Gases

Elements that exist as gases at 25 0 C and 1 atmosphere 5.1

Gases assume the _________ and the __________ of their containers. Gases are the most ____________ state of matter. Gases will mix __________ and completely when confined to the same container. Gases have much lower __________ than liquids and solids. 5.1 Physical Characteristics of Gases

Units of Pressure 1 pascal (Pa) = 1 N/m 2 1 atm = 760 mm Hg = 760 torr = 101,325 Pa = 14.7 psi = in. Hg 5.2 Barometer Pressure = Force Area (force = mass x acceleration)

Sea level1 atm 4 miles0.5 atm 10 miles0.2 atm 5.2

Boyle’s Law P α 1/V This means Pressure and Volume are _________ 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 ( ). Son of Earl of Cork, Ireland.

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!If one temperature goes up, the volume goes up! Jacques Charles ( ). Isolated boron and studied gases. Balloonist.

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

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 R2D2

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

Avogadro’s Law V  number of moles (n) V = constant x n V 1 /n 1 =______ 5.3 Constant temperature Constant pressure

Ideal Gas Equation 5.4 Charles’ law: V  T  (at constant n and P) Avogadro’s law: V  n  (at constant P and T) Boyle’s law: V  (at constant n and T) 1 P V V  nT P V = constant x = R nT P P R is the gas constant PV =______

The conditions 0 0 C and 1 atm are called standard temperature and pressure (STP). PV = nRT R = PV nT = (1 atm)(22.414L) (1 mol)( K) R = L atm / (mol K) 5.4 Experiments show that at STP, 1 mole of an ideal gas occupies L.

Density (d) Calculations d = m V = PMPM RT m is the mass of the gas in g M is the molar mass of the gas Molar Mass ( M ) of a Gaseous Substance dRT P M = d is the density of the gas in g/L 5.4 Solve for n, which = mass/molar mass Or,

A 2.10-L vessel contains 4.65 g of a gas at 1.00 atm and C. What is the molar mass of the gas? 5.3 n = 2.10 LX 1 atm x K Latm molK M = 54.6 g/mol PV = nRT n = mol g M n =

Gas Stoichiometry What is the volume of CO 2 produced at 37 0 C and 1.00 atm when 5.60 g of glucose are used up in the reaction: C 6 H 12 O 6 (s) + 6O 2 (g) 6CO 2 (g) + 6H 2 O (l) g C 6 H 12 O 6 mol C 6 H 12 O 6 mol CO 2 V CO 2 V = = 4.76 L 5.5

Dalton’s Law of Partial Pressures V and T are constant P1P1 P2P2 P total = ______ 5.6

Consider a case in which two gases, A and B, are in a container of volume V. P A = n A RT V P B = n B RT V n A is the number of moles of A n B is the number of moles of B P T = P A + P B X A = nAnA n A + n B X B = nBnB n A + n B P A = X A P T P B = X B P T P i = X i P T 5.6 mole fraction (X i ) = nini nTnT

A sample of natural gas contains 8.24 moles of CH 4, moles of C 2 H 6, and moles of C 3 H 8. If the total pressure of the gases is 1.37 atm, what is the partial pressure of propane (C 3 H 8 )? P i = X i P T X propane = P T = 1.37 atm = P propane = x 1.37 atm= atm 5.6

2KClO 3 (s) 2KCl (s) + 3O 2 (g) Bottle full of oxygen gas and water vapor P T = P O + P H O

Chemistry in Action: Scuba Diving and the Gas Laws PV Depth (ft)Pressure (atm)

Kinetic Molecular Theory of Gases 1.The molecules can be considered to be points; that is, they possess mass but have negligible volume. 2. Collisions among molecules are perfectly elastic. 3.Gas molecules exert neither ________ forces on one another. 4.The average kinetic energy of the molecules is proportional to the temperature of the gas in ____. Any two gases at the same temperature will have the same average kinetic energy 5.7

Kinetic theory of gases and … Compressibility of Gases Boyle’s Law P  collision rate with wall Collision rate  number density Number density  1/V P  1/V Charles’ Law P  collision rate with wall Collision rate  average kinetic energy of gas molecules Average kinetic energy  T P  TP  T 5.7

Kinetic theory of gases and … Avogadro’s Law P  collision rate with wall Collision rate  number density Number density  n P  nP  n Dalton’s Law of Partial Pressures Molecules do not attract or repel one another P exerted by one type of molecule is unaffected by the presence of another gas P total =  P i 5.7

Deviations from Ideal Behavior 1 mole of ideal gas PV = nRT n = PV RT = Repulsive Forces Attractive Forces

Effect of intermolecular forces on the pressure exerted by a gas. 5.8

Van der Waals equation nonideal gas P + (V – nb) = nRT an 2 V2V2 () } corrected pressure } corrected volume

The distribution of speeds for nitrogen gas molecules at three different temperatures The distribution of speeds of three different gases at the same temperature 5.7 Velocity of a Gas

Gas diffusion is the gradual mixing of molecules of one gas with molecules of another by virtue of their kinetic properties. 5.7 NH 3 17 g/mol HCl 36 g/mol NH 4 Cl

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

GAS DIFFUSION AND EFFUSION ______ is the gradual mixing of molecules of different gases.______ is the gradual mixing of molecules of different gases. ______ is the movement of molecules through a small hole into an empty container.______ is the movement of molecules through a small hole into an empty container.

GAS DIFFUSION AND EFFUSION Graham’s law governs effusion and diffusion of gas molecules. Graham’s law governs effusion and diffusion of gas molecules. KE=1/2 mv 2 Thomas Graham, Professor in Glasgow and London. Rate of effusion is inversely proportional to its molar mass.

GAS DIFFUSION AND EFFUSION Molecules effuse thru holes in a rubber balloon, for example, at a rate (= moles/time) that is proportional to Tproportional to T inversely proportional to M.inversely proportional to M. Therefore, He effuses more rapidly than O 2 at same T. He

Graham’s Law Problem 1 1 mole of oxygen gas and 2 moles of ammonia are placed in a container and allowed to react at 850 degrees celsius according to the equation: 4 NH 3 (g) + 5 O 2 (g) --> 4 NO(g) + 6 H 2 O(g) Using Graham's Law, what is the ratio of the effusion rates of NH 3 (g) to O 2 (g)?

Graham’s Law Problem 2 What is the rate of effusion for H 2 if ml of CO 2 takes 4.55 sec to effuse out of a container?

Graham’s Law Problem 3 What is the molar mass of gas X if it effuses times as rapidly as N 2 (g)?