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Real And Ideal Gases

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Published in: Chemistry
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This presentation describes the behavior of real gases as well as that of ideal gases. It also describes Ideal gas equation with some worked examples. Enjoy your learning.

Athumani R / Dubai

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Qualification: Bachelor Of Pharmacy

Teaches: Others, Biology, Chemistry, Physics

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  1. REAL AND IDEAL GASES PREPARED BY: ATHUMANI R KAWAMBWA PHARMICIST (BPHARM)
  2. OUTLINES Behavior of Ideal gases Kinetic Theory of gases The Pressure and Volume of an Ideal Gases The Ideal gas law Behavior of Real Gases Real molecules Occupy space Real gases can approach ideal behavior
  3. Gases have the following properties:- i. They fill all space open to them ii. They expand when heated iii. They exert pressure on the walls of their containers iv. The pressure of gases change as their temperature changes In this subtopic we are going to learn about these properties of gases in more details.
  4. Also I shall explain the properties of real gases such as hydrogen, oxygen, methane and so on. *But real gases are so complicated so we first discuss the simplified model of a gas. We will discuss the gases that do not exist in real world. • That gases we call Ideal gases
  5. BEHAVIOR OF IDEAL GASES •:• Ideal gases have some properties similar to real gases. Ideal gases is a gas in which there is no intermolecular forces, and in which molecules do not take up any space themselves Ideal gases do not change their total kinetic enerH.y when they collide into each other. This mean that the co ISion of ideal gases is perfect elastic. There is no real gas that is also an ideal gas.
  6. KEY ASSUMPTIONS ABOUT IDEAL GASES Ideal gas is the one: The molecules have mass, but negligible size. There is no intermolecular forces Whose collision between it's molecules is perfect elastic.
  7. Intermolecular forces Is the force of attraction between molecules of the different Force of attraction that exist between molecules of water an ethanol when they come in contact. Perfect Elastic Collision Is the type of collision in which the kinetic energy and momentum of colliding molecules are conserved. This means that total initial kinetic energy is equal to total final kinetic energy, and total momentum before collision is equal to the total momentum after collision.
  8. KINETIC THEORY OF GASES The kinetic theory of gases main idea is: Gases consist of molecules in a constant state of motion. This means that the molecules of gases are always in random motion, unlike other states such as liquids and solids whose molecules are fixed in a specific position.
  9. ii. KINETIC THEORY OF GASES Related ideas: The pressure of gases is due to collisions of the molecules with the walls of container. The molecules of gases travel in straight lines until they collide with one another, or with the walls of container. iii. In theses collisions, the total kinetic energy of the molecules does not change.
  10. Some gases such as Helium have ideal properties. We can compare ideal gases with real gases, and from their different characteristics we can learn great deal about real gases.
  11. KINETIC THEORY OF GASES The molecules of gases are in constant random motion. • This is feature is main foundations of the kinetic theory of gases. The statement that " motion of gases molecules is random" means that many molecules of gases moving in one direction as in any other direction. The average speed of the gas molecules is in order of 500 ms—I at room temperature. The lighter the molecule, the higher average speed and vice versa.
  12. The hydrogen molecules have an average speed somewhat above 1500 ms-I, while for carbon dioxide have an average speed nearer to 350 ms-I + There is a wide range of energies among the molecules of in a gas. Some move very rapidly, and much faster than average, some move very much lower speed than average. 'When a gas is heated, on average all molecules increase their kinetic energy. But not all molecules will increase their kinetic energies. Some will have more than average.
  13. A CURVE OF DISTRIBUTION OF KINETIC ENERGY OF MOLECULES IN A GAS CHANGES WITH TEMPERATURE. r sample
  14. INTERPRETATION OF THE CURVE The shapes are not symmetrical, the curve stretches out more faster at higher than at lower energies. As the temperature goes up, the average energy of all molecules increases, but the distribution of speed speeds, kinetic energies, spreads out.
  15. QUIZ: Use idea of kinetic theory of gases to answer these two questions: a). What happens to average kinetic energy of the molecules in a gas as the temperature increases? b). What might happen to the kinetic energy of any individual molecule in a gas as a temperature increases?
  16. THE REASSURE AND VOLUME OF IDEAL GASES The pressure of the gases molecules is caused by collisions of the gases molecules with the walls of container. • By doing some Mathematics, it is possible show that the pressure of Ideal gas depends on three factors: The number of molecules per unit volume. The mass of the molecules ii. Their speed iii.
  17. • This should make sense because, if there are more molecules present in a given volume, there should be more collisions with the walls, so the pressure should increase. Also if the molecules have greater momentum the harder they will bounce off the walls. Therefore, they will exert greater force on the wall and cause pressure will increase. *Momentum is the product of mass and speed. Momentum = Mass x Speed
  18. IDEAL GAS LAW The behavior of ideal gases is represented by ideal gas equation: PV = nRT Where by: P is pressure, measured in Pascals (Pa) V is volume, measured in metre cubic (m3) n is number of moles of a gas, in moles T is temperature in Kelvin R is a gas constant R = 8.314 JK-1mol-1
  19. If pairs of measurements of V and P taken at room temperature are plotted and their line extended back, they meet at -273 0 C. this temperature, the volume of gases appear to reduce to zero. This is impossible for real gases, but none the less the graphs the temperature is of great importance . We can use the -273 oc point on the graph to define the zero of new scale temperature. •This is absolute scale or Kelvin scale
  20. UNITS CONVERSION l. DEGREE CELSIUS TO KELVIN K = 273 + oc Convert the following temperatures in to Kelvin scale. a). 265 oc b). - 100 oc c). 90 oc
  21. ll. METRE CUBIC TO DECIMETRE CUBIC 1 m3 = 1000 dm3 or 1 Ix 103dm3 Ill. LITRE TO CENTIMETRE CUBIC 1 Litre 1 dm3 Hence 1 Litre = 1 dm = 1000 cm3 = 1000 cm3 3
  22. What is the volume, given in dm3, of 1 mol of ideal gas at 20 oc and 100 kPa? Data given: Number of moles, n = I mol Pressure, P = 100 kPa = 100000 Pa Gas constant, R = 8.314 Temperature, T = 20 oc = (20+273) K = 293 K
  23. Answer to Example 2: Required to find volume Apply ideal gas equation PV = nRT On making V subject 1 mol x 8.314 x293 K 100000 N/m2 V = 0.024 m3
  24. V - 0.024 So, it is converted into dm3 by multiplying it by 1000. Hence, V = 0.024 x 1000 dm3 Therefore, volume is 24 dm3 We usuallä ake approximation that 1 mol of gas occupies 24 m at room temperature and pressure.
  25. If a balloon contained 1 dm3 of helium at 20 oc and 100 kPa pressure. How many moles of helium would be present? Data given: Volume, V= 1 dm3 Pressure, P = 100 kPa = 100000 Pa Gas constant, R = 8.314 Temperature, T = 20 oc = (20+ 273) K = 293 K
  26. Answer to Example 3: number of moles Apply ideal gas equation PV = nRT On making n subject Required to find n PV 3 100000 N/m2x1 dm 8.314 K
  27. n = 41.1 moles Therefore, there are 41.1 moles of the gas QUIZ: A weather balloon may have an 'envelope' of material that may contain total volume of 1000 dm3 when it is fully expanded. However, the volume of helium put in the balloon when it is released into atmosphere is only a fraction of this volume. Why is the balloon not fully inflated before it is released?
  28. THE BEHAVIOR OF REAL GASES One of the difference between real gas and ideal gas is that real gases liquify when the temperature is low enough and pressure is high enough. Lowering the temperature of real gases allows the intermolecular forces to overcome motion of molecules. Squeezing the molecules together by using high pressure has a similar effect. + Bringing the molecules closer together allows the intermolecular forces to be more effective.
  29. A very exaggerated impression of molecules taking up space. O
  30. Only ideal gases would strictly obey the ideal gas equation. All real gases show deviations from ideal behavior for two reasons: i. Molecules in real gases take up space. In real gasecla molecule takes up very small volume (of the order 10- m ) , which is then not available for another molecule to move in. There are so many molecules that the total volume they occupy can not be ignored. : If three balls of 20 cm3 are introduced in a box cm . The balls will not take all space of the box. o Approximately 10 % of the space of the box will be free. The situation is the same for the gas molecules when introduced to a container.
  31. ii. Real gasses have intermolecular forces between the molecules. Intermolecular forces bring the molecules together, these forces are always present In gases, even though a gas is only a gas because the intermolecular forces are not stronger enough to prevent the molecules bouncing apart when they collide. : If a molecule is moving toward the wall of con alner, e majority of molecules will be attracting it from behind or from its side. This tends to slow the molecule and prevent it colliding the wall of container with much force as it would do if there were no intermolecular forces. Therefore: The pressure exerted by a real gas is less than it would be if the gas were ideal gas.
  32. TABLE OF SUMMARY Molecules in Molecules in Effect in Real Ideal gas Real gas gas Occupation of space by molecules Intermolecula r forces None None Occupy space Present and can be strong Reduces volume from ideal value Reduces pressure from ideal value
  33. REAL GASES CAN APPROACH IDEAL BEHAVIOUR The two conditions for ideal behavior were that the molecules were negligible size and that there were intermolecular forces. We can sometimes come close to these conditions for real gases if: We use gases that have very small intermolecular forces between molecules. ii. We use gases at very law pressure. Gases like hydrogen and helium fulfil the first condition: and by using very low pressure, the molecules of gas spend a great deal of their time far apart from each other.
  34. Then, pressure results in the intermolecular forces not having a chance to work effectively. It also means that because there are very few molecules in a given space at low pressures, the volume that the molecules do occupy is a very small proportion of the total such that their own pressure does become nearly negligible.
  35. QUIZ: 1. In your own words, explain why, at low pressures, real gases begin to behave more like ideal gases? 2. Assume that the effective volume of an oxygen molecule is 64x 10 30m3 a. Estimate the volume occupied by 1 mol of oxypen molecules. Avogadro's number = 6.02 X 1023 mol- b. What percentage of volume of 1 mol of oxygen gas is this at room temperature and pressure?
  36. Volume of oxygen Number of molecules = Number of moles x Avogadro's number = 1 mol x 6.02 x 1023 mol-I = 6.02 x 1023 molecules Therefore there are 6.02 x 1023 molecules of oxygen in 1 mol. Volume = Volume of one molecule x Number of molecules = 10-30m3 x 6.02 x 1023 = 3.8528 x 10-5 Volume is 3.8528 X 10 —5 mg
  37. . : Percentage of the volume to volume arp Volume at rtp = 0.024 Tn3 From Volume Percentage Volume = x 100% Volume at rtp 0.000038528 Percentage Volume = x 100% 0.024 = 0.16% The percentage volume is 0.16%
  38. NEXT SUBTOPIC STATE OF MATTER 2
  39. REFERENCES • AS Level and A Level Chemistry Brian Ratcliff et el 10th Edition Online Verified Sources

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