Would liquid propane (bp = - 42 o C) be similarly useful? Answer: Vapor Pressure and Boiling Point Boiling occurs when the vapor pressure exceeds the atmospheric pressure in an open system. When this occurs under standard atmospheric conditions, it is referred to as the normal boiling point. Boiling refers to the phenomenon where vapor can form within the bulk of a liquid bubbling occurs. If atmospheric pressure has not been exceeded, vapor can only be lost from the surface, since vapor molecules can mix with air molecules When Pliquid > Patm, the compressive force is less than the force of expansion and the liquid boils 8

9 Question: Why does food cook slower at 10,000 ft? Answer: Relative Humidity: Relative humidity is the ratio of the actual vapor pressure due to water compared to the equilibrium vapor pressure of water at a particular temperature Temperature ( o C) Vapor Pressure (mm Hg) Question: What is the relative humidity on an 86 o F day if the vapor pressure exerted by water is 28.6 mm Hg? Question: Humans can exist briefly in saunas at temperatures that can cook steak: 110 o C. However, sitting in a sauna at 40 o C/55 mm Hg H2O vapor pressure would be deadly. Why the difference in allowed temperatures? The temperature on surface of the skin initially is lower than 40 o C, so the equilibrium vapor pressure is lower than 55 mm Hg and moisture condenses on the person and his or her temperature rises. o Essentially, since the sauna is at 100 relative humidity, the person in the sauna is under the dew point o No way to take advantage of the heat of vaporization H 2O = 540 cal/g 9