Gases

Characteristics of Gases

• Unlike liquids and solids, they– Expand to fill their containers.– Are highly compressible.– Have extremely low densities.

Composition of the Atmosphere

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo

when seen from space.

Nitrogen 78.0842%

Oxygen 20.9463%

Argon 0.93422%

Carbon dioxide

0.03811%

Water vapor about 1%

Other 0.002%

• Pressure is the amount of force applied to an area.

Pressure

• Atmospheric pressure is the weight of air per unit of area.

P =FA

Ideal-Gas Equation

The Ideal-Gas concept assumes that the gas molecules behave according to the conditions in the Kinetic Molecular Theory, which will be discussed later in this unit.

PV = nRT

Kelvin

• All motion within an atom ceases at zero Kelvin (K) -- this point is called absolute zero

• 0 K = -273 C• Absolute zero has never been reached in a lab • Space is at an average of 2.73K (no joke) due

to big bang explosion!

Oxygen boiling point: 90.15 K (-183.0 °C )Oxygen melting point: 54.36 K (-218.79 °C, -361.82 °F). Nitrogen boiling point 77.35 K (-195.8 °C)Nitrogen melting point 63.14 K (-210 °C or -346 °F)

Sample Data for Hall attendees and absentees

1. Length of Mg 3.00cm

1.422g=100.cm5a. 20.0ᵒC5b. 748 torr6. 30.0mL

Kinetic-Molecular Theory

This is a model that aids in our understanding of what happens to gas particles as environmental conditions change.

KMT• 1. Gases consist of large numbers of molecules that are in

continuous, random motion.• 2. The combined volume of all the molecules of the gas is

insignificant relative to the total volume in which the gas is contained. REAL GASES TAKE UP SOME VOLUME

• 3. Attractive and repulsive forces between gas molecules are insignificant. REAL GASES HAVE SOME ATTRACTIVE FORCES

• 4. Energy can be transferred between molecules during collisions, but the average kinetic energy of the molecules does not change with time, as long as the temperature of the gas remains constant. NOT ALL COLLISIONS ARE ELASTIC

• 5. The average kinetic energy of the molecules is proportional to the absolute temperature in Kelvin.

Main Tenets of Kinetic-Molecular Theory

1. Gases consist of large numbers of molecules that are in continuous, random motion.

Main Tenets of Kinetic-Molecular Theory

• 2. The combined volume of the gas molecules is insignificant relative to the total volume in which the gas is contained.

…REAL GASES TAKE UP SOME VOLUME• 3. Attractive and repulsive forces between gas

molecules are so small that they are ignored.…REAL GASES HAVE small ATTRACTIVE and REPULSIVE FORCES

Main Tenets of Kinetic-Molecular Theory

4. Collisions between gas molecules are assumed to be elastic, in that no energy is lost during collisions.

...NOT ALL COLLISIONS ARE ELASTIC

Main Tenets of Kinetic-Molecular Theory

5. The average kinetic energy of the molecules is directly proportional to the absolute temperature, in Kelvin.

Kinetic molecular theory

• Pressure is caused by collisions of molecules on the walls of the container. High pressure results from high force of collisions and great frequency of collisions. (total pressure is the sum of the forces for all collisions)

• Temperature approximates molecular motion. With increase temperature there is an increase in KE and increase of force of collisions and therefore greater pressure.

• What is the effect of a volume increase at constant temp?– Molecules collide less often with other molecules

and walls, so pressure decreases• Effect of a temp increase at constant volume

– Increase in KE (KE=1/2mv2), and increase of force of collisions (F=ma), therefore pressure increases (P=F/A)

EffusionThe escape of gas molecules through a tiny hole into an evacuated space.

The trend: The smaller the gas’s molar mass the faster the gas moves and the faster the effusion

Why is helium losing more gas through the pores of the ballon

than nitrogen

Diffusion

The spread of one substance throughout a space or throughout a second substance.

Again gases with smaller molar masses move faster

Real GasesIn the real world, the behavior of gases only conforms to the ideal-gas equation at relatively high temperature and low pressure.

Why? At high temperatures molecules are moving very fast and any loss of speed from resulting collisions or intermolecular forces are less significant. At low pressures it is assumed to have very few molecules and therefore individual gas molecule’s volume, energy lost during collisions and intermolecular forces are less significant.

Deviations from Ideal Behavior

• The assumptions made in the kinetic-molecular model break down at high pressure and/or low temperature.

• We assume gases occupy no space and have no attractions.• Real molecules do occupy space and attract one another.• Usually only seen under high pressure conditions –useful for

industry which moves lots of gases around under high pressure

R- The Universal Gas ConstantR can be calculated from values of STP and the Ideal Gas Law equation:PV=nRT

Ideal Gas Law ProblemDetermine the volume in L of a gaseous system with the following properties: 295K, 2.05moles, 775torr

Ideal Gas Law ProblemDetermine the Temperature in Celsius of a gaseous system with the following properties: 100.0L, 2.50moles, 1.25atm

Ideal Gas Law ProblemDetermine the Pressure in torr of a gaseous system with the following properties: 12.5L, 0.0995moles, at 125ᵒC

Ideal Gas Law ProblemDetermine the mass of a gaseous system containing only Oxygen with the following properties: 50.0L, 400.K, 105kPa

Ideal Gas Law Problem with Stoich.Determine the volume of water vapor formed if 2.50moles of C3H8 reacts with 10.0moles of Oxygen gas at 25.0ᵒC and 767mmHg.

Ideal Gas w/ Stoich.2.50L of Hydrogen gas at 1.05atm and 290.K reacts with excess oxygen gas. How many grams of water vapor could be formed theoretically?

Ideal Gas Stoich1.50L of hydrogen gas at 752torr and 303K reacts with 0.250moles of oxygen gas. Determine the maximum mass of water vapor that can be formed.

Ideal Gas w/ stoich10.0g of C2H6 reacts with 10.0L of Oxygen at 1.30atm and 298K. Determine the maximum mass of Carbon dioxide that can be formed.

Lab CalculationsData table #3 convert mm Mg ribbon to moles

mmgmol use 1.4266g=1000mm

Data Table #5a Lab Temp convert to Kelvin celsius + 273 = Kelvin

Data table#7 P(lab)=P(Hydrogen) +P(water vapor)so… P(hydrogen) = P(lab) – P(water vap)

Look up Pwater as a function of temp in Table 12.2

Mole and Molar Volume Lab#8 Use the Combined Gas Law to solve for V2 use STP

conditions for T2=273K and P2 =760 torr P1V1/T1 = P2V2/T2 then V2= (P1V1T2)/(T1P2)

-once you have solved for V2 convert your answer from mL to L to use in #9

#9 (Your answer from #8 in L) / (moles of H2) Moles of H2 is determined from the balanced

equation and moles of Mg that reacted (#3) you are to assume that Mg is the limiting reactant and HCl is in excess. Since the mole ratio is 1:1 then moles of Mg=moles of H2