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**During this unit you will be able to answer the following questions:**

The Gas Laws During this unit you will be able to answer the following questions: Why does pumping a tire become difficult as it becomes more inflated? Why does a hot air balloon lift off the ground and the air inside is heated? Why does soda eventually go flat, and what could you do to slow that process down? Why do your car ties seem like they have lost air, or go flat in the cold winter months, but then as spring/summer approaches, they seem to have re-inflated on their own?

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What do you know? What is the name of the gas, mostly responsible for the greenhouse effect? Carbon Dioxide

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**The Atmosphere Made of a sea of gases**

The air is made of 78% nitrogen, 21% oxygen, 0.9% argon, and 0.033% carbon dioxide. The trapping of heat energy in the atmosphere is called the greenhouse effect. Increasing the greenhouse gases could be increasing the worlds average temperature.

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**Warmer temps. mean the ice caps would melt and cause coastal flooding**

CO2 in the atmosphere is increasing due to the combustion of fossil fuels and loss of forests

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**The ozone is another main component of our atmosphere.**

Ozone is the molecule O3 Shields us from UV rays given off by the sun. These UV rays cause cancer, and are called a mutagen. The ozone is approximately 3 mm thick

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**Ozone is being depleted by chlorofluorocarbons (CFC’s)**

CFC’s can be found in air conditioners, refrigerators, and aerosol cans 1 CFC molecule can destroy millions of ozone molecules

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**The hole of the ozone is centered over Antarctica**

CFC’s have been banned, but it’s estimated they won’t be completely out of the atmosphere until 2060

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Sept 13, 2007 March 3, 2012

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**Kinetic Molecular Theory**

Predicts the behavior of gases and assumes that gases are ideal gases Ideal gases are gases that travel in a straight line and only change direction when they collide another gas molecule. There is no attractive forces between ideal gases.

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The average kinetic energy of gas particles is directly proportional to temperature in Kelvin. –THIS IS WHY WE USE KELVIN FOR ALL GAS LAW PROBLEMS Meaning: if gas molecules move faster, and hit each other more often, then the temperature will increase. If the molecules move slowly and hit each other less often, the temperature will decrease. Temperature depends on how often and how hard molecules hit each other.

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**Different gases at the same temperature move at different speeds. Why?**

They have different masses. Lighter molecules/compounds, such as H2, move faster than heavier molecules/compounds such as N2 or CO2

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**Temperatures for gases are always given in Kelvin**

Temperatures for gases are always given in Kelvin. Remember that if the temperature is given in Celsius use the equation K = ºC

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**Fluidity Gases are in constant motions and flow like liquids**

Gases are in constant motion and collide with each other and their container. The kinetic molecular theory assumes that the collisions are elastic This means energy is not lost after a collision happens.

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Density Gases have a density that is less than solids and liquids because there’s more space between atoms. The kinetic molecular theory says that the volumes of the individual molecules can be ignored when compared to the total volume of the gas

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**Volume and Compressibility**

Remember, volume is the amount of space something takes up. If you apply a pressure to a gas, the gas molecules come closer together. This decreases the volume of the gas.

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**Volume = length * width * height**

Common units of volume include: liters (L), milliliters (mL), cubic meters (m3), or cubic centimeters (cm3)

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Avogadro’s principle states that equal volumes of gases under the same conditions have equal numbers of molecules. The volume of 1 mole of any gas at STP is 22.4 liters. This is called molar volume

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Pressure When a gas fills a container it puts a pressure on the container in all directions. Think of blowing air in a balloon. Pressure is caused by the collision of gases running into each other The more collisions occurring, the more the pressure increases Pressure is defined as force exerted over a specific area

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**Pressure = force/unit area The metric unit of force is the pascal (Pa)**

1 pascal = 1 Newton/meter2

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**All the air above your exerts (gives) a pressure.**

This is called atmospheric pressure 1 atmosphere of pressure causes the a column of mercury 760 mm high. 1 atmosphere = 760 mmHg

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Pressure Abbreviation Equivalant Pascel Pa Bar bar 1 bar = 100,025 Pa Millimeter of mercury mmHg 1 mmHg = Pa Pound per square inch psi 1 psi = Pa Torr torr 1 torr = Pa Atmosphere atm 1 atm = 101,325 Pa

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**STP = standard temperature and pressure**

Values include 0ºC and 1 atm OR 273 K and kPa.

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What do you know? The average kinetic energy (temperature) of a gas molecule depends on _____________________. Pressure is caused by the ___________________ of gas molecules against each other and the container. HOW FAST THE MOLECULES MOVE Collisions

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**Properties of Gases They don’t always behave ideally**

At high pressures gas molecules get close and attractive forces are significant. Same thing occurs at low temperatures Two assumptions that the kinetic molecular theory doesn’t always follow: The volume of gas molecules is negligible There are no attractive forces between gas molecules.

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**Low temperatures & high pressure cause gases to misbehave.**

If the temperature is low enough, and the pressure is high enough, then molecules can condense.

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Depending on the temperature and the amount of water in the air, water can be in a gas form and a liquid form at almost the same time. Water changes from liquid to gas and back to liquid. This is called fog. H2O(l) H2O(g)

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**The two opposing arrows tells us that the system is in equilibrium.**

In a closed container the pressure caused from water evaporating reaches a maximum value Ex: butane lighter. Gas evaporates until it can’t

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**Equilibrium vapor pressure is just called vapor pressure.**

As temperature increases, vapor pressure increases

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**Liquids boil when its vapor pressure equals the atmospheric pressure.**

The higher you are, the smaller the atmospheric pressure, so it takes less heat to boil water on top of a mountain than sea level. Water boils at 94ºC in the mountains of Denver. Water boils instantly in outer space. Which substance from the graph has the highest boiling point? Which ahs the lowest?

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**Evaporation Vs. Boiling point**

Evaporation happens at temperatures lower than boiling point Evaporation happens at the surface of the liquid. Random molecules gain enough energy to break free of intermolecular forces

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**Boiling happens when bubbles of vapor form inside the liquid.**

The pressure inside the liquid has to be greater than the pressure outside the liquid.

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**Shows the temperatures and pressures a substance exists in different phases.**

X-axis is temperature and Y-axis is pressure Phases are in equilibrium with one another at each line Equilibrium means that the phases are changing back and forth at equal times. Phase Diagram

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**In each region the substance exists in one phase**

If you increase the temperature, vapor increases. At the same time the density of liquid decreases. Over time, the liquid and vapor will have the same density, so vapor won’t exist This is called the critical point

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**At one point of the diagram, solid liquid and gas all exist at the same point (point A)**

This is called the triple point

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**The normal boiling point is where the liquid-gas line crosses 1 atm of pressure.**

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**Phase Diagrams Practice**

Label the diagram: a. triple point b. solid-vapor equilibrium c. solid-liquid equilibrium d. critical point e. normal boiling point f. normal freezing point

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**Phase Diagrams Practice**

What’s Happening?: 77. A? 78. D? 79. E? 80. F? Triple point- all three phases exist in equilibrium Critical point- no longer tell diff btween L and G, vapor not exist… Normal boiling point- liquid-gas line crosses 1atm Normal freezing/melting point- solid/liquid line crosses 1atm

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**Phase Diagrams Practice**

s, l or g ?: 81. 82. 83. 85. 86. gas liquid solid liquid gas liquid

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**Simple Gas Laws You MUST know what these variables represent**

P = Pressure ( what are units for pressure? ) V = Volume ( what are units for volume? ) n = moles T = Temperature in Kelvin Calculations with temperature must be made in Kelvin

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**Charles’ Law Relates temperature and volume**

If the temperature increases, then the volume must increase The opposite is also true. If the volume decreases, the temperature must also decrease This is called a direct relationship

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**Charles’ Law Heating a gas will make it expand.**

Gas molecules move faster when heat is added, striking the container walls with more force. More frequent and forceful collisions make the volume of a flexible container like a balloon increase.

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**If heating a gas makes it expand, what will cooling it do?**

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**Charles’ equation A. V1 = starting volume V2 = ending volume**

T1 = starting temp. T2 = ending temp.

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**T1 and T2 must have the same units**

V1 and V2 must also have the same units Beware: some problems will give you extra information that you don’t need, such as how many moles are in the container or what the pressure is

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**Charles Law Example 1: 7. Basic Charles Law example:**

A chamber has an initial temperature of 298 K and is 2.5 L. The chamber volume changes to 1.75 L. What is the new temperature?

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**Charles Law Example 2: Intermediate problem:**

A container has an initial volume of 17 L at a temperature of 313 K and a pressure of 1 atm. If the temperature changes to 350K and the pressure stays the same, what would the new volume have to be? Since the pressure didn’t change, we don’t have to worry about it. We use the same equation!

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**Charles Law Example 3: Advanced problem:**

A chamber has 12 moles of a gas, a volume of 25 L, and a temperature of 300 K. Assuming the number of moles stays the same, what is the new volume if the temperature increases 25ºC? (K = ºC + 273)(ºC = K – 273)

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A. To solve this problem, you’ll need to convert ºC to K in order to make the units the same. Once the units are the same, you need to use your new T2 temperature in the equation. The number of moles is unnecessary information.

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25ºC = X 298 = X 300K K = 598 K

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**Boyle’s Law Relates pressure and volume**

If pressure increases, then volume will decrease If pressure decreases, then volume will increase This is called an inverse relationship

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If we plot the same information, but with the inverse of volume, we get a straight line…. THUS the relationship is INVERSE!!! 1/V P

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**Equation for Boyle’s Law:**

P1V1 = P2V2

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**Boyle’s Law Example 1: Basic example problem:**

A container has a volume of 6 L and under 3 atm of pressure. If the atmospheric pressure is cut in half, then what is the final volume?

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P1V1 = P2V2 (3 atm)(6 L) = (1.5 atm)(V2) 12 L = V2

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**Boyle’s Law Example 2: Intermediate problem:**

A container with an unknown pressure has a 12 moles of gas and a volume of 35 L. When the volume is decreased to 20 L, the pressure is 5 atm. What is the initial pressure? The number of moles is not important in this problem

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P1V1 = P2V2 (P1)(35 L) = (5 atm)(20 L) P1 = 2.9 atm

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**Boyle’s Law Example 3: Advanced problem:**

A container is 2.7 L and is under 2017 torr of pressure. If the volume changes to 1500 mL, what is the new pressure in units of atm? (760 torr = 1 atm)(1000 mL = 1 L) To solve, convert 2017 torr to atm, then convert 1500 mL to L. Once done, plug into the Boyle’s Law equation.

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P1V1 = P2V2 (2.654 atm)(2.7 L) =(P2)(1.5 L) 4.8 atm = P2

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**Other Simple Gas Equations to Know**

Gay-Lussac There is a direct relationship between pressure and temperature, when other variables are constant. Avogadro’s Law There is a direct relationship between moles and volume, when other variables are constant.

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**The total pressure of the gas in a mixture can be found.**

The pressure of a single gas in a mixture is called its partial pressure. Adding up all the partial pressures gives the total pressure. Simply add up the pressures of the other gasses in the mixture. Ptotal=PA+PB+PC… Daltons Gas Law

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Combined Gas Laws As long as the number of molecules in the gas doesn’t change (the moles stay constant) the combined gas law can be used to find a missing volume, temperature, or pressure. Combined Gas Law Equation:

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**Combined Gas Law Example 1:**

Example problem: A weather balloon containing helium with a volume of L rises in the atmosphere and is cooled from 27°C to -27°C. The pressure on the gas is reduced from kPa to 25.0 kPa. What is the volume of the gas at the lower temperature and pressure? Remember to convert ºC to K

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P1= P2= V1= V2= T1= T2= 110.0 kPa 25.0 kPa 410.0 L ? 300 K 246 K

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L = V2

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Ideal Gas Law When you combine all the gas laws together you get the ideal gas law PV = nRT n = moles R = gas constant

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Gas constant: Number was determined by using STP values and solving for R Due to the units of R, all variables must be in atm OR kPa, L, mol, and K

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Ideal Gas Law Example 1: A sample of carbon dioxide with a mass of grams is placed in a mL container at 400.0K. What is the pressure exerted by the gas? You need pressure, volume, moles, and temperature. Your missing variable is pressure. Find moles by converting your grams of CO2 to moles (use the molar mass of CO2) and then convert mL to L

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= 5.68x10-3 mol of CO2 = 0.350 L

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PV = nRT P = atm

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Volume Conversions Equal volumes of gases contain equal volumes of molecules at the same temperature and pressure. The chemical equation gives a conversion factor or mole ratio to solve the problem. The mole ratio from the chemical equation is the same as the volume ratio.

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1 N2 (g) + 3 H2 (g) 2 NH3 (g) 3L of H2 will contain 3 times as many molecules as 1 L of N2. 2L of NH3 will contain twice as many molecules as 1 L of N2.

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**Volume Conversion Example 1:**

N2 (g) + 3 H2 (g) 2 NH3 (g) How many liters of H2 are needed to react completely with 22L of N2? 66 L H2 =

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**Quick Review Volume vs. ________ or ________? Inverse or Direct?**

What Law/s does this represent? Charle’s Law – with Temperature OR Avogadro’s Law – with moles Both are DIRECT

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**Quick Review Volume vs. ______________? Inverse or Direct?**

What Law does this represent? Boyle’s Law- Pressure Inverse

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**Quick Review Pressure vs. ______________? Inverse or Direct?**

What Law does this represent? Gay-Lussac’s Law- Temperature Direct

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