GASES

THREE STATES OF MATTER

A windy day or a still day is a result of the difference in pressure of gases in two different locations. A fresh breeze on a mountain peak is a study in basic gas laws. 3

Important Characteristics of Gases 1) Gases are highly compressible An external force compresses the gas sample and decreases its volume, removing the external force allows the gas volume to increase. 2) Gases are thermally expandable When a gas sample is heated, its volume increases, and when it is cooled its volume decreases. 3) Gases have low viscosity (flow easily) Gases flow much easier than liquids or solids. 4) Most Gases have low densities Gas densities are on the order of grams per liter whereas liquids and solids are grams per cubic cm, 1000 times greater. 5) Gases are infinitely miscible Gases mix in any proportion such as in air, a mixture of many gases. 4

Kinetic Molecular Theory A gas consists of a collection of small particles traveling in straight-line motion and obeying Newton's Laws. The molecules in a gas occupy no volume (that is, they are points). Collisions between molecules are perfectly elastic (that is, no energy is gained or lost during the collision). There are no attractive or repulsive forces between the molecules. The average kinetic energy of a molecule depends on temperature. http://www.chm.davidson.edu/vce/kineticmoleculartheory/basicconcepts.html

The Nature of Gases Three basic assumptions of the kinetic theory as it applies to gases: 1. Gas is composed of particles- usually molecules or atoms Small, hard spheres Insignificant volume; relatively far apart from each other No attraction or repulsion between particles 6

2. Particles in a gas move rapidly in constant random motion The Nature of Gases 2. Particles in a gas move rapidly in constant random motion Move in straight paths, changing direction only when colliding with one another or other objects Average speed of O2 in air at 20 oC is an amazing 1660 km/h! (1.6km=1mile) 7

The Nature of Gases 3. Collisions are perfectly elastic- meaning kinetic energy is transferred without loss from one particle to another- the total kinetic energy remains constant Newtonian Cradle- Where the collisions between the balls elastic? Yes, because kinetic energy was transferred with each collision 8

THE KINETIC THEORY OF GASES Remember the assumptions Gas consists of large number of particles (atoms or molecules) Particles make elastic collisions with each other and with walls of container There exist no external forces (density constant) Particles, on average, separated by distances large compared to their diameters No forces between particles except when they collide

What happens to a ball when it drops? …..but in reality the ball loses height and eventually stops bouncing The potential energy of the ball Which is converted to potential energy in the ball Which is converted into the potential energy of the ball………….. Is converted to kinetic energy in the ball Which is converted to kinetic energy in the ball Why does this happen? 10

How does the bouncing ball lose energy? Through friction with the air (air resistance) Through sound when it hits the floor Through deformation of the ball Through heat energy in the bounce 11

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Ideal Gas Model   Kinetic Molecular Theory (KMT) for an ideal gas states that all gas particles: are in random, constant, straight-line motion. are separated by great distances relative to their size; the volume of the gas particles is considered negligible. have no attractive forces between them. have collisions that may result in the transfer of energy between gas particles, but the total energy of the system remains constant.

Ideal vs. Non-Ideal Gases x x Kinetic Theory Assumptions Point Mass No Forces Between Molecules Molecules Exert Pressure Via Elastic Collisions With Walls (courtesy F. Remer)

Ideal vs. Non-Ideal Gases Violates Assumptions Volume of molecules Attractive forces of molecules (courtesy F. Remer)

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)

Atmospheric Pressure Weight of column of air above your head. We can measure the density of the atmosphere by measuring the pressure it exerts.

Pressure = Force per Unit Area P = (Weight of column)/(Area of base) Atmospheric Pressure Pressure = Force per Unit Area Atmospheric Pressure is the weight of the column of air above a unit area. For example, the atmospheric pressure felt by a man is the weight of the column of air above his body divided by the area the air is resting on P = (Weight of column)/(Area of base)

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

Pressure Column height measures Pressure of atmosphere 1 atmosphere (atm) * = 760 mm Hg (or torr) * = 29.92 inches Hg = 14.7 pounds/in2 (psi) = 101.3 kPa (SI unit is PASCAL) * = about 34 feet of water!

Pressure Conversions 540mm Hg = 0.711 atm S: 540mmHg 101.3 kPa What is 540 mm Hg expressed in atm? U: atm, kPa k: 540 mmHg p: 1 atm = 760 mmHg S: 1 atm 760 mm Hg B. What is the pressure in kPa 540mm Hg = 0.711 atm S: 540mmHg 101.3 kPa 760 mm Hg = 72.0 kPa

Pressure Conversions A. What is 920 torr expressed in mmHg?