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Properties of Gases Chpt. 10

Properties of Gases Chpt. 10. A Quick Review Matter is anything that occupies space and has mass There are three states of matter. Particles slide over each other. Particles vibrate about a fixed position. Particles have almost complete freedom of movement.

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Properties of Gases Chpt. 10

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  1. Properties of Gases Chpt. 10

  2. A Quick Review • Matter is anything that occupies space and • has mass • There are three states of matter Particles slide over each other Particles vibrate about a fixed position Particles have almost complete freedom of movement

  3. In this chapter we will be looking at the third state of matter - GAS. Gases have distinct properties that distinguish them from solids and liquids. These properties may be explained in terms of the particles (atoms, molecules, ions) of a gas having more freedom of movement than the particles of a solid or liquid.

  4. Some Properties of Gases: • Gases DO NOT have a definite shape or size and will spread throughout any container they are placed in – DIFFUSION Diffusion: is the movement of particles from an area of high concentration to an area of low concentration. Example: NH3 and HCl Ammonium Chloride Smoke particles travelling throughout the air Example liquids (much slower): Spreading of ink throughout a beaker of water

  5. A gas is defined as a substance that has no well-defined boundaries but diffuses rapidly to fill any container in which it is placed.

  6. 2. Gases do not have a fixed volume – they fill any space into which they are put, therefore the volume of the gas is the volume of the container in which they are placed. The volume of a gas is influenced by two factors: - temperature - pressure Increasing Temperature – gas expands and occupies greater volume Increasing Pressure – gas becomes compressed and occupies a smaller volume

  7. Jacques Charles and Robert Boyle were two scientists who investigated how the volume of a gas changes with temperature and how the volume of a gas changes with pressure respectively. Charles (volume & temperature) Boyle (volume & pressure)

  8. In this chapter we shall be studying the laws that gases obey and why they obey these laws. This also involves a study of the work of Charles and Boyle

  9. 3 Main properties of a fixed amount of gas Temperature Pressure Volume Note: Before studying the laws that gases obey we must first understand how to measure the above three properties

  10. Temperature: Temperature is a measure of the degree of hotness of an object. Two Scales Celsius (centigrade) scale Kelvin (absolute) scale Two fixed points: - 0oC – freezing point of water - 100OC – boiling point of water - 0 K – absolute zero (temperature at which a gas would occupy no volume)

  11. Relationship between Celsius Scale and Kelvin Scale O0C 10O0C -273O C 0 K 273 K 373 K Absolute Zero

  12. Temperature can be converted from the Celsius scale to the Kelvin scale by adding 273 Celsius Kelvin *0oC = 0 + 273 = 273 K 30oC = 30 + 273 = 303 K 50oC = 80oC = 100oC = 273oC =

  13. *Note: - size of a degree on the Celsius scale is the same as that on the Kelvin scale i.e. rise in temperature of 10oC is same as rise in temperature of 10 K - SI unit of temperature – Kelvin*

  14. Pressure: • Pressure of a gas is the force it exerts on each unit area of its container • SI unit of pressure is N/m2 (Nm-2) or *Pascal (Pa) • We will be dealing with pressure in terms of • atmospheric pressure: • Normal atmospheric pressure: • 1.013 x 105 N/m2 • 1.013 x 105 Pa • 101,325 Pa • 101kPa (*1kPa – 1000 Pa)

  15. Note: Old method of expressing pressure of gases used millimetres of mercury or atmospheres: Normal Atmospheric Pressure: 760 mm Hg = 1 atm = 1.013 x 105Pa

  16. Volume: • The volume of a sample of gas is the same as the volume of the container in which the sample is held • SI unit of volume is m3 • Laboratory units: • cm3 • Litres (L) • A litre is also called a cubic decimetre (dm3= 1/10 of metre) • 1L = 1000cm3 = 1dm3

  17. Relationship between m3, cm3 and Litres 1m3 = 1 x 106 cm3 *N.B. To change cm3 to m3 multiply by 10-6 To change litres to m3 multiply by 10-3

  18. Summary of measuring three main properties of gases Temperature – unit Kelvin ( convert from celsius) Pressure – unit Pascal Volume – unit cubic metre (convert from cm3 and litres)

  19. Standard Temperature and Pressure (s.t.p.) As previously noted the volume of a gas varies with temperature and pressure. Thus in order to compare volumes occupied by gases, it is necessary to measure all volumes at the same temperature and pressure: Standard Temperature = 273 K (0OC) Standard Pressure = 1.013 x 105 Pa or 101,325 Pa or 101kPa

  20. Five Main Gas Laws • Boyles Law • Charles Law • The Combined Gas Law • Gay-Lussac’s Law of combing Volumes • Avogadro’s Law

  21. Boyles Law Irish scientist Robert Boyle experimented with the relationship between pressure and volume of gases. He set up a J-shaped tube and added mercury to see what it did to the volume of a trapped gas, kept at a constant temperature

  22. As pressure increases, volume decreases

  23. Boyles Experimental Results *This relationship is inversely proportional, when one increases the other decreases.

  24. The volume is inversely proportional to the pressure *Note:see fig. 10.5(a) and fig. 10.5(b) pg. 110

  25. Boyles Law: At a constant temperature , the volume of a fixed mass of gas is inversely proportional to its pressure V α1 P The proportionality symbol can be replaced by a constant k which gives us a mathematical equation: V = k1 pV = k P p = pressure V = volume k = proportionality constant

  26. Knowing that the pressure of a gas multiplied by its volume is always a constant value gives another way of expressing Boyle’s Law: p1V1 = p2V2 Thus, it is possible to calculate the volume of a gas at one pressure when its volume at another pressure is known. *Note: see pg. 111 table 10.1 and fig. 10.7 for further explanation

  27. Boyle’s Air Pump

  28. Boyles Law Summary V α1 P pV = k p1V1 = p2V2 Must also be familiar with associated graphs (3)!!!!!

  29. Charles Law French scientist, Jacques Charles, investigated the relationship between the volume and temperature of a fixed mass of gas at constant pressure

  30. Charles Law French physicist Jacques Charles was the first to fill a balloon with hydrogen gas and make a solo flight. He showed that the volume of a gas increases when the temperature increases (at a constant pressure)

  31. Charles Law Experiment

  32. Charles Law Experimental results *Note: see figure 10.10 pg. 112

  33. In previous graph straight line does not go through the origin therefore one cannot say that the volume of the gas is directly proportional to the temperature measured in O C. However, if the line is continued backwards, it cuts the x-axis at -273oC i.e. absolute zero in terms of the Kelvin Scale

  34. *Note: see figure 10.11 pg. 112 -273oC O K Using the Kelvin scale of temperature a direct relationship between volume and temperature can be seen i.e. volume is directly proportional to temperature

  35. Charles Law: At constant pressure , the volume of a fixed mass of gas is directly proportional to its temperature measured on the Kelvin scale V α T V = ktV = k T The proportionality symbol can be replaced by a constant k which gives us a mathematical equation: V = volume k = proportionality constant T = temperature (Kelvin)

  36. Knowing that volume divided by temperature always gives a constant value allows the volume of a gas at any given temperature to be calculated provided that its volume at some other temperature is known: V1 = V2 T1 T2 *Note: see pg. 113 table 10.2 and fig. 10.13 for further explanation

  37. Charles Law Summary V α T V = k T V1 = V2 T1 T2 Must also be familiar with associated graphs (3)!!!!!

  38. The Combined Gas Law (The General Gas Law) The results of Boyle’s and Charles’ law can be combined into a single expression which takes the form: p1 V1= p2 V2 T1 T2 Using this equation, the volume of a gas at any temperature and pressure can be calculated provided that its volume at some other given temperature and pressure is known.

  39. *Points to Note: • Since combined gas law derived from Charle’s law MUST convert all temperatures to the KELVIN SCALE • Units on both sides of equation must be consistent e.g. if using kPa on left side must use kPa on right side

  40. Example 1: A certain mass of gas was found to occupy a volume of 269cm3 when the temperature was 17o C and the pressure 99.7kPa. What volume would the gas occupy at s.t.p.?

  41. Example 2: A sample of hydrogen of volume 100cm3 at a pressure of 1 x 105 Pa is compressed to 55cm3 at constant temperature. What is the new pressure of the gas?

  42. Gay-Lussac’s Law of Combining Volumes Following on from work done by Henry Cavendish on the composition of water (electrolysis), Joesph Gay-Lussac confirmed that when hydrogen reacts with oxygen, 2 volumes of hydrogen always react with 1 volume of oxygen 1808 – Gay-Lussac stated his law of combining volumes

  43. He studied the reactions of other gases to further investigate whether they also reacted in simple ratios Hydrogen + Oxygen Steam 2 volumes 1 volume 2 volumes Hydrogen + Chlorine Hydrogen Chloride 1 volume 1 volume 2 volumes Nitrogen + Oxygen Nitrogen Dioxide Monoxide 2 volumes 1 volume 2 Volumes *Note: Please read through experiment outline pg’s 114-115

  44. In 1808, Gay-Lussac was able to state his law of combining volumes: Gay-Lussac’s Law of Combining Volumes In a reaction between gases, the volumes of the reacting gases and the volumes of any gaseous products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure.

  45. Avogadro’s Law An explanation of Gay-Lussac’s law depends on the idea that gases consist of particles. Gay-Lussac’s and Daltons Atomic Theory were published at the same time (1808).However attempts to explain Gay-Lussac’s theory using Dalton’s atomic theory failed. Amedeo Avogadro, (Professor of Physics in 19th century Italy) put forward a hypothesis, which explained Gay-Lussac’s law, relating molecules and volumes.

  46. Avogadro showed experimentally that 100 cm3 of hydrogen react exactly with 100 cm3 of chlorine. This indicates that there must be the same number of molecules of hydrogen and chlorine in each volume: Hydrogen + Chlorine Hydrogen Chloride 1 volume 1 volume 2 volumes Applying Avogadro’s Law: n molecules + n molecules 2n molecules Hydrogen Chlorine Hydrogen Chloride 1 molecule + 1 molecule 2 molecules H2 Cl2 2HCl

  47. Two volumes of hydrogen contain twice as many molecules ……… …. as one volume of oxygen Each oxygen atom bonds with two hydrogen atoms to form a molecule of water

  48. Similarly, 2H2 + O2 2H2O 2NO + O2 2NO The ratio in which the volumes of gases combine is the same as the ratio in which the molecules of gases combine. Thus, when dealing with gaseous reactions the words volume and molecule can always be interchanged.

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