The Property of Gases – Kinetic Molecular Theory

And Pressure

Kinetic Molecular Theory of Gases

•The word kinetic refers to motion.

•Kinetic energy is the energy an object has because of its motion.

•Kinetic Molecular Theory makes assumptions about:▫Size▫Motion▫Energy of gas particles

1. According to the KMT all matter consists of tiny particles that are in constant, random motion

• Move in a straight line until they collide with other particles or with the walls of the container.

2. Gas particles are much smaller than the distances between them. Most of a gas consists of empty space.

Because they are so far apart, there are no attractive or repulsive forces between the gas molecules

The motion of one particle is independent of the motion of other particles

3. No kinetic energy is lost when gas particles collide with each other or with the walls of the container (elastic collision)

The total amount of kinetic energy remains constant.

4. All gases have the same average kinetic energy at a given temperature Temperature is a measure of average kinetic energy of particles

in a sample of matter. Kinetic energy and temperature are directly related

The higher the temperature, the greater the kinetic energy

Absolute Zero

•The greater the atomic and molecular motion, the greater the temperature is of a substance.

• If all atomic and molecular motion would stop, the temperature would be at absolute zero (0 Kelvin or -273 oC)

•273 + _____oC = _______Kelvin

Diffusion and Effusion

•Diffusion – describes the movement of one material through another▫Particles diffuse from an area of high concentration to low

concentration•Effusion – gas escapes through a tiny opening.

•The heavier the molecule, the slower it will effuse or diffuse

Diffusion and Effusion

Diffusion Effusion

Pressure•Pressure is the force per unit area

•Gas pressure is the force exerted by a gas per unit surface area of an object.

Gas pressure is the result of billions of collisions of billions of gas molecules with an object

•Atmospheric pressure (air pressure) results from the collisions of air molecules with objects.

The air pressure at higher altitudes is slightly lower than at sea level because the density of the Earth’s atmosphere decreases as elevation increases.

•Vacuum - Empty space with no particles and no pressure

Measuring Pressure

•Barometer – an instrument used to measure atmospheric pressure

Measuring Pressure

•Manometer – an instrument used to measure gas pressure in a closed container

Units of Pressure and STP

•Average atmospheric pressure is 1 atm

•STP (Standard Temperature and Pressure) ▫1 atm and 0oC or 1 atm and 273 K

Conversion Factors for Pressure

1 atm = 760 torr = 760 mmHg = 101.3 kPa

Example 1Convert 2.5 atm into torr, mmHg, kPa

2.5 atm 760 torr1 atm

= 1900 torr

2.5 atm 760 mmHg1 atm

= 1900 mmHg

2.5 atm 101.3 kPa1 atm

= 250 kPa

Example 2Convert 215 kPa into torr, mmHg, atm

215 kPa 760 torr101.3 kPa

= 1610 torr

215 kPa 760 mmHg101.3 kPa

= 1610 mmHg

215 kPa 1 atm101.3 kPa

= 2.12 atm

Dalton’s Law of Partial Pressures

Ptotal = P1 + P2 + P3 … + Pn

•Dalton’s law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the pressures of all the gases in the mixture.

Example 1

Ptotal = PO2 + PCO2 + PN2

0.97 atm = PO2 + 0.70 atm + 0.12 atm

PO2 = 0.15 atm

A mixture of O2, CO2, and N2 has a total pressure of 0.97 atm. What is the partial pressure of O2, if the partial pressure of CO2 is

0.70 atm, and the partial pressure of N2 is 0.12 atm?

Example 2

Ptotal = PO2 + PCO2 + PCO

Ptotal = 0.563 atm + 2.32 atm + 0.599 atm

Ptotal = 3.48 atm

235 kPa 1 atm101.3 kPa

= 2.32 atm

455 torr 1 atm760 torr

= 0.599 atm

**You first have to put everything in the same units!

There is a mixture of CO2, O2, and CO in a container. What is the total pressure if the pressure in atm if O2 is 0.563 atm, CO2 is 235

kPa, and CO is 455 torr?

COCl2

C2H2AsCl3

Cl3CNO2

C4H8Cl2S