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Gases

Gases. Properties of ideal gases. Gases are evenly distributed in a volume. They have very weak IMF, intermolecular forces. (van der Waal) They have frequent elastic collisions They have high kinetic energy due to their high velocities.

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Gases

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  1. Gases

  2. Properties of ideal gases • Gases are evenly distributed in a volume. • They have very weak IMF, intermolecular forces. (van der Waal) • They have frequent elastic collisions • They have high kinetic energy due to their high velocities. • Most of the volume they occupy is empty space; they are easily compressed.

  3. Kinetic molecular theory of gases • Kinetic energy (KE) is the energy a particle has as a result of its molecular movement. • KE=1/2(molar mass)velocity2 • Temperature is directly proportional to KE. For example: an increase in molecular movement causes an increase in temperature. • This theory is best applied to gases since they are free moving particles and are subject to great changes in velocity. • http://www.youtube.com/watch?v=EzWRvK0zhQk KE and Temp.

  4. Applying the kinetic molecular theory to gases • An example: Two gases, He and H2 are at the same temperature of 20C, have the same mole value, but are in separate but identicalcontainers. • Both gases have the same KE due to their same temperature. • Both gases will exert the same pressure since there is the same number of molecules of each gas. (same mole value) and are in the same volume. • The hydrogen will move faster than the He since it is a lighter gas H2= 2g/mol, He=4g/mol

  5. Applying the kinetic molecular theory to gases continued • Summary: • Remember KE= 1/2mv2. So if gases are at the same temp. then they have the same KE. • If gases have different masses, then their velocities must be different. The rates of diffusion will differ. • Regardless of the mass of the gas, gases at the same temperature have the same force of collision/molecule. Heavy gases hit just as hard as light gases if they are at the same temp. • Heavier gases have slower velocities, lighter gases have higher velocities. This equalizes the force of collision.

  6. Kinetic energy videos. • http://www.youtube.com/watch?v=UNn_trajMFo KE and size of molecules • http://www.youtube.com/watch?v=EzWRvK0zhQk KE and Temp. (too long)

  7. Applying the kinetic molecule theory to solids and liquids. • This theory does not apply as easily to solids and liquids due to their restricted motion. • It is safer to apply the KE theory to solids and liquids if they are at the same temp. • H2O and alcohol at 20C have similar KE • Or apply it to the same solid or liquid. • H2O at 20C has less KE than H2O at 100C

  8. Kelvin temperature scale/ absolute temperature scale. • Properties of this temperature scale • Based upon the molecular movement of gases • At zero Kelvin there is no molecular movement. Since temp. is based upon molecular movement, O Kelvin is the coldest temp. possible anywhere. • O Kelvin = -273°C • Add 273 to C to get Kelvin

  9. Non Ideal Gas- SF6 http://www.youtube.com/watch?v=xQo-v_F1P9U • Mythbusters inhale SF6

  10. Properties of gases

  11. Pressure • Pressure= force/area • For gases: the force is created by the strength and the number of collisions. • The strength of the collisions is determined by the kinetic energy of the molecules. Higher KE = higher velocity= greater force of collisions. • The number of collisions is determined by: the mole value/volume of the gas and the velocity of the gas. • For gases: the area is defined by the pressure unit. (walls of the container).

  12. Units of pressure • Millimeters of mercury (mmHg) = based upon the height of a mercury column in a barometer. Pressure at sea level=760mmHg • Torr (named after Torricelli) is the same unit as the mmHg. You will see this unit in the texts. 760 torr= pressure at sea level. • Atmospheres (atm)= 1atm is equal to the pressure at sea level.

  13. Pascal (Pa)= SI unit, =1newton/meter2, 101,325 Pa = pressure at sea level. kPa is most often used. 101.3kPa • Pounds per square inch (psi) =14.7 psi is the pressure at sea level.

  14. Mercury Barometer

  15. Summary of pressure at sea level • 1 atm • 101,325 Pa or 101.3kPa • 760mmHg • 760 torr • 14.7psi (will not be using this unit.)

  16. Standard temperature and pressure: STP • For research and convenience a standard temp and pressure are used when working with gases. • Standard temperature: 273K or 0 celcius • Standard pressure: sea level= 1atm, 760mmHg, 760torr, 101.3 kPa, 14.7psi • MEMORIZE STP!!!!!!!!!!!

  17. Avogadro’s Hypothesis • Equal volumes of gases at the same temperature and pressure have the same number of molecules. • At S.T.P. the volume of one mole of any ideal gas is 22.4L. • At STP the volume of a gas can be easily determined using a proportion as long as the amount (moles) of gas is known.

  18. Pressure and volume relationship • An increase in pressure will decrease volume, or decreasing the volume will increase the pressure. • When the volume is decreased, the number of collisions increase per time. The force ofcollisions stays the same. • Pressure and volume are inversely related. The equation: P=k/V or P1V1=P2V2 when moving the gas into another container. Amount and temperature of gas remains constant.

  19. Pressure and volume

  20. Pressure and volume: Boyles law

  21. Effects of temperature on pressure and volume • Temperature and pressure in a fixed volume • Increasing the temperature increases the pressure. • Molecules have more KE which means their velocity increases. (mass stays constant) • Higher velocity= more collisions and greater force in the collisions so the pressure increases

  22. Effects of temperature on pressure and volume • Temperature and volume (container can expand) • Increasing temperature will increase the volume, the pressure remains constant. • Temperature is directly proportional to volume. T=kV (charles law) • Molecules have more KE which means their velocity increases. • Higher velocity=more collisions and greater force in the collisions. This expands the container. The container will expand until the pressure inside=the pressure outside.

  23. Gas Laws so far that you will be using on your homework. • Boyle’s Law: P1V2 = P2V2 • Charle’s Law: V1/T1 =V2/T2 • Gay-Lussac law: P1/T1 =P2/T2 • Combined Gas Law: uses the above laws: P1V1/T1 =P2V2/T2 • Important note: you must use the Kelvin temperature scale for all “T’s”

  24. IDEAL GAS LAW: uses only ideal gases and is the only law that allows for a change in moles. • PV=nRT • P=pressure • V=volume(L) • n=moles of gas • R=constant • T=temperature(K)

  25. R Constants

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