Packet #8: Physical Behavior of Matter: Gases Reference Table: Tables A, H, and T

Reference Table A  STP = Standard Temperature & Pressure  Standard temperature = 273 °K or 0°C  Standard pressure = kPa (kilopascal) or 1 atmosphere; atm.  The atmosphere exerts a pressure of kPa or 1 atmosphere on everything.  Other units of measurement for pressure: 1 atm = mm Hg = torr = psi = 101,300 Pa = kPa

RECALL... GASES: No definite shape or volume. They take on the shape and volume of the container they are in.  Solid  Gas = Sublimation (CO 2, I 2 )  Gas  Solid = Deposition  Liquid  Gas = Evaporation  Gas  Liquid = Condensation  An open glass of water left standing around will eventually evaporate even with out being heated.  When water evaporates, it changes from a liquid to a gas called water vapor.

Vapor Pressure  Water vapor takes up more space than an equal mass of liquid water.  As a result, in a closed container, the vapor that forms can exert a significant amount of pressure on the sides of the container.  This pressure is known as vapor pressure.

Vapor Pressure  A gas exerts pressure on its surroundings  For example, when you blow up a balloon, the air inside pushes against the elastic sides of the balloon to keep it inflated.

Table H: Vapor Pressures of Four Liquids: AAAAs the Temperature Increases, the Vapor Pressure Increases WWWWhen the vapor pressure reaches atmospheric pressure, the liquid BOILS (when vapor pressure equals atmospheric pressure) TTTThe weaker the intermolecular forces are, the easier it is to become a gas (the more gas there is the greater the vapor pressure)

Intermolecular Forces & Vapor Pressure  As you increase the temperature, you also increase  As you increase the temperature, you also increase Kinetic Energy  As you increase kinetic energy, molecules can begin to overcome the intermolecular forces holding the molecules together and escape to form a gas.  Thus the weaker the intermolecular forces are, the easier it is to become a gas  Weak intermolecular forces = high vapor pressure (Propanone)  Strong intermolecular forces = low vapor pressure (Ethanoic Acid)

Intermolecular Forces Example…  Look at propanone at 50°C and Ethanoic Acid at 50°C  Propanone at 50°C exerts more vapor pressure than ethanoic acid at 50°C because, which allows propanone ’ s molecules to escape more easily to form a gas faster.  Propanone at 50°C exerts more vapor pressure than ethanoic acid at 50°C because Propanone has weaker intermolecular forces holding its molecules together than ethanoic acid, which allows propanone ’ s molecules to escape more easily to form a gas faster.  So Table H also shows you which liquids are stronger (harder to break apart).

As the Temperature Increases, the Vapor Pressure Increases  Temp and Pressure have a.  Temp and Pressure have a Direct Relationship.  Example: Vapor Pressure of H 2 O. 25°C = 3 kPa 50°C = 12 kPa 70°C = 31 kPa  As a liquid is heated, more of it turns into vapor and the vapor pressure increases.  Thus as the amount of gas increase, the vapor pressure increases

Boiling Water in a Vacuum  vacuum-demo-boiling-water-30c vacuum-demo-boiling-water-30c vacuum-demo-boiling-water-30c

 Mt. McKinley, located in Alaska, is the highest mountain peak in North America with a summit elevation of 20,237 feet (6,168 m) above sea level. elevationsea levelelevationsea level  Atmospheric pressure is 606 torr.  How many kPa is that equivalent to?  At what temperature would water boil at the summit? Making Pasta in Mt. McKinley

Combined Gas Law P 1 V 1 = P 2 V 2 T 1 T 2 T 1 T 2  Out of the six variables, you must be given five.  Temperature MUST BE IN °K  If the question gives you °C, then you must convert it to °K (°K = °C + 273)  REMEMBER STP!!! – If a test questions states STP, the question has already given you two (2) variables, temperature (K) and pressure (atm or kPa).

Practice Problems  A gas has a pressure of kPa at 288  K and fills a 1.5L container. What is the pressure exerted by the gas after the volume and temperature are increased to 2.5L and 303  K, respectively?  A sample of neon gas occupies 4.61 L at 2.83 atm and 277  K. What will be the pressure of the gas in atm when 8.3 L are occupied at 293  K?  A sample of gas has a volume of L when its temperature is 25.0°C and its pressure is atm. What will the new volume be at STP?

Boyle ’ s Law:  At constant temperature, the volume of a gas is inversely proportional to pressure.  The more pressure you have on the gas = the smaller the volume of the gas.  So if I have 3 times as much pressure on a gas, the volume of that gas is 1/3  Theoretically, if I double the pressure of a gas, then the volume of the gas should be _____?

 Pressure vs. Volume  Is a hyperbola  Shows the inverse relationship

BOYLE ’ S LAW FORMULA: P 1 V 1 = P 2 V 2 P 1 = old pressureP 2 = new pressure V 1 = old volumeV 2 = new volume V 1 = old volumeV 2 = new volume Combined Gas Law (Table T) P 1 V 1 = P 2 V 2 T 1 T 2 T 1 T 2  Boyle ’ s Law is at constant temperature – simply remove temperature from the CGL.

Practice Problems A gas occupies 12.3 liters at a pressure of 40.0 mm Hg. What is the volume when the pressure is increased to 60 mm Hg? A gas occupies 12.3 liters at a pressure of 40.0 mm Hg. What is the volume when the pressure is increased to 60 mm Hg? If a gas at 25.0°C occupies 3.60 liters at a pressure of 1.00 atm, what will be its volume at a pressure of 2.50 atm? If a gas at 25.0°C occupies 3.60 liters at a pressure of 1.00 atm, what will be its volume at a pressure of 2.50 atm? To what pressure must a gas be compressed in order to get into a 3.00 cubic foot tank the entire weight of a gas that occupies cubic feet. at standard pressure? To what pressure must a gas be compressed in order to get into a 3.00 cubic foot tank the entire weight of a gas that occupies cubic feet. at standard pressure?

Charles ’ Law:  At constant pressure, volume is directly proportional to temperature.  The higher the temperature, the higher the volume, the lower the temperature the lower the volume  Temperature must be in Kelvin.  So if I increase the volume of a gas by 3 the temperature of that gas also increases by 3

 Volume vs. Temperature  Straight line  Shows a Direct Relationship

CHARLES ’ LAW FORMULA: V 1 = V 2 T 1 T 2 T 1 T 2 V 1 = old volumeV 2 = new volume V 1 = old volumeV 2 = new volume T 1 = old temp. (°K)T 2 = new temp. (°K) T 1 = old temp. (°K)T 2 = new temp. (°K) Combined Gas Law (Table T) P 1 V 1 = P 2 V 2 T 1 T 2 T 1 T 2  Charles ’ s Law is at constant pressure – simply remove temperature from the CGL.

Practice Problems Calculate the decrease in temperature when 2.00 liters at 20.0°C is compressed to 1.00L Calculate the decrease in temperature when 2.00 liters at 20.0°C is compressed to 1.00L ml of air is at 20.0°C, what is the volume at 60.0° C? ml of air is at 20.0°C, what is the volume at 60.0° C? A gas occupies ml at a temperature of A gas occupies ml at a temperature of 27.0°C. What is the volume at 132.0°C? What change in volume results if 60.0 ml of gas is cooled from 33.0°C to 5.00°C. What change in volume results if 60.0 ml of gas is cooled from 33.0°C to 5.00°C.

Gas Laws in Action…  Many important gases contain a mixture of components. One notable example is air.  Scuba divers who are going deeper then 150 feet use another important mixture, helium and oxygen.  Normal air is not used because the nitrogen present dissolves in the blood in large quantities as a result of the high pressures experienced by divers under several hundred feet of water

Dalton ’ s Law helps Divers Avoid “ the bends ”  When the diver returns too quickly to the surface, the nitrogen bubbles out of the blood just as soda fizzes when its opened and the divers gets “ the bends ” a very painful and potentially fatal condition.  This is why helium is mixed with oxygen in the diver ’ s tank. Helium will not dissolve into the diver ’ s blood even at high pressures

Avogadro ’ s Law:  Equal volumes of different gases at the same temperature and pressure contain an equal number of molecules.  Basically As long as temperature and pressure do not change the gas with the larger volume consists of a greater number of molecules  One mole of any gas at STP has a volume of 22.4 liters and contains 6.02x10 23 molecules

Avogadro ’ s Hypothesis…

Graham ’ s Law  Relates the rate of diffusion of gases to their molar masses  Diffusion is movement of gas particles from an area of high concentration to an area of low concentration  If someone in a room uses a deodorant spray, it doesn't take long for everyone else in the room to smell it. This is because of diffusion.

Graham ’ s Law  The GREATER the molar mass of a gas the SLOWER it diffuses  Gases with small molar masses diffuse very rapidly  Gases with large molar masses diffuse slowly

Scientists construct models to explain the behavior of substances…  While gas laws describe HOW gases behave, they do not explain WHY gases behave the way they do  The KINETIC MOLECULAR THEORY (KMT) explains the behavior of gases using the IDEAL GAS MODEL

Kinetic Molecular Theory ( “ Ideal Gas Laws ” ) 1.A gas is composed of particles that are in continuous random straight-line motion. 2.When gas molecules collide with each other or with the walls of the container, t here is a transfer of energy between colliding particles; the total energy remains constant. 3.The volume of gas particles is negligible in comparison with the volume of space they are in. There is a lot of space between the particles. 4.Gas particles are considered as having no force of attraction for each other.

Kinetic Molecular Theory  “ Real gases ” deviate from the “ ideal gas laws ”  There are deviations from the four key points made in the KMT.  Point 3 Deviation: The volume of gas particles is significant. Gas particles do have some volume.  Point 4 Deviation: Gas particles do have a force of attraction.

IDEAL GASES:  Exist under conditions of low pressure & high temperature.  At low pressure, gas particles are far apart from one another, and at high temperature the particles are moving very fast making the ideal gas laws valid. REAL GASES:  Exist under conditions of high pressure & low temperature.  At high pressure, gas particles are very close together, and at low temperature, the particles are moving slowly making the two deviations valid.

 HYDROGEN and HELIUM are two “ real gases ” that behave the most like IDEAL gases. Kinetic Energy vs. Attractive Force:  The Higher the Temperature, the more kinetic energy particles have and the more they can overcome the attractive forces holding the particles together. The particles can then begin to change from solid to liquid to gas.

Review Questions 1) The average kinetic energy of the molecules of an ideal gas is directly proportional to the A) number of moles present B) temperature measured on the Kelvin scale C) pressure at standard temperature D) volume occupied by individual gas molecules 2) Under what conditions does a real gas behave most like an ideal gas? A) at high temperatures and low pressures B) at low temperatures and high pressures C) at low temperatures and low pressures D) at high temperatures and high pressures

3) If the pressure on a given mass of gas in a closed system is decreased and the temperature remains constant, the volume of the gas will A) increase B) decrease C) remain the same 4) At which temperature would the molecules in a one gram sample of water have the lowest average kinetic energy? A) 5°C B) -100°C C) 100°K D) 5°K

5) When a sample of a gas is heated at constant pressure, the average kinetic energy of its molecules A) decreases, and the volume of the gas decreases B) increases, and the volume of the gas increases C) decreases, and the volume of the gas increases D) increases, and the volume of the gas decreases 6) As the temperature of a sample of a gas increases at constant pressure, the volume of the gas sample A) increases B) decreases C) remains the same

7) According to the Vapor Pressure of Four Liquids chemistry reference table, what is the boiling point of ethanoic acid at 80 kPa? A) 100°C B) 125°C C) 28°C D) 111°C 8) As the pressure of a gas at kPa is changed to kPa at constant temperature, the volume of the gas A) increases B) decreases C) remains the same

9) A real gas behaves more like an ideal gas when the gas molecules are A) far apart and have strong attractive forces between them B) lose and have weak attractive forces between them C) lose and have strong attractive forces between them D) far apart and have weak attractive forces between them 10) Which gas will most closely resemble an ideal gas at STP? A) H 2 B) Cl 2 C) SO 2 D) NH 3

11.At STP, which sample contains the same number of molecules as 11.2 liters of CO 2 (g) at STP? A liters of N 2 (g) B. 5.6 liters of NO 2 (g) C. 7.5 liters of H 2 (g) D liters of CO(g) 12) Based on the Vapor Pressure of Four Liquids chemistry reference table, which substance has the weakest intermolecular forces? A) propanone B) ethanoic acid C) ethanol D) water