1 GAS Properties SAVE PAPER AND INK When

1 GAS Properties SAVE PAPER AND INK!!! When you print out the notes on Power. Point, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

2 Importance of Gases • Airbags fill with N 2 gas in an accident. • Gas is generated by the decomposition of sodium azide, Na. N 3 inside the airbag. • 2 Na. N 3 ---> 2 Na + 3 N 2

THREE STATES OF MATTER 3

Characteristics of Gases • Gases expand to fill any container uniformly and completely. – Why? Gas molecules have random motion and are loosely held together • Gases are fluids (like liquids). • Gases diffuse and mix rapidly • Gases have very low densities. – There is a lot of “free” space in a gas 4

5 Characteristics of Gases • Gases can be compressed. – lots of empty space between molecules • Gases undergo diffusion & effusion. – random motion – Gases have PRESSURE!

6 What is Pressure? • Pressure is the force produced by the gas on the walls of its container divided by the surface area of the container. • P = F/A • Force is in Newtons (N) • Area is in meters squared.

What causes the pressure of a gas in a closed container? 7 Impacts of gas molecules with the walls of the container. Microscopic View Anything that increases the number of impacts per second or the force of each impact increases the pressure.

Light molecules move faster and hit the walls more often but with less force. 8 Heavy molecules move more slowly but hit the walls with greater force. These 2 effects exactly balance out. **Gas pressure doesn’t depend on the identity of the gas. ** It depends on Number of impacts per unit time and the Force of each impact

9 Pressure Depends on 1) the concentration or # of gas molecules per unit volume and 2) the temperature.

How fast do the molecules in the air move? • Depends on the mass. • Light molecules are faster than heavy molecules at the same temperature. • Temperature = measure of the ave. translational K. E. of the particles of a system. 10

11 Kinetic Molecular Theory • Particles in an ideal gas… – have no volume. – have elastic collisions. – are in constant, random, straight-line motion. – don’t attract or repel each other. – have an avg. KE directly related to Kelvin temperature.

12 Properties of Gases Gas properties can be modeled using math. The Model depends on— • V = volume of the gas (L) • T = temperature (K) – ALL temperatures in the entire chapter MUST be in Kelvin!!! No Exceptions! K = C + 273 • n = amount (moles) • P = pressure (atmospheres)

13 Temperature • Always use absolute temperature (Kelvin) when working with gases. ºF -459 ºC -273 K 0 32 212 0 100 273 373 K = ºC + 273

Pressure Atmospheric Pressure can be 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. NEWTON’S 3 rd LAW 14

Pressure Units Pressure is measured in many units. • 1 standard atmosphere (atm) = 760 mm Hg (or torr) = 29. 92 inches of Hg = 14. 7 pounds/in 2 (psi) = 101. 3 k. Pa (SI unit is PASCAL) = about 34 feet of water! 15

Pressure Conversions A. The pressure of a tire is measured as 32. 0 psi. What is this pressure in atm ? 1 atm = 14. 7 psi x atm 32. 0 psi Cross multiply and divide ! 32 psi-atm = 14. 7 x So, x = 32 psi-atm / 14. 7 psi x = 2. 18 psi 16

17 STP Standard Temperature & Pressure 0°C 1 atm 273 K -OR- 101. 325 k. Pa

18 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. P 1 V 1 = P 2 V 2 Robert Boyle (1627 -1691). Son of Early of Cork, Ireland.

Boyle’s Law and Kinetic Molecular Theory P proportional to 1/V 19

20 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.

21 Charles’s Law If n and P are constant, then V α T V and T are directly proportional. V 1 V 2 T 1 = T 2 • If one temperature goes up, the volume goes up! Jacques Charles (17461823). Isolated boron and studied gases. Balloonist.

22 Charles’s original balloon Modern long-distance balloon

23 Charles’s Law

24 Gay-Lussac’s Law If n and V are constant, then P α T P and T are directly proportional. P 1 P 2 T 1 = T 2 • If one temperature goes up, the pressure goes up! Joseph Louis Gay. Lussac (1778 -1850)

Gas Pressure, Temperature, and Kinetic Molecular Theory P proportional to T 25

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! P 1 V 1 P 2 V 2 = T 1 T 2 No, it’s not related to R 2 D 2 26

27 Combined Gas Law If you should only need one of the other gas laws, you can cover up the item that is constant and you will get that gas law! P 1 V 1 = T 1 P 2 V 2 T 2 Boyle’s Law Charles’ Law Gay-Lussac’s Law

28 Combined Gas Law Problem A sample of helium gas has a volume of 0. 180 L, a pressure of 0. 800 atm and a temperature of 29°C. What is the new temperature(°C) of the gas at a volume of 90. 0 m. L and a pressure of 3. 20 atm? Set up Data Table P 1 = 0. 800 atm V 1 = 180 m. L P 2 = 3. 20 atm V 2= 90 m. L T 1 = 302 K T 2 = ? ?

29 Calculation P 1 = 0. 800 atm P 2 = 3. 20 atm P 1 V 1 = P 2 V 2 T 1 T 2 V 1 = 180 m. L V 2= 90 m. L T 1 = 302 K T 2 = ? ? P 1 V 1 T 2 = P 2 V 2 T 1 P 1 V 1 T 2 = 3. 20 atm x 90. 0 m. L x 302 K 0. 800 atm x 180. 0 m. L T 2 = 604 K - 273 = 331 °C = 604 K

30 Learning Check A gas has a volume of 675 m. L at 35°C and 0. 850 atm pressure. What is the temperature in °C when the gas has a volume of 0. 315 L and a pressure of 802 mm Hg?

One More Practice Problem A balloon has a volume of 785 m. L 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? 31

And now, we pause for this commercial message from STP 32 OK, so it’s really not THIS kind of STP… STP in chemistry stands for Standard Temperature and Pressure Standard Pressure = 1 atm (or an equivalent) Standard Temperature = 0 deg C (273 K) STP allows us to compare amounts of gases between different pressures and temperatures

33 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?

Avogadro’s Hypothesis Equal volumes of gases at the same T and P have the same number of molecules. V = n (RT/P) = kn V and n are directly related. twice as many molecules 34

Avogadro’s Hypothesis and Kinetic Molecular Theory The gases in this experiment are all measured at the same T and V. P proportional to n 35

IDEAL GAS LAW P V = n R T Brings together gas properties. Can be derived from experiment and theory. BE SURE YOU KNOW THIS EQUATION! 36

Using PV = n. RT P = Pressure V = Volume T = Temperature N = number of moles R is a constant, called the Ideal Gas Constant Instead of learning a different value for R for all the possible unit combinations, we can just memorize one value and convert the units to match R. L • atm R = 0. 0821 Mol • K 37

Using PV = n. RT How much N 2 is required to fill a small room with a volume of 960 cubic feet (27, 000 L) to 745 mm Hg at 25 o. C? Solution 1. Get all data into proper units V = 27, 000 L T = 25 o. C + 273 = 298 K P = 745 mm Hg (1 atm/760 mm Hg) = 0. 98 atm And we always know R, 0. 0821 L atm / mol K 38

Using PV = n. RT How much N 2 is req’d to fill a small room with a volume of 960 cubic feet (27, 000 L) to P = 745 mm Hg at 25 o. C? Solution 2. Now plug in those values and solve for the unknown. PV = n. RT RT RT n = 1. 1 x 103 mol (or about 30 kg of gas) 39

40 Learning Check Dinitrogen monoxide (N 2 O), 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 (mm Hg) in the tank in the dentist office?

Learning Check 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? 41

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. 42

43 Gases in the Air The % of gases in air Partial pressure (STP) 78. 08% N 2 593. 4 mm Hg 20. 95% O 2 159. 2 mm Hg 0. 94% Ar 7. 1 mm Hg 0. 03% CO 2 0. 2 mm Hg PAIR = PN + PO + PAr + PCO = 760 mm Hg 2 2 Total Pressure 2 760 mm Hg

Dalton’s Law of Partial Pressures 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) 0. 32 atm 0. 16 atm What is the total pressure in the flask? Ptotal in gas mixture = PA + PB +. . . Therefore, Ptotal = PH 2 O + PO 2 = 0. 48 atm Dalton’s Law: total P is sum of PARTIAL pressures. 44

45 Dalton’s Law John Dalton 1766 -1844

Health Note When a scuba diver is several hundred feet under water, the high pressures cause N 2 from the tank air to dissolve in the blood. If the diver rises too fast, the dissolved N 2 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 O 2 in scuba tanks used for deep descents. 46

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. 47

Table of Vapor Pressures for Water 48

49 Solve This! A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the gas? 768 torr – 17. 5 torr = 750. 5 torr

50 GAS DENSITY 22. 4 L of ANY gas AT STP = 1 mole High density Low density

Gases and Stoichiometry 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) Decompose 1. 1 g of H 2 O 2 in a flask with a volume of 2. 50 L. What is the volume of O 2 at STP? Bombardier beetle uses decomposition of hydrogen peroxide to defend itself. 51

Gases and Stoichiometry 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) Decompose 1. 1 g of H 2 O 2 in a flask with a volume of 2. 50 L. What is the volume of O 2 at STP? Solution 1. 1 g H 2 O 2 1 mol O 2 22. 4 L O 2 34 g H 2 O 2 2 mol H 2 O 2 1 mol O 2 = 0. 36 L O 2 at STP 52

53 Gas Stoichiometry: Practice! A. What is the volume at STP of 4. 00 g of CH 4? B. How many grams of He are present in 8. 0 L of gas at STP?

54 What if it’s NOT at STP? • 1. Do the problem like it was at STP. (V 1) • 2. Convert from STP (V 1, P 1, T 1) to the stated conditions (P 2, T 2)

Try this one! 55 How many L of O 2 are needed to react 28. 0 g NH 3 at 24°C and 0. 950 atm? 4 NH 3(g) + 5 O 2(g) 4 NO(g) + 6 H 2 O(g)

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. 56

HONORS GAS DIFFUSION AND only EFFUSION Graham’s law governs effusion and diffusion of gas molecules. Rate of effusion is inversely proportional to its molar mass. Thomas Graham, 1805 -1869. Professor in Glasgow and London. 57

HONORS only DIFFUSION AND EFFUSION GAS 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 O 2 at same T. He 58

HONORS only Gas Diffusion relation of mass to rate of diffusion • HCl and NH 3 diffuse from opposite ends of tube. • Gases meet to form NH 4 Cl • HCl heavier than NH 3 • Therefore, NH 4 Cl forms closer to HCl end of tube. 59