GASES. Unit 1 Chapter 1

Transcription

1 GASES Unit 1 Chapter 1

2 otion of particles: In solids the particles: are moving relatively slowly. have low kinetic energy In liquids the particles: molecules move faster. have higher kinetic energy. In gases, the particles: move fastest, have high kinetic energy.

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4 4 Kinetic Theory odel of States Solid Particles vibrate but don t flow. Strong molecular attractions keep them in place. Liquid Particles vibrate, rotate, tumble and flow, but cohesion (molecular attraction) keeps them close together. Gas Particles move freely through container. The wide spacing means molecular attraction is negligible.

5 5 Particles can have 3 types of motion: Vibrational kinetic energy (vibrating) Rotational kinetic energy (tumbling) Translational kinetic energy (flying around)

6 6 Kinetic Theory of s, l & g. When it is cold, molecules move slowly In solids, they move so slowly that they are held in place (only vibrational energy) In liquids they move a bit faster, they can tumble and flow, but they don t escape from the intermolecular attraction with other molecules (mostly rotational energy, some vibration & translation) In gases they move so quickly that can overcome the intermolecular attractions and leave the container (more translational energy, with a little bit of rotation & vibration).

7 7 Plasma, the Fourth State When strongly heated, or exposed to high voltage or radiation, gas atoms may lose some of their electrons. As they capture new electrons, the atoms emit light they glow. This glowing, gas-like substance is called plasma

8 Properties of gases: Gases: can be atoms or molecules I Have No Bright Or Clever Friends I H N Br O Cl F are all diatomic gases, have mass, no definite volume or shape, are compressible & can expand. Therefore properties of gasses can only be compared under specific conditions. Gas particles are very spaced out!

9 Find the properties of some common gases: Finish for homework! Read pages from your textbook. For N, O, CO, radon & methane gas: List their: Abundance General use (Do we breath it? Do plants use it etc) Technological applications For O 3 how is it a useful and harmful gas?

10 10 Fun Gases (of no real importance) Nitrous Oxide (N O) AKA: Laughing gas, Happy gas, Nitro, NOS Uses anaesthetic in dentist offices, this sweet-smelling gas reduces pain sensitivity and causes euphoric sensations. It is an excellent oxidizer, reigniting a glowing splint much like oxygen would. It is used in racing where it is injected into the carburetor to temporarily increase an engine s horsepower. Sulfur Hexafluoride One of the densest gases in common use. Fun with Sulfur hexafluoride

11 atch the gas with the problem it causes Gas Carbon Dioxide CFCs ethane Carbon monoxide Sulfur dioxide Problem Ozone layer depletion Climate Change Toxic poisoning Noxious smell Acid Rain

12 Last class: We talked about the motion of molecules: Vibrational Rotational Translational Gasses have mostly translational motion. As you increase temperature their motion also increases. Increase in translational movement = increase in velocity

13 Pressure in gases: A force applied over a unit of area. For a gas, pressure results from gas molecules colliding with the wall of its container. easured in Pascals (Pa) or kilopascals (kpa)

14 Adding a gas: Adds more gas molecules ore collisions Increased net pressure Ex. Double # of molecules = If container is not strong enough walls can rupture double pressure

15 Removed a Gas: Removes gas molecules Less collisions Pressure decreases If container is not strong enough walls can collapse

16 Change Size of Container: Decrease container size Decreases space for molecules to move Increases collisions Increases pressure

17 Change Size of Container: Increase container size Increases space for molecules to move Decreases collisions Decreases pressure

18 Heating a Gas: Gas molecules absorb heat olecules move more rapidly Increase collisions Increase pressure

19 Cooling a Gas: Gas molecules release heat olecules move more slowly Decrease collisions Decrease pressure

20 Gases exert a pressure as they collide with the walls of containers. The total pressure is dependent on magnitude & quantity of collisions. Concentration: Add more gas Remove gas Container size: decrease increase Temperature: Increase Temp Decrease Temp Pressure Pressure Pressure Pressure Pressure Pressure

21 Kinetic olecular Theory (K..T): 1. A gas is composed of particles. Gas particles move rapidly & are in constant random motion 3. All collisions are perfectly elastic 4. Kinetic energy is proportional to temperature

22 Textbook questions Do Page 6 # 3,4,5,6, To be done for next class

23 Kinetic energy & temperature: As temperature increases molecules move faster & have a greater KE. Not all molecules are moving at the same speed. The KE of moving objects is expressed by: Ek 1 mv This shows that the KE of molecules is dependent on both their mass & velocity.

24 Increasing # molecules mode mean The range of kinetic energies can be represented as a bell curve. axwell s Velocity Distribution Curve. ost molecules The mean & mode can help establish average molecules Slow molecules Average molecules Average kinetic energy Fast olecules Increasing kinetic energy

25 Number of molecules Average kinetic energy of colder molecules Average kinetic energy of molecules Average kinetic energy of warmer molecules Distribution of Particles Around Average Kinetic Energies. Conclusion: As temperature increases. Curve broadens. Average KE increases. Slower than average molecules Faster than average molecules Kinetic Energy of molecules (proportional to velocity of molecules)

26 Two different gases at the same temperature will: = ½ mv Lighter molecules will move faster. Heavier molecules will move slower. Have the same AVERAGE E K Fun Fact The average speed of oxygen molecules at 0 C is 1656km/h. At that speed an oxygen molecule could travel from ontreal to Vancouver in three hours If it travelled in a straight line.

27 Observing gases As scientists observed gases, they saw mathematical relationships that very closely, but not perfectly, described the behaviour of many gases. They have developed theories & mathematical laws that describe a hypothetical gas, called ideal gas. It is an approximation that helps us model and predict the behavior of real gases.

28 Kinetic Theory for ideal gases. 1. Particles of a gas are infinitely small. Explains effusion & compressibility.. Particles of a gas are in constant motion, and move in straight lines. Until they run into another particle or wall. Explains diffusion. 3. The particles do not attract or repel each other. Explains why gases expand to fill a space. 4. The average kinetic energy of the particles are proportional to the absolute temperature. Explains observed changes in pressure. Fun fact Each air molecule has about ten billion collisions per second 8

29 Thomas Graham ( ) Graham studied the speed of diffusion & effusion. Diffusion is when gas molecules spread throughout a container until they are evenly distributed Effusion is when gas molecules pass through tiny opening in container. He derived his law from E k = ½ mv we will use E k = ½v And in my spare time I invented dialysis, which has saved the lives of thousands of kidney patients Write this! m = mass (kg) v = velocity (m/s) = molar mass (g/mol)

30 Graham s Law Rate of diffusion of a gas is inversely related to the square root of its molar mass. The equation shows the ratio of Gas 1 s speed to Gas s speed. v1 v 1 Same as v v 1 1 v = velocity = molar mass (Leave a space for variants)

31 Ex. 1 Determine the relative rate of diffusion for krypton and bromine. vkr Br v v1 v Br Relative rate means find the ratio v 1 /v! 1 Kr g/mol g/mol Ans: Kr diffuses times faster than Br.

32 Ex. Oxygen gas molecules have an average speed of 1.3 m/s at a given temp and pressure. What is the average speed of hydrogen molecules under the same conditions? v1 v 1 v H 1.3 m/s 3.00 g/mol.0 g/mol v v H O O H v H 1.3 m/s v H 49.0 m/s C. Johannesson

33 Derive an equation that allows you to solve for. Distance, if time is kept constant Time, if distance is kept constant

34 Graham s Law Version #, Effusion Time It can be easier to measure the time it takes for a gas to effuse completely, rather than the speed. Add the equations! t 1 1 d1 t d 1 It can be useful to know the distance that a gas would spread to in a certain amount of time.

35 Ex. 3 An unknown gas diffuses 4.0 times faster than O. Find its molar mass. The ratio v 1 /v is 4.0. v X O v v1 v O 1 X Square both sides to get rid of the square root sign. X 4.0 X g/mol g/mol 3.00 g/mol X.0 C. Johannesson g/mol