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Evaporation

Evaporation. Model 1 The complete evaporation of 5 mol of water is displayed in a rigid, closed container. The energy supplied to cause the phase change is in the form of heat from a Bunsen burner. Each mole of water is depicted by a single Lewis structure of water. D.

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Evaporation

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  1. Evaporation Model 1 • The complete evaporation of 5 mol of water is displayed in a rigid, closed container. Theenergy supplied to cause the phase changeis in the form of heat from a Bunsen burner. Each mole of water is depicted by a single Lewis structure of water. D Critical Thinking Questions • Label each container as either the liquid or gas phase. • What is meant by the capital delta (D) in model 1? • Have the bonds of the water molecules been noticeably disturbed via the process depicted? Explain? • Have the intermolecular forces of the water molecules been noticeably disturbed via the process depicted? Explain? Model 2 • A liquid, at 30ºC, held together by intermolecular forces. Three particular molecules have been singled out and have been numbered, as well as darkened, for clarity. T = 30ºC 3 2 Critical Thinking Question • Do all the molecules in this container have the same energy? Explain. 1 Chapter 11

  2. Evaporation Information • It is reasonable to model the solid and liquid phase with a velocity distribution similar to the Maxwell-Boltzmann distribution, as shown in model 3. • It is reasonable to expect the velocity, and thus kinetic energy,of molecules in the condensed phases to be distributed fromthe very slow (low energy) to the very fast (high energy). Model 3 • A Maxwell-Boltzmann distribution for a liquid. The dashed, vertical line displays the velocity (energy) at which the IM forces can be broken. Critical Thinking Questions • In model 3, would a molecule with velocity to the right or left of the given vertical line have a higher energy? • Suppose that molecules 1, 2, and 3 from model 2 have avelocity that is to the right of the given line in model 3. • Would molecule 1 vaporize? Why or why not? • Would molecule 2 vaporize? Why or why not? • Would molecule 3 vaporize? Why or why not? Information • For a liquid molecule to evaporate, not only must the molecule have enough energy to overcome the IM forces attracting it, but it must also be at the surface of the container. Chapter 11

  3. Evaporation Critical Thinking Questions • To the figure in model 3, add a curve that represents the liquid after being heated to a temperature of 60ºC. Label the appropriate curves, one as “T=30ºC” and the other as “T=60ºC” • According to your curve for the higher temperature system, are more or less molecules at sufficient energy to break the IM forces of the liquid state? Explain. • Has increasing the temperature created more positions at the surface for the liquid molecules? • Has increasing the temperature increased the rate of evaporation? Explain. • Suppose I replace the molecules in model 2 ( ) with another pure substance ( ) that has stronger intermolecular forces between molecules. • Draw a dotted vertical line on model 3 that represents the minimum energy to overcome the IM forces. • How would the rate of evaporation compare between the molecules and ? Chapter 11

  4. Vapor Pressure Model 4 • A closed container of liquid is monitored over time. The temperatureinside is 25ºC. Initially, all the gas molecules were removed.A plot of pressure vs. time has been created from the data. 0.5 P (atm) Time (sec) 20 Critical Thinking Questions • Why is the pressure zero inside the container at the beginning of the experiment depicted in model 4? • The container shown in panel (a) is immediately following the closing of the container. What does the upward pointing (or red) arrow represent? • Panel (b) of model 4 introduces a downward facing (blue) arrow. What does this type arrow represent? • What do the grey spheres represent in panels (b) and (c)? Chapter 11

  5. Vapor Pressure Critical Thinking Questions • Are there more upward or downward facing arrows in panel (b)? Where on the graph might represent panel (b)—mark your estimate on the graph itself? Information • The vapor pressure is the pressure created when the rate of evaporation is equal to the rate of condensation. • The vapor pressure is a constant for a substance that depends only on the temperature. Critical Thinking Questions • According to model 4, what is the vapor pressure of the liquid in the container at 25ºC? • If the temperature were raised to 30ºC in model 4, would the rate of evaporation increase, decrease, or stay the same? Justify our choice. • Complete the following phrase and justify your choice: ”as the temperature increases the vapor pressure ________.” Chapter 11

  6. Exercises • Consider the following molecules all with similar mass: • Formaldehyde H2C=O • Methanol CH3-OH • Ethane CH3-CH3 All three are currently at a temperature and pressure at which they are in the liquid phase. • Rank these three species from lowest to highest evaporation rate. • Rank these three species from lowest to highest vapor pressure. Chapter 11

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