Gas Laws Unit (part 1): Chapter 10 Gas Behavior Basics Kinetic-Molecular Theory Temperature and Pressure relationships Gas Laws Boyle’s Law, Charle’s Law , Gay – Lussac’s Law Combined Gas Law

Learning Objectives… To describe the characteristics and behaviors that all gases have in common To use Kinetic Molecular Theory (KMT) to describe how the behavior of gas particles explains the physical properties of gases

Gas Basics… Gases are all around us but we can’t always see them A gas is usually not a single element or compound but a mixture of them

Gas Basics… Air is a mixture of gases that acts like a single gas 78% Nitrogen 21% Oxygen 1% Other gases

Gas Basics… Most gases are made of molecules Can be diatomic or polyatomic MOLECULES Examples: N2, O2, CO2, CH4, etc. Some gases exist as single atoms He, Ne, Ar, Kr, Xe, Rn We use the term “gas particles” to refer to an individual atom OR a molecule of gas.

How Do Gases Behave? Regardless of their chemical identity, gases have very similar behavior Remember: 1 mole of any Gas at 0o C & 1 atm occupies A volume of 22.4 L STP = 0o C & 1 atm. Kinetic Molecular Theory (KMT)explains the behavior of gases

Temperature time… Get up out of your seats and find a partner from some other area of the room and discuss the following question… What are the temperatures at which water freezes and boils in Celsius, Fahrenheit, AND Kelvin temperature scales? The next slide will help your discussion and or calculations…

Temperature conversions… Kelvin temperature from Celsius: 0° C + 273 = kelvin temp Temperatures in Celsius and Fahrenheit [(° F) – 32] x (5/9) = ° C (oC x 9/5) + 32 = o F Make sure you write down the chart on the whiteboard in front of the classroom…

Kinetic-Molecular Theory… 1. Gases consist of small particles that have mass Evidence of mass a flat b-ball weighs less than one with air in it

Kinetic-Molecular Theory… 2. The particles in a gas are separated from each other by large distances The volume of gas particles themselves is considered to be zero b/c they are so small compared to the volume of their containers

Kinetic-Molecular Theory… Evidence of low density Gases are easy to compress. Moderate squeezing (pressure) on a gas will decrease its volume. Compressing a gas changes Its density…

Kinetic-Molecular Theory… 3. The particles of a gas are in constant, rapid, random motion They travel in straight lines, changing direction only when they collide with each other or the edges/walls of the container KE (Kinetic Energy depends only on temp Hotter = faster

Kinetic-Molecular Theory… Evidence of constant random motion Gases fill their containers completely when a balloon fills with air, the air is distributed evenly throughout the balloon Nowhere around you is there an absence of air

Kinetic Molecular Theory… Gases “diffuse” and “effuse” Diffusion = when gas molecules move through each other from areas of higher to lower concentration This is why you can smell food cooking in another area of a house Effusion = when gas molecules move through a hole so tiny that they have to pass through one particle at a time This is why balloons deflate slowly even when tied off The lighter the gas,(or smaller the atom) the faster it will diffuse/effuse (Graham’s Law) Nowhere around you is there an absence of air The movement of one substance through another is called diffusion

Kinetic-Molecular Theory … Which picture shows diffusion? Effusion? Nowhere around you is there an absence of air The movement of one substance through another is called diffusion (b) (a)

Kinetic Molecular Theory… Do these show diffusion? Effusion? Both? (a) (b)

Kinetic-Molecular Theory … A balloon filled with which gas will deflate the quickest and why? O2: 32 g/mol N2: 28 g/mol H2: 2 g/mol He: 4 g/mol

Kinetic-Molecular Theory… 4. Gases exert pressure because they constantly collide with each other and with the walls of the container in which they are held. Gas particles undergo thousands of collisions with each other and with the walls of their container. When they collide with the container, the impact exerts a force on the inside wall. The force exerted over that area of wall space = pressure (pressure = force per unit of area)

Pressure… Get up out of your seats and find a partner from some other area of the room and discuss any units of pressure (air pressure) that you may be aware of or familiar with…

Conversions for Pressure Units… One (1) atmosphere of pressure equals… 1 atmosphere = 101.325 kPa (kilopascals) 1 atm. = 760 mm Hg 1 atm. = 14.7 lbs./in2 (or psi) 1 atm. = 760 torr 1 torr = 1 mm Hg 101,325 Pascals = 101.325 kPa Put this slide into your notes!...

Kinetic-Molecular Theory … Every time a gas particle collides with a container wall it exerts an outward push or force on the wall The outward force spread over the area of the container is called “pressure” e.g., the pressure a gas exerts on the inner walls of a balloon gives the balloon its shape Nowhere around you is there an absence of air The movement of one substance through another is called diffusion

Kinetic-Molecular Theory… Evidence of pressure a. The pressure a gas exerts on the inner walls of a balloon gives the balloon its shape. b. Pressure increases when more air is added to a basketball More gas particles  more collisions with the walls  greater force  increased pressure

Kinetic-Molecular Theory… Assuming a constant volume and temp., the total pressure of a mixture of gases is the sum of the pressures of the component gases. Dalton’s Law of Partial Pressures Dalton observed that the particles of different gases in a mixture exert the same force on the walls of the container as if they were alone at the same temperature

Kinetic-Molecular Theory The pressure of a gas depends on its temp Higher temp = higher pressure (& vice-versa) Higher temp = faster moving particles = collisions are more frequent and at higher speeds = more forceful = increased pressure. Each of these balloons has the same amount of gas inside! Why would you need to check your tire pressure in winter? Nowhere around you is there an absence of air The movement of one substance through another is called diffusion Low Temp Med Temp High Temp

Kinetic-Molecular Theory… 5. When gas particles collide, they do not sloooow dooooown (slow down) Collisions are “elastic.” In a perfectly elastic collision, no kinetic energy is lost

Kinetic-Molecular Theory… 6. Gas particles exert no force on one another. Gas particles don’t slow down and condense into a liquid because the attractive forces between them are very weak

Real vs. Ideal Gases Most of the time, gases behave exactly as the KMT predicts they will When they do, we call them “Ideal Gases” However, KMT makes some faulty assumptions… 1) gases have no attraction for each other (no Intra-molecular forces) 2) gases have no volume For explanations of deviations from ideal behavior are on p. 445 in Prentice Hall

Real vs. Ideal Gases So under certain conditions, real gases do not behave like KMT predicts they will At very low temps, gas particles slow down enough that the IMFs they supposedly don’t have between them become significant At very high pressures, the volume they supposedly don’t have is significant and compression becomes very difficult

Real vs Ideal Gases Even though ideal gases do not really exist… “real gases” behave like ideal gases under most conditions so we can still use the gas laws to describe them and their behaviors. For explanations of deviations from ideal behavior are on p. 445 in Prentice Hall