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GASES Chemistry I – Chapter 14 Chemistry I Honors – Chapter 13

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1 GASES Chemistry I – Chapter 14 Chemistry I Honors – Chapter 13
SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

2 GAS: One of Three STATES OF MATTER

3 General Properties of Gases
There is a lot of “free” space in a gas. Gases can be expanded infinitely. Gases fill containers uniformly and completely. Gases diffuse and mix rapidly.

4 Importance of Gases Airbags fill with N2 gas in an accident.
Gas is generated by the decomposition of sodium azide, NaN3. 2 NaN3 ---> 2 Na N2

5 Our Atmosphere: Gravity holds the atmosphere close to the Earth
Our Atmosphere: Gravity holds the atmosphere close to the Earth. It is divided into various layers according to differences in temperature: The Troposphere: We live in the Troposphere It contains more than 75% of the atmosphere’s mass The Stratosphere: Most of the clouds and rain are in this layer It extends up to 48km above the earth! The ozone layer is between the Stratosphere and Mesosphere The Mesosphere: This layer extends to 80km above the Earth! The Thermosphere Here the air is very thin Over 99.99% of the atmosphere lies below this layer It is comprised of:

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8 The Ionosphere: Reflects radiowaves back to Earth so signals can be sent around the world
The Exosphere: This layer begins at ~480km above the Earth and fades away into space

9 Did you know... About 99% of the atmosphere is made up of oxygen (21%) and nitrogen (78%). The remainder is made up of Ar (argon), CO2 and very small amounts of H2, NH3, O3 (ozone), CH4, CO, He, Ne, Kr and Xe also gases such as SO2, NO2 are put into the air from vehicles & industry About 20 % of the Earth's population breathes severely contaminated air, largely CO2 and SO2 resulting from industrial processes. This increases the number of respiratory conditions, especially amongst children and elders. 13 % of the British children experience asthma caused by air contamination

10 Properties of Gases Gas properties can be modeled using math. Model depends on— V = volume of the gas (L) T = temperature (K) ALL temperatures in the entire chapter MUST be in Kelvin!!! No Exceptions! n = amount (moles) P = pressure (atmospheres)

11 Temperature Average kinetic energy of the particles of a substance
Without particles there is no temperature No temperature in a vacuum Must be measured in Kelvin to avoid negative values while doing calculations C° = K

12 PRESSURE: A FORCE per unit area Measured in pascals (Pa) = 1 N/m2
= 100 g / m2 A normal day has a pressure of 100 kPa = 100 x 1000 Pa x 100 g / m2 1 kPa = kg / m2 Atmospheric pressure varies with altitude the lower the altitude, the longer and heavier is the column of air above an area of the earth.

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14 Pressure Pressure of air is measured with a BAROMETER (developed by Torricelli in 1643) Hg rises in tube until force of Hg (down) balances the force of atmosphere (pushing up). (Just like a straw in a soft drink) P of Hg pushing down related to Hg density column height

15 Pressure Column height measures Pressure of atmosphere
* 1 standard atmosphere (atm) = 760 mm Hg (or torr) = inches Hg = 14.7 pounds/in2 (psi) = * kPa (SI unit is PASCAL) = about 34 feet of water! * Memorize these!

16 Pressure Conversions The air pressure in a cave underwater is atm, what is the pressure in kPa? 101.3 kPa B. What is 475 mm Hg expressed in kPa? 760 mm Hg 2.30 atm x = 233 kPa 1 atm 475 mm Hg x = kPa

17 Pressure Conversions The air pressure in a cave underwater is atm, what is the pressure in kPa? 101.3 kPa B. What is 475 mm Hg expressed in kPa? 760 mm Hg 2.30 atm x = 233 kPa 1 atm 475 mm Hg x = kPa

18 Pressure Conversions A. What is 3.71 atm expressed in kPa?
B. The pressure of a tire is measured as 830 torr. What is this pressure in atm?

19 Crushed Can Demo: Why did the can implode? What happened to the pressure inside once the temperature decreased? Lower temperature = slower moving particles = less collisions with the wall T P Did the # of particles/moles change? Did the air pressure change(# particles colliding)? Did the volume change? Only because the pressure outside is much greater then inside – the ouside pressure crushed the can. T α P

20 Charles’ Law http://www.youtube.com/watch?v=tPH57yp0x1U
Gay-Lussac’s Law If n and V are constant, then P α T P and T are directly proportional. P P2 = T T2 If temperature goes up, the pressure goes up! P1 = T1 P T2 Joseph Louis Gay-Lussac ( ) Charles’ Law

21 Gas Pressure, Temperature, and Kinetic Molecular Theory
As the temperature increases the particles move faster  increasing # of collisions = ↑ pressure P proportional to T

22 A gas has a pressure of 3. 0 atm at 127º C
A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C? T1 = 127°C = 400K P1 = 3.0 atm T2 = 327°C = 600K P2 = ? What law applies to this situation? T and P – Gay-Lussac’s Law: 3.0 atm = P P2 = 3.0 atm x 600 K P2 = 4.50 atm 400K K K P1 = P2 T T2

23 Charles’s Law If n and P are constant, then V α T
V and T are directly proportional. V V2 = T T2 If temperature goes up, the volume goes up! V1 = T1 V T2 Jacques Charles ( ). Isolated boron and studied gases. Balloonist.

24 Charles’s original balloon
Modern long-distance balloon

25 Boyle’s Law – Vacuum Demo - http://www.youtube.com/watch?v=N5xft2fIqQU
Charles’s Law 0 K -273 °C Boyle’s Law – Vacuum Demo -

26 A gas has a volume of 3.0 L at 127°C. What is its volume at 227 °C?
T1 = 127°C = 400K V1 = 3.0 L T2 = 227°C = 500K V2 = ? Which law applies to this situation? T and V - Charles’s Law: 3.0 L = V V2 = 3.0 L x 500 K V2 = 3.75 L 400 K K K V1 = V2 T T2

27 Boyle’s Law P α 1/V This means Pressure and Volume are INVERSELY PROPORTIONAL if moles and temperature are constant (do not change). For example, P goes up as V goes down. P1V1 = P2 V2 Robert Boyle ( ). Son of Earl of Cork, Ireland.

28 As Pressure Increases Volume decreases

29 Boyle’s Law and Kinetic Molecular Theory
P proportional to 1/V Smaller volume means the particles are forced closer together – increasing collisions with each other and with the walls of the container.

30 Boyle’s Law A bicycle pump is a good example of Boyle’s law.
As the volume of the air trapped in the pump is reduced, its pressure goes up, and air is forced into the tire.

31 Boyle’s Law: - Volume PV = k Temperature is constant Pressure

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33 A gas has a volume of 3.0 L at 2 atm. What is its volume at 4 atm?
P1 = 2 atm V1 = 3.0 L P2 = 4 atm V2 = ? Which law applies to this situation? P and V = Boyle’s Law 2 atm x 3.0 L = 4 atm x V2 V2 = 2 atm x 3.0 L 4 atm V2 = 1.5 L P1 V1 = P2 V2

34 SUMMARY of Gas Laws LAW Charles’ V T V1/T1 = V2/T2 P, n Gay-Lussac’s
RELAT-IONSHIP CONSTANT Charles’ V T V1/T1 = V2/T2 P, n Gay-Lussac’s P T P1/T1 = P2/T2 V, n Boyle’s P V P1V1 = P2V2 T, n

35 Combined Gas Law The good news is that you don’t have to remember all three gas laws! Since they are all related to each other, we can combine them into a single equation. BE SURE YOU KNOW THIS EQUATION! V1 P V2 P2 = T T2

36 Combined Gas Law If you only need one of the gas laws, you can cover up the item that is constant and you will get that gas law! = P1 V1 P2 Boyle’s Law Charles’ Law Gay-Lussac’s Law V2 T1 T2

37 Combined Gas Law Problem
A sample of helium gas has a volume of L, a pressure of atm and a temperature of 29°C. What is the new temperature(°C) of the gas at a volume of 90.0 mL and a pressure of 3.20 atm? Given: P1 = atm V1 = 180 mL T1 = 29°C + 273 = 302 K P2 = atm V2= 90 mL T2 = ??

38 Calculation P1 = atm V1 = 180 mL T1 = 302 K P2 = atm V2= 90 mL T2 = ?? P1 V P2 V2 = P1 V1 T2 = P2 V2 T1 T T2 T2 = P2 V2 T1 P1 V1 T2 = 3.20 atm x mL x 302 K atm x mL T2 = 604 K = °C = K

39 Learning Check A gas has a volume of 675 mL at 35°C and atm pressure. What is the temperature in °C when the gas has a volume of L and a pressure of 802 mm Hg?

40 One More Practice Problem
A balloon has a volume of 785 mL on a fall day when the temperature is 21°C. In the winter, the gas cools to 0°C. What is the new volume of the balloon?

41 And now, we pause for this commercial message from STP
STP in chemistry stands for Standard Temperature and Pressure STP allows us to compare amounts of gases between different pressures and temperatures Standard Pressure = 1 atm (or an equivalent) Standard Temperature = 0 deg C (273 K)

42 Try This One A sample of neon gas used in a neon sign has a volume of 15 L at STP. What is the volume (L) of the neon gas at 2.0 atm and –25°C?

43 Ideal Gases, Avogadro’s Theory and the Ideal Gas Law
What are three things that make the behaviour of a gas ideal? How do they collide? 2) Does their size matter? 3) How do they move?

44 What are three things that make the behaviour of a gas ideal?
How do they collide? ELASTICALLY  no attraction, no energy loss, perfect bounces 2) Does their size matter? NO too small, spaces between too big 3) How do they move? straight lines and randomly

45 FYI: What you will see in university

46 Avogadro’s Theory Volume is proportional to moles
Equal volumes of gases at the same T and P have the same number of molecules. Volume is proportional to moles V and n are directly related V α n twice the volume twice as many molecules

47 Ideal Gas Law: Generally speaking, an ideal gas obeys all gas laws perfectly under all conditions: 1) Charles’s Law V α T (V / T = k ) 2) Gay-Lussac’s Law P α T (P / T = k ) 3) Boyle’s Law V α 1 / p ( V x P = k ) 4) Avogadro’s Theory V α n In other words, V α n α T α 1/p All these variables can be combined to include an equation with moles, n using a constant… V = constant x n x T x 1/p The constant is the letter R

48 Ideal Gas Law: The Constant R: Is called the Ideal GAS CONSTANT
Relates P, T, V, and moles Is found by substituting in known values for P, T, V and n (moles) Ex. You could sub in values at STP: T = 273 K, P = 1 atm, 1 molegas= 22.4 L R with different units : These #’s are always provided . J / K • mol or J K-1 mol-1 L•atm/K•mol  mainly used Pa m3 K-1 mol-1 L Torr K-1 mol-1

49 Multiply both sides by p, rearrange n…
IDEAL GAS LAW V= R x n x T x 1/p Multiply both sides by p, rearrange n… P V = n R T Brings together gas properties. Can be derived from experiment and theory. BE SURE YOU KNOW THIS EQUATION!

50 Using PV = nRT L • atm Mol • K P = Pressure V = Volume T = Temperature
N = number of moles R is the constant called the Ideal Gas Constant R = L • atm Mol • K

51 1 standard atmosphere (atm) 1 atm = 760 mm Hg (or torr)
REMINDER: 1 standard atmosphere (atm) 1 atm = 760 mm Hg (or torr) = kPa (SI unit is PASCAL) Ex kPa = kPa 101.3 kPa / atm = atm Ex torr = torr mm Hg / atm = atm

52 Using PV = nRT How much N2 is required to fill a small room with a volume of 27,000 L to 745 mm Hg at 25 oC? Solution 1. Given: convert to proper units V = L T = 25 oC = 298 K P = 745 mm Hg (760 mm Hg/atm) = 0.98 atm And we always know R = L•atm / mol•K

53 Using PV = nRT How much N2 is required to fill a small room with a volume of L to P = 745 mm Hg at 25 oC? Solution 1. Divide both sides by RT to solve for n. 2. Now plug in those values and solve for the unknown. PV = nRT n = PV / RT RT RT n = 1.1 x 103 mol (or about 30 kg of gas)

54 Learning Check EX. Dinitrogen monoxide (N2O), laughing gas, is used by dentists as an anesthetic. If 2.86 mol of gas occupies a 20.0 L tank at 23°C, what is the pressure in atm in the tank in the dentist office? Given: n=2.86 mol, V=20.0L T=23°C K P=? PV = nRT P = nRT / V P = 2.86 mol ( L atm/mol K)(296 K) L P = 3.48 atm

55 Using PV = nRT When Given Mass
0.78 grams of hydrogen gas is produced at 22°C and 125 kPa. What volume of hydrogen is expected? Given: T = 22°C = 295 K, P = 125 kPa = 125 kPa/101.3 = 1.23 atm n = find n using molar mass first n = mass PV = nRT Mm V = nRT/P n = 0.78g = mol( L atm/mol K)295K g/mol atm = mol = 0.76 L

56 A 5. 0 L cylinder contains oxygen gas at 20. 0°C and 735 mm Hg
A 5.0 L cylinder contains oxygen gas at 20.0°C and 735 mm Hg. How many grams of oxygen are in the cylinder? Step 1) find moles using PV = nRT n = mol Step 2) find mass using m = n x Mm m = 40.3 g

57 Deviations from Ideal Gas Law
Real molecules have volume. The ideal gas consumes the entire amount of available volume. It does not account for the volume of the molecules themselves. There are intermolecular forces. An ideal gas assumes there are no attractions between molecules. Attractions slow down the molecules and reduce the amount of collisions. Otherwise a gas could not condense to become a liquid.

58 Gases in the Air The % of gases in air Partial pressure (STP)
78.08% N mm Hg 20.95% O mm Hg 0.94% Ar mm Hg 0.03% CO mm Hg PAIR = PN + PO + PAr + PCO = 760 mm Hg Total Pressure mm Hg

59 Dalton’s Law of Partial Pressures
2 H2O2 (l) ---> 2 H2O (g) + O2 (g) 0.32 atm atm What is the total pressure in the flask? Ptotal in gas mixture = PA + PB + ... Therefore, Ptotal = PH2O + PO2 = atm Dalton’s Law: total P is sum of PARTIAL pressures.

60 Dalton’s Law John Dalton

61 Health Note When a scuba diver is several hundred feet under water, the high pressures cause N2 from the tank air to dissolve in the blood. If the diver rises too fast, the dissolved N2 will form bubbles in the blood, a dangerous and painful condition called "the bends". Helium, which is inert, less dense, and does not dissolve in the blood, is mixed with O2 in scuba tanks used for deep descents.

62 Collecting a gas “over water”
Gases, since they mix with other gases readily, must be collected in an environment where mixing can not occur. The easiest way to do this is under water because water displaces the air. So when a gas is collected “over water”, that means the container is filled with water and the gas is bubbled through the water into the container. Thus, the pressure inside the container is from the gas AND the water vapor. This is where Dalton’s Law of Partial Pressures becomes useful.

63 Table of Vapor Pressures for Water

64 Solve This! A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the H2 gas? 768 torr – 17.5 torr = torr

65 GAS DENSITY Low density 22.4 L of ANY gas AT STP = 1 mole High density

66 Gases and Stoichiometry
2 H2O2 (l) ---> 2 H2O (g) + O2 (g) Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP? Bombardier beetle uses decomposition of hydrogen peroxide to defend itself.

67 Gases and Stoichiometry
2 H2O2 (l) ---> 2 H2O (g) + O2 (g) Decompose 1.1 g of H2O2 in a flask with a volume of 2.50 L. What is the volume of O2 at STP? Solution 1.1 g H2O mol H2O mol O L O2 34 g H2O mol H2O mol O2 = 0.36 L O2 at STP

68 Gas Stoichiometry: Practice!
A. What is the volume at STP of 4.00 g of CH4? B. How many grams of He are present in 8.0 L of gas at STP?

69 What if it’s NOT at STP? 1. Do the problem like it was at STP. (V1)
2. Convert from STP (V1, P1, T1) to the stated conditions (P2, T2)

70 Try this one! How many L of O2 are needed to react 28.0 g NH3 at 24°C and atm? 4 NH3(g) + 5 O2(g) NO(g) H2O(g)

71 GAS DIFFUSION AND EFFUSION
HONORS only GAS DIFFUSION AND EFFUSION diffusion is the gradual mixing of molecules of different gases. effusion is the movement of molecules through a small hole into an empty container.

72 GAS DIFFUSION AND EFFUSION
HONORS only GAS DIFFUSION AND EFFUSION Graham’s law governs effusion and diffusion of gas molecules. Rate of effusion is inversely proportional to its molar mass. Thomas Graham, Professor in Glasgow and London.

73 GAS DIFFUSION AND EFFUSION
HONORS only GAS DIFFUSION AND EFFUSION Molecules effuse thru holes in a rubber balloon, for example, at a rate (= moles/time) that is proportional to T inversely proportional to M. Therefore, He effuses more rapidly than O2 at same T. He

74 Gas Diffusion relation of mass to rate of diffusion
HONORS only Gas Diffusion relation of mass to rate of diffusion HCl and NH3 diffuse from opposite ends of tube. Gases meet to form NH4Cl HCl heavier than NH3 Therefore, NH4Cl forms closer to HCl end of tube.


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