1 mr. shieldsregents chemistry u07 l02 and boiling points

Download 1 Mr. ShieldsRegents Chemistry U07 L02 And Boiling Points

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Slide 2 1 Mr. ShieldsRegents Chemistry U07 L02 And Boiling Points Slide 3 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 its KE 3) As Temperature increases, velocity increases so the frequency of collisions increases - i.e. Pressure increases Slide 4 3 KE and Temp KMT Assumption 5 Avg. KE of a gas is directly proportional to Temp This equation tells us that if m2 is > m1 then v2 must be < v1 to maintain the above equality. Lets look at some data KE = mv 2 So At any specific temperature The KE of gas 1 = The KE of gas 2 so m 1 v 1 2 = m 2 v 2 2 Slide 5 4 Molecular Velocities MoleculeH2H2 HeH2OH2ON2N2 O2O2 CO 2 Avg. Speed (m/sec) 19601360650520490415 Molecular weight KineticEnergy (x10 6 ) 3.8423.6993.8033.7863.8423.789 But molecules do not have one specific velocity KE is the same for each inc dec Slide 6 5 Distribution of Molecular Velocities At a fixed T Note: molecules of a given gas are not moving at One specific velocity. There is a distribution of velocities (lighter) heavier Slide 7 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. Slide 8 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. Slide 9 8 Evaporation Weve 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? Slide 10 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. Slide 11 10 Evaporation and Vaporization A word derived from the word vaporization is Vapor is a term that describes the gaseous state of a Substance that is normally liquid at room temperature. VAPOR Slide 12 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 someones Personality! Right? Slide 13 12 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? Energy is needed to overcome the intermolecular Attractive between molecules. E Slide 14 13 Evaporation These forces of attraction are known collectively as the van der Waals 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 Slide 15 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. Slide 16 15 Evaporation Those molecules with sufficient KE can overcome the Attractive forces between its 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? Slide 17 16 Evaporation 1.A vg Velocity of the molecules decreases 2.A s Avg. Velocity decreases so does avg KE 3.T herefore 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? Slide 18 17 Evaporation Remember Evaporation can occur at any temp. Even When its 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. Slide 19 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. Slide 20 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. Slide 21 20 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 Rate of evap. at given T Time Rate condensation * * * * * Fig B * * * * * Fig B * * Fig A Fig C Slide 22 21 Dynamic Equilibrium Increasing Gas phase Concentration Time 0 Fig AFig B Fig C Slide 23 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 liquids molecules in the Gas phase is called the VAPOR PRESSURE of the liquid Slide 24 23 VP of H 2 0 760 Normal BP = 100 deg 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! Slide 25 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. Slide 26 25 HO-CH2-CH2-OH x Ethylene Glycol 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? Slide 27 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 AlcoholsMW BP deg C AlkanesMW Methanol3264.7Methane16-162 Ethanol4678.3Ethane30-89 Propanol6097.2Propane42-42 Butanol74117.7Butane54-0.5