1. Gases

2. What are the properties of gases? Why are gases important? we live in a gas each moment of our lives we need gas to live (O 2 ) and indirectly, CO 2 the behavior of the gases of the atmosphere is very important: it determines weather, climate extreme weather, such as hurricanes, tornadoes, heat and cold, excess rain etc. climate determines where people live, what they eat, wear, etc. many machines need gases to function – internal combustion engines

3. What kinds of physical properties describe gases? Odor Color Pressure Volume Temperature

4. How are gases different from liquids and solids? no specific volume or shape

5. The Scientific History of Gases Initial studies of gases occurred 1600’s-1800’s. We will look at the work of 3 scientists Robert Boyle (1600’s) Irish Jacques Charles (late 1700’s)French Amedeo Avagadro (early 1800’s)Italian

6. Robert Boyle (1627-1691) The Hg J-Tube Experiment

7. When you squeeze a balloon, what happens? Pressure increases, volume decreases. What kind of relationship is this? Pressure is inversely proportional to volume.

8. Boyle found that when temperature is held constant, where k is a constant or PV = k So if we take a sample of gas P 1 V 1 and change the pressure P 2, the volume changes V 2. P 1 V 1 = kP 2 V 2 = k These equations can be written: P 1 V 1 = P 2 V 2

9. Pressure Pressure is the force exerted per unit area (N/m 2 ) The SI unit is called a Pascal – very small! Atmospheric pressure at sea level: 101.3 kPa Very high pressures are measured in atmospheres; 1 atm = 101.3 kPa Other system to measure pressure: mm Hg in a mercury barometer

10. Mercury Barometer 1 atm = 101.3 kPa = 760 mm Hg = 760 torr All three units are used; you need to be able to convert units from one to another.

11. Pressure Unit Conversions 1 atm = 760 mm Hg(torr) = 101.3kPaP 1 V 1 = P 2 V­ 2 Converting one unit to another is easy if you remember the following: Unit You Have * Unit You Want Unit You Have a) 0.5 atm  kPaRelationship between units: 1 atm = 101.3 kPa 0.5 atm* 101.3 kPa = 50.65 kPa 1 atm

12. Other Pressure Conversions b) 400 kPa  atm c) 550 mmHg  atm d) 2.3 atm  torr e) 50 kPa  torr f) 800 torr  kPa

13. Boyle’s Law Problems A sample of gas under a pressure of 720 mm Hg has a volume of 300 ml. The pressure is then increased to 800 mm Hg. What volume will the gas then occupy, assuming temperature is constant throughout?

14. Problem 3 A quantity of gas under pressure of 1.3 atm has a volume of 600 ml. The volume is increased to 2.5 L at constant temperature. What is the new pressure of the gas?

15. Problem 4 A quantity of gas has a volume of 400 ml when confined under a pressure of 250 kPa. What will be the new volume of the gas if the pressure is reduced to 100 kPa at constant temperature?

16. Problem 5 Under a pressure of 800 mm Hg, a confined gas has a volume of 750 ml. At constant temperature, the pressure is increased until the gas has a volume of 600 ml. What is the new pressure?

17. Problem 6 A quantity of gas has a volume of 120 liters when confined under a pressure of 1.75 atm at a temperature of 20ºC. At what pressure will its volume be 30 liters at 20ºC?

18. Problem 7 The pressure of 200 ml of a gas at constant temperature is changed from 380 torr (mm Hg) to 760 torr. What is the new volume of the gas?

19. Problem 8 A gas with a volume of 26 L has a pressure of 10 MPa. The pressure is lowered to atmospheric pressure. What is the volume?

20. Lab 6: The Effect of Pressure on the Volume of Gases Boyle’s Law Instructions You will study the relationship between pressure and volume. The primary piece of equipment in this lab is a large syringe used to prepare Thanksgiving turkey. The syringe is sealed at the bottom with a tip cap or modeling clay to act as a piston.

21. Procedure Take off the tip cap and set the syringe at 50 ml. Assume the pressure inside the syringe is 101.3 kPa. Now squeeze the syringe as hard as you can. What is the smallest volume you can get? Calculate the pressure that you exerted on the gas inside the syringe using Boyle’s Law.

22. Is PxV really a constant? In the next experiment, we will examine whether or not PV=k for this experimental system. Place a textbook on the syringe. The pressure can be calculated for each trial by taking atmospheric pressure and adding the pressure exerted by the textbooks. The pressure exerted by each textbook is approximately 26.7 kPa.

23. Measure the change in volume when stacking books on the syringe piston. One student should carefully balance the books on the syringe. Calculate the increased pressure 0 books = atmospheric pressure 1 book = atm + pressure of book 2 books = atm + pressure of two books Fill in the data table.

24. Data Table BooksPressure (kPa) Volume (ml) PV=kBooksPressure (kPa) Volume (ml) PV=k 0 101.3 5 1 128.1 6 2 7 3 8 4 9

25. Make a graph of pressure on the x axis and volume on the y axis. Label the axes and give the graph a title.

26. Questions Several complete sentences, on a separate sheet of paper How are gases different from solids and liquids? Why are gases compressible? This experiment was originally performed with different equipment. Describe the original experiment and explain why it is no longer performed in schools.

27. What is temperature? Temperature is not heat! Temperature is a measure of the average kinetic energy of molecules

28. Kelvin Scale The magnitude of the units of the kelvin scale are identical to celsius. Relationship: K = C + 273 Zero degrees C = 273 K Zero degrees K = -273K – lowest temperature possible!!

29. Celsius and Fahrenheit scales are not true measures of average kinetic energy! Atoms at zero degrees F and C still have considerable kinetic energy – they are moving all over the place! The Kelvin scale makes the assumption of an impossible state of matter – absolute zero – where atoms and molecules have no energy of motion whatsoever. The Kelvin scale is a true measure of average kinetic energy

30. Very Cold: Bose-Einstein Condensate Very Hot: Plasma A fifth state of matter, a Bose-Einstein condensate, occurs when atoms are cooled to within a few millionths of a degree of absolute zero.Bose-Einstein condensate At very high temperatures, electrons are so excited that they are not associated with any atoms – a plasma.high temperaturesplasma Both of these states are nearly impossible to visuallize!

31. Charles’ Law In the late 18 th century, Jacques Charles studied the effects of temperature on the volume of a gas at constant pressure. Every experiment yielded the same result – increasing the temperature of a gas also increased the volume.

32. Charles’ Law

33. Comparing Boyle’s Law and Charles’ Law

34. Charles’ Law Charles noticed that a graph of volume versus temperature gave a straight line for many gases. William Thompson (Lord Kelvin) noticed that the extrapolation to 0 always intersected the same point on the temperature axis: -273ºC, which he called absolute zero, where the average KE of the particles would be zero.

35. Charles’ Law Problems A sample of gas has a volume of 150 ml when its temperature is 17ºC. If its temperature is increased to 32ºC, what will its volume become, assuming the pressure is constant throughout?

36. A quantity of gas has a volume of 200 ml at a temperature of –3ºC. If the temperature of the gas is raised to 27ºC at constant pressure, what volume will the gas occupy?

37. A quantity of gas has a volume of 100 ml at 200ºK. The temperature of the gas is raised to 400ºK, while the pressure remains constant. What is the new volume of the gas?

38. What volume will a quantity of gas occupy at – 23ºC if its volume is 500 ml at 52ºC?

39. A quantity of gas occupies a volume of 800 ml at 127ºC. At constant pressure, at what temperature will the volume of the gas be 600 ml?

40. Joseph Gay-Lussac

41. Gay-Lussac’s Law What is the relationship between temperature and pressure at constant volume? When air is heated, its particles move faster, which leads to increased pressure. What kind of relationship is this? A direct proportion: P=kTwhich leads to Gay-Lussac’s Law

43. The Combined Gas Law The three gas laws can be combined into a single expression called the combined gas law: The other laws can be seen easily by holding one variable constant.

44. STP Standard Temperature and Pressure A standard set of conditions is often set as a reference: STP STP is given on your reference tables as Table A

45. Using the Combined Gas Law Example #1: This type of combined gas law problem (where everything goes to STP) is VERY common: 2.00 L of a gas is collected at 25.0°C and 745.0 mmHg. What is the volume at STP? You have to recognize that five values are given in the problem and the sixth is an x. Remember to change the Celsius temperatures to Kelvin.

46. 2.00 L of a gas is collected at 25.0°C and 745.0 mmHg. What is the volume at STP? When problems like this are solved in the classroom, use a data table, like this:

47. Next step: fill it in with data from the problem. Here is the right-hand side filled in with the STP values:

48. Filling in the data from the problem: Insert the values in their proper places in the combined gas law equation: P 1 V 1 / T 1 = P 2 V 2 / T 2