Gases

Characteristics of Gases Gases are fluids Gases are fluids –In other words, they can flow. Gases have low density Gases have low density –Most of the volume occupied by gases are empty space. Gases are highly compressible Gases are highly compressible Completely fills a container Completely fills a container

Pressure Pressure is defined as the force divided by area. Pressure is defined as the force divided by area. So, it is the force on an area. So, it is the force on an area. That is why scientists derived the pascal to measure pressure. That is why scientists derived the pascal to measure pressure. A pascal is defined as one newton (the SI unit for force) applied over an area of one square meter. A pascal is defined as one newton (the SI unit for force) applied over an area of one square meter. Pressure is caused when a gas collides with the walls of a container. Pressure is caused when a gas collides with the walls of a container. Collisions = force, container walls give us the area over which the force is applied Collisions = force, container walls give us the area over which the force is applied

How do we measure pressure? One way is with a barometer One way is with a barometer The most common type use a liquid to measure the atmospheric pressure. The most common type use a liquid to measure the atmospheric pressure.

How is pressure recorded? There are several different pressure units that can be used. There are several different pressure units that can be used. UnitAbbreviation Equivalent number of pascals Atmosphereatm 1 atm = 101,325 Pa Barbar 1 bar = 100,025 Millimeter of mercury mm Hg 1 mm Hg = PascalPa1 Pounds per square inch psi 1 psi = x 10 3 Pa Torrtorr 1 torr =

Example problems Convert each of the following pressures into the specified unit atm  pascals 72.7 atm  pascals kPa  millimeters of mercury (hint kPa means 1000 Pa) kPa  millimeters of mercury (hint kPa means 1000 Pa) kPa  atmospheres kPa  atmospheres 44.7 psi  pascals 44.7 psi  pascals

Answers pascals pascals 92.5 millimeters of mercury 92.5 millimeters of mercury atmospheres atmospheres pascals pascals

Kinetic Molecular Theory Book Definition - A theory that explains that the behavior of physical systems depends on the combined actions of the molecules constituting the system. Book Definition - A theory that explains that the behavior of physical systems depends on the combined actions of the molecules constituting the system. Another way to say this – the behavior of a system depends on the actions of the molecules that make it up. Another way to say this – the behavior of a system depends on the actions of the molecules that make it up.

Concepts of the Kinetic Molecular Theory All matter is composed of particles. All matter is composed of particles. All matter is in constant motion All matter is in constant motion Gases particles are in constant, rapid, and random motion Gases particles are in constant, rapid, and random motion This motion represents the kinetic energy (KE = ½ mv 2 ) of the system. This motion represents the kinetic energy (KE = ½ mv 2 ) of the system. Particles of a gas are very far apart relative to their size. Particles of a gas are very far apart relative to their size. All collisions are perfectly elastic. In other words, there is no loss or gain in energy. All collisions are perfectly elastic. In other words, there is no loss or gain in energy.

Temperature and Kinetic Energy The average kinetic energy of random motion is proportional to the absolute temperature (temperature in Kelvin) The average kinetic energy of random motion is proportional to the absolute temperature (temperature in Kelvin) So, if the temperature changes; the random motion changes. So, if the temperature changes; the random motion changes. If temperature increases; random motion increases. If temperature increases; random motion increases. Note: this does not mean that all particles are moving at the same speed, but overall the rates increase the average. Note: this does not mean that all particles are moving at the same speed, but overall the rates increase the average.

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