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Vapor Pressure

Vapor Pressure. And Boiling Points. Mr. Shields Regents Chemistry U07 L02. Origins of Vapor Pressure. Recall, from our prior discussion of gases the following: 1) Pressure is the result of the frequent collisions of molecules with the walls of the container

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Vapor Pressure

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  1. Vapor Pressure And Boiling Points Mr. Shields Regents Chemistry U07 L02

  2. Origins of Vapor Pressure Recall, from our prior discussion of gases the following: 1) Pressure is the result of the frequent collisions of molecules with the walls of the container 2) The frequency of collisions is a function of temperature - Temperature is a measure of the avg. KE and - The velocity of a molecule is a function of it’s KE 3) As Temperature increases, velocity increases so … the frequency of collisions increases - i.e. Pressure increases

  3. KE and Temp – KMT Assumption 5 Avg. KE of a gas is directly proportional to Temp KE = ½ mv2 So … At any specific temperature The KE of gas 1 = The KE of gas 2 so… ½ m1v12 = ½ m2v22 This equation tells us that if m2 is > m1 then v2 must be < v1 to maintain the above equality. Let’s look at some data …

  4. Molecular Velocities dec inc KE is the same for each But molecules do not have one specific velocity

  5. Distribution of Molecular Velocities At a fixed T heavier (lighter) Note: molecules of a given gas are not moving at One specific velocity. There is a distribution of velocities

  6. KE and Temp And … as the temp increases Avg KE increases Avg velocity shifts right and the whole distribution of velocities also shifts right and Flattens out.

  7. Evaporation This change in molecular velocity with changing T is True for all states of matter, not just gases. And the distribution of velocities for any given temp Is also true for any state of matter. As we will see, this is important in understanding Both liquid phase evaporation and its vapor pressure.

  8. Evaporation We’ve all seen puddles dry up on warm summer days Or the morning dew disappears shortly after The rays of the sun warm the blades of grass. How does this happen?

  9. Evaporation and Vaporization Evaporation is a term that describes the escape of Molecules from the surface of a liquid or solid into The Gas phase and it occurs at All temperatures This is not the same as boiling! Why? However, whether discussing evaporation or boiling the Phase change that occurs (liquid to gas) is known as Vaporization.

  10. Evaporation and Vaporization A word derived from the word vaporization is … “VAPOR” Vapor is a term that describes the gaseous state of a Substance that is normallyliquid at room temperature.

  11. Evaporation and Vaporization The term “Gas” is usually used only for those Substances that normally exist as a gas a room temp. And a liquid that can Evaporate quickly at room Temp is said be volatile. A word that is sometimes also used to describe someone’s Personality! Right?

  12. E Evaporation and Vaporization So … What is needed to vaporize a molecule from the Liquid Phase into the gaseous phase? (Think back to the phase change diagram). Would this phase transition be exo or endothermic? Endothermic – absorption of energy. Why do we need to provide energy? Energyis needed to overcome the intermolecular Attractive between molecules.

  13. Evaporation These forces of attraction are known collectively as the van der Waal’s forces London dispersion (induced dipole) Dipole-dipole Hydrogen bond They are the forces that hold molecules together in the liquid phase that must be overcome to move molecules from the liquid phase into the gas phase The energy necessary to overcome these forces is supplied by the… KE of the molecule

  14. Evaporation Remember… Molecules at a given T have a distribution of Velocities. Those at the far right are moving faster Than those further to the left.

  15. Evaporation Those molecules with sufficient KE can overcome the Attractive forces between it’s neighbors and therefore Leave the liquid phase. At higher temps there Is a greater % of Molecules with sufficient Energy to escape. What do you think happens to average KE of the Remaining molecules in the liquid when higher KE molecules leave?

  16. Evaporation • Avg Velocity of the molecules decreases • As Avg. Velocity decreases so does avg KE • Therefore Temperature of the liquid decreases • Have you ever rubbed alcohol on your skin? • What happens? • The same is true when you • perspire. Evaporating sweat • cools down remaining sweat • It in turn cools the skin. • Why?

  17. Evaporation Remember … Evaporation can occur at any temp. Even When it’s cold outside puddles eventually “dry up”. There Are always some no. of molecules with enough energy To overcome the attractive forces. But what happens if the liquid is confined in a sealed container? In a short while, a DYNAMIC EQUILIBRIUM is reached.

  18. Dynamic Equilibrium Molecules with sufficient energy leave the liquid. In an Open container This continues Until all the liquid Is gone. In a closed system The molecules in The gas phase Remain above The liquid.

  19. Dynamic equilibrium In the closed system some of the gas molecules return To the liquid phase. This is called condensation. At first condensation is close to zero since there are Few molecules in the gas phase. As more molecules move Into the gas phase the Rate of condensation Rapidly increase as Molecules lose energy to The system.

  20. Rate of evap. at given T Fig C * * * * * Fig B * * Fig A Rate condensation Time  Dynamic Equilibrium At any given T the rate of evaporation is constant. The Rate of condensation rapidly increases until it is Equal to the rate of evaporation When the two processes Are equal we have Reached a state of Dynamic Equilibrium

  21. Fig A Fig B Fig C Dynamic Equilibrium Time 0 Increasing Gas phase Concentration Dynamic Equilibrium

  22. Vapor Pressure Remember … molecules in the gas phase have a specific KE Which is a function of the temperature These molecules collide with the container walls and Produce a pressure This pressure produced by the liquid’s molecules in the Gas phase is called the… VAPOR PRESSURE of the liquid

  23. 760 Normal BP = 100 deg VP of H20 BP at 290 mm = 75 deg Vapor Pressure & B.P. VP has a fixed value For a given Temperature. The higher the Temp the Higher the VP When the VP equals Atmospheric pressure Boiling occurs The “Normal BP” is the Temp at which a liquid Boils at Std Pressure(1 atm) Note how BP changes with Pressure!

  24. VP & BP of Various Liquids VP is dependent Only Upon the Temp of The liquid and The forces of Attraction that are Prominent for that Liquid & NOT HOW MUCH LIQUID THERE IS Forces of attraction For these liquids inc. From Left to right so VP dec. in the same Direction.

  25. Ethylene Glycol HO-CH2-CH2-OH x VP & BP of Various Liquids H- Bond plays A key role in Determining The BP of these Liquids with the exception of Diethyl ether. Why?

  26. BP and Molecular Weight Lastly, as mentioned earlier Intermolecular forces Increase with increasing MW Therefore, within the same family of compounds, as MW increases VP dec. and BP inc. For example…

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