Nature of Gases Kinetic Molecular Theory

We will now discuss the gas phase So far we have primarily focused on reactions that happen in the liquid phase We will now discuss the gas phase

Kinetic Molecular Theory States Gas Particles: 4. Have no interaction between atoms or molecules of a gas. Don’t attract or repel each other. 6. Have an avg. KE directly related to Kelvin temperature. 2. Go in a straight trajectory until 5. an elastic collision occurs 3. Have no volume: Mostly empty space 1. Consist of widely separated atoms or molecules in constant, random straight line motion

Properties of gases Gases have a unique set of physical properties: Gases are translucent or transparent. Gases have very low densities when compared to liquids or solids. Gases are highly compressible compared to liquids and solids. Gases can expand or contract to fill any container. Mostly empty space

Properties of gases Gases have a unique set of physical properties: Gases are translucent or transparent. Gases have very low densities when compared to liquids or solids. Gases are highly compressible compared to liquids and solids. Gases can expand or contract to fill any container. These can be explained by the kinetic molecular theory – (refer to your reading material for the KMT postulates)

Gases consist of atoms or molecules with a lot of space in between, that are in constant, random motion kinetic molecular theory: the theory that explains the observed thermal and physical properties of matter in terms of the average behavior of a collection of atoms and molecules.

This empty space makes gases good insulators (example: windows, coats)

Pressure Gases have pressure Pressure = force per unit of area A gas can easily change shape. When you push down on a balloon, sideways forces push the sides of the balloon outward.

Pressure and the number of molecules are directly related More molecules means more collisions. Fewer molecules means fewer collisions. Gases naturally move from areas of high pressure to low pressure because there is empty space to move in.

Collisions of Gas Particles 10

Kinetic theory of pressure Pressure comes from the collisions of the many, many atoms inside and outside the balloon. If the collisions occur frequently, the pressure is high. If the collisions don’t occur often, the pressure is low.

*Example: Blowing ______ air into a balloon makes it larger. Here is the _____________ relationship between the # of gas particles in a container and the volume and pressure of the container: As the # of gas particles _____________, the volume of a flexible container will ____________ if the temperature and pressure of the container remain constant. # particles ___, V ___ *Example: Blowing ______ air into a balloon makes it larger. qualitative increase increase ↑ ↑ more

As the # of gas particles ____________, the pressure of a rigid container will ____________ if the temperature and volume of the container remain constant. # particles ___, P ___ *Examples: Pushing the button on an aerosol can releases the gas and ___________ the pressure in the container. Adding too much gas into a rigid container could make it ___________ from too much pressure! increase increase ↑ ↑ decreases explode

# of Gas Particles vs. Pressure

Units of pressure Convert 800 torr of pressure to atmospheres (atm). Standard pressure = 1.000 atm = 760.0 torr = 760.0 mmHg = 101.305 KPa Convert 800 torr of pressure to atmospheres (atm). Relationships: Standard pressure = 760 torr = 1 atm Solve: 𝟖𝟎𝟎 𝒕𝒐𝒓𝒓 𝟏 𝒂𝒕𝒎 = 1.05 torr 𝟕𝟔𝟎 𝒕𝒐𝒓𝒓

A gas will expand to fill any size Kinetic theory of pressure Remember: Gases consist of atoms or molecules with a lot of space in between, that are in constant, random motion A gas will expand to fill any size (or shaped) container

Kinetic Molecular Theory Because we assume molecules are in motion, they have a kinetic energy. At the same Temperature, all gases have the same average KE. As Temperature goes up, KE also increases — and so does speed of the molecules. Temperature = measure of the average kinetic energy of the particles of a substance

Kinetic theory of pressure The average force from molecular impacts depends on two things: 1. Faster (hotter) molecules mean more force per impact and higher pressure. Higher temperature means harder collisions and higher pressure

Kinetic theory of pressure The average force from molecular impacts depends on two things: 1. Faster (hotter) molecules mean more force per impact and higher pressure. 2. More molecules per cubic centimeter (higher density) mean more impacts and therefore higher pressure. Higher density means more collisions and higher pressure

Volume of Gas In a smaller container, molecules have less room to move. Hit the sides of the container more often. As volume decreases, pressure increases. (think of a syringe)

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph!

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph! Random motion A typical air molecule has 140,000 collisions in a centimeter

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph! The average speed of an oxygen molecule (O2) in air is 390 m/s, or 873 mph Random motion Heavier molecules move slower

Diffusion diffusion: the spreading of molecules through their surroundings through constant collisions with neighboring molecules.

No interaction between atoms or molecules, except during collisions The basis of kinetic molecular theory, which explains gas behavior Straight trajectory until a collision occurs Mostly empty space Gases consist of widely separated atoms or molecules in constant, random motion

Pressure increases when: Standard pressure = 1.000 atm = 760.0 torr = 760.0 mmHg = 101.305 KPa Pressure increases when: the temperature (speed of molecules) increases. the density (number of molecules) increases. The energy of molecules only depends on temperature therefore, heavier molecules move slower. Diffusion is the slow spreading of one type of molecules within another type.

Gases consist of widely separated atoms or molecules in constant, random motion Mostly empty space No interaction between gas atoms or molecules except in collisions Straight trajectories until collision occurs

Gases consist of widely separated atoms or molecules in constant, random motion Mostly empty space No interaction between gas atoms or molecules except in collisions Straight trajectories until collision occurs Because gases can expand and contract they behave differently from solid and liquids.

Gas pressure is increased by more frequent and/or harder collisions

You can affect the gas pressure by changing: 1. the density More molecules means more impacts and a higher pressure.

You can affect the gas pressure by changing: 1. the density More molecules means more impacts and a higher pressure. 2. the volume of the container With less space to move around, there are more collisions and a higher pressure.

How KMT Explains the Gas Laws You can affect the gas pressure by changing: 1. the density More molecules means more impacts and a higher pressure. 2. the volume of the container With less space to move around, there are more collisions and a higher pressure. 3. the temperature With more kinetic energy, the molecules move faster. The collisions are harder and more frequent.

Atmospheric pressure The air pressure at the top of Mt. Everest is only one third the air pressure at sea level (0.33 atm compared to 1.00 atm)

Earth is covered with a thin layer of air Pressure Earth is covered with a thin layer of air Image credit: NASA (photo from Apollo 7 spacecraft)

Atmospheric pressure A barometer Empty space Gravity pulls mercury down the tube Air pressure pushes mercury up the tube A barometer As air pressure changes, the height of the mercury column changes. barometer: an instrument that measures atmospheric pressure.

Variations in atmospheric pressure affect the weather