Gases

The Nature of Gases Gases expand to fill their containers Gases are fluid – they flow Gases have low density 1/1000 the density of the equivalent liquid or solid Gases are compressible Gases effuse and diffuse

1) Which statement about atmospheric pressure is false?  A) As air becomes thinner, its density decreases. B) Air actually has weight. C) With an increase in altitude, atmospheric pressure increases as well. D) The colder the air, the lower the atmospheric pressure

Ideal Gases Ideal gases are imaginary gases that perfectly fit all of the assumptions of the kinetic molecular theory. Gases consist of tiny particles that are far apart relative to their size. Collisions between gas particles and between particles and the walls of the container are elastic collisions No kinetic energy is lost in elastic collisions

Ideal Gases (continued) Gas particles are in constant, rapid motion. They therefore possess kinetic energy, the energy of motion There are no forces of attraction between gas particles The average kinetic energy of gas particles depends on temperature, not on the identity of the particle.

Pressure Pressure is the force created by the collisions of molecules with the walls of a container Unit Symbol Definition/Relationship Pascal Pa SI pressure unit 1 Pa = 1 newton/meter2 Millimeter of mercury mm Hg Pressure that supports a 1 mm column of mercury in a barometer Atmosphere atm Average atmospheric pressure at sea level and 0 C Torr torr 1 torr = 1 mm Hg

2) A gas at a pressure of 10.0 Pa exerts a force of __________ N on an area of 5.5 m2 B) 0.55 C) 5.5 D) 1.8 E) 18

Standard Pressure 1 standard atmosphere (atm) 101.3 kPa (kilopascals) 14.7 lbs/in2 760 mm Hg (millimeters of mercury) 760 torr

3) A pressure of 1.00 atm is the same as a pressure of __________ of mm Hg. A) 193 B) 101 C) 760 D) 29.9 E) 33.0

Measuring Pressure The first device for measuring atmospheric pressure was developed by Evangelista Torricelli during the 17th century. The device was called a “barometer” Baro = weight Meter = measure

An Early Barometer The normal pressure due to the atmosphere at sea level can support a column of mercury that is 760 mm high.

4) In a Torricelli barometer, a pressure of one atmosphere supports a 760 mm column of mercury. If the original tube containing the mercury is replaced with a tube having twice the diameter of the original, the height of the mercury column at one atmosphere pressure is __________ mm.   A) 380 B) 760 C) 1.52x103 D) 4.78 x 103 E) 121

The Kelvin Scale

Standard Temperature and Pressure “STP” Either of these: 273 Kelvin (273 K) 0 C Standard Temperature and Pressure “STP” And any one of these: 1 atm 101.3 kPa 14.7 lbs/in2 (psi) 760 mm Hg 760 torr

A) a gas is made up of molecules 4) One significant difference between gases and liquids is that __________. A) a gas is made up of molecules B) a gas assumes the volume of its container C) a gas may consist of both elements and compounds D) gases are always mixtures E) All of the above answers are correct.