# Unit 9 - States of Matter and Gas Behavior

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Unit 9 - States of Matter and Gas Behavior
Chemistry Chapters 12-13

The Behavior of Gases Gases expand, diffuse, exert pressure, and can be compressed because they are in a low density state consisting of tiny, constantly-moving particles. Kinetic-molecular theory explains the different properties of solids, liquids, and gases. Atomic composition affects physical and chemical properties. The kinetic-molecular theory describes the behavior of matter in terms of particles in motion.

The Behavior of Gases Gases consist of small particles separated by empty space. Gas particles are too far apart to experience significant attractive or repulsive forces. Gas particles are in constant random motion. An elastic collision is one in which no kinetic energy is lost. Kinetic energy of particles is directly proportional to temperature

The Behavior of Gases Kinetic energy of a particle depends on mass and velocity. Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

The Behavior of Gases Great amounts of space exist between gas particles. Compression reduces the empty spaces between particles.

Gas Pressure Pressure is defined as force per unit area.
Gas particles exert pressure when they collide with the walls of their container. The particles in the earth’s atmosphere exert pressure in all directions called air pressure. There is less air pressure at high altitudes because there are fewer particles present, since the force of gravity is less.

Gas Pressure Barometers are instruments used to measure atmospheric air pressure.

Gas Pressure The average height of a mercury column in a barometer at sea level is 760 mm (760 mmHg) There are other units of pressure as well with the following relationships: 1.00 atm = 760 mmHg = 760 torr = kPa

Gas Pressure Dalton’s law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the pressures of all the gases of the mixture. The partial pressure of a gas depends on the number of moles, size of the container, and temperature and is independent of the type of gas.

Gas Pressure Ptotal = P1 + P2 + P3 +…Pn
Partial pressure can be used to calculate the amount of gas produced in a chemical reaction.

Gas Pressure Gases are often collected by water displacement, but water vapor is introduced to the gas sample (mixture of gases) You can use Dalton’s law to get the partial pressure of the gas without the water vapor

Forces of Attraction Intermolecular forces—including dispersion forces, dipole-dipole forces, and hydrogen bonds—determine a substance’s state at a given temperature. Attractive forces between molecules cause some materials to be solids, some to be liquids, and some to be gases at the same temperature.

Forces of Attraction Prefix INTRA means “within”, where INTER means “between”

Forces of Attraction Dispersion forces are weak forces that result from temporary shifts in the density of electrons in electron clouds. Weakest of all intermolecular forces Dispersion forces become increasingly stronger as molecule gets more electrons, which explains why iodine is a solid but fluorine is a gas at room temperature

Forces of Attraction Dipole-dipole forces are attractions between oppositely charged regions of polar molecules.

Forces of Attraction Hydrogen bonds are special dipole-dipole attractions that occur between molecules that contain a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair of electrons, typically fluorine, oxygen, or nitrogen.

Forces of Attraction

Liquids and Solids The particles in solids and liquids have a limited range of motion and are not easily compressed. Liquids and solids are called “condensed phases” because their particles are very close together and do not have the energy necessary to “escape” the sample…their motion is limited

Liquids and Solids Solid particles are arranged in a crystal arrangement where they are fixed in position. This arrangement results in a substance that has a defined shape and volume.

Liquids and Solids Liquid particles have enough energy to “slide” by other particles, but do not have the energy to escape the sample itself. This results in a substance that has a fixed volume but takes the shape of its container.

Phase Changes Matter changes phase when energy is added or removed. Heat is the transfer of energy from an object at a higher temperature to an object at a lower temperature.

Phase Changes When ice is heated, the ice eventually absorbs enough energy to break the hydrogen bonds that hold the water molecules together. When the hydrogen bonds break, the particles move apart and ice melts into water. The melting point of a crystalline solid is the temperature at which the forces holding the crystal lattice together are broken and it becomes a liquid.

Phase Changes Vaporization is the process by which a liquid changes to a gas or vapor. Evaporation is vaporization only at the surface of a liquid.

Phase Changes In a closed container, the pressure exerted by a vapor over a liquid is called vapor pressure.

Phase Changes The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure.

Phase Changes Evaporation vs Boiling Summary:
Evaporation is the result of surface level molecules obtaining enough energy to escape the sample as a gas Boiling occurs throughout the sample when the vapor pressure of the liquid is the same as the atmospheric pressure

Phase Changes Sublimation is the process by which a solid changes into a gas without becoming a liquid. As heat flows from water to the surroundings, the particles lose energy. The freezing point is the temperature at which a liquid is converted into a crystalline solid. As energy flows from water vapor, the velocity decreases. The process by which a gas or vapor becomes a liquid is called condensation. Deposition is the process by which a gas or vapor changes directly to a solid, and is the reverse of sublimation.

Phase Changes A phase diagram is a graph of pressure versus temperature that shows in which phase a substance will exist under different conditions of temperature and pressure.

Phase Changes

Phase Changes The triple point is the point on a phase diagram that represents the temperature and pressure at which all three phases of a substance can coexist. The critical point is the highest temperature the substance can exist as a liquid, regardless of pressure Normal melting point and Normal boiling point are the temperatures at which these phase changes occur under standard pressure (760 torr)

Gas Laws For a fixed amount of gas, a change in one variable—pressure, temperature, or volume—affects the other two.

Gas Laws Boyle’s law states that the volume of a fixed amount of gas held at a constant temperature varies inversely with the pressure. P1V1 = P2V2 where P = pressure and V = volume

Gas Laws Example Problem. A sample of oxygen gas has a volume of 150. mL when its pressure is atm. What will the volume of the gas be at a pressure of atm if the temperature is constant?

Gas Laws Givens and Unknown P1=0.947 atm V1=150. mL P2=0.987 atm V2=?

Gas Laws Equation  P1V1=P2V2 Solve for unknown, V2

Gas Laws As temperature increases, so does the volume of gas when the amount of gas and pressure do not change. Kinetic-molecular theory explains this property.

Gas Laws

Gas Laws Celsius and Fahrenheit use ARBITRARY zero points, where any temperature lower than these points are considered to be negative Absolute zero is zero on the Kelvin scale. Remember - temperature is a measure of kinetic energy of the particles. Particles CAN NOT HAVE less kinetic energy than zero. Therefore, at absolute zero, all motion ceases.

Gas Laws Conversions: Kelvin T (K) = Celsius T (C) + 273
Celsius T (C) = Kelvin T (K) - 273

Gas Laws Charles’s law states that the volume of a given amount of gas is directly proportional to its kelvin temperature at constant pressure.

Gas Laws IMPORTANT! Your temperatures MUST BE IN KELVIN when using Charles’ Law (or any gas law). A rearrangement of the Charles’ Law equation that is often easier to use (no fraction) is V1T2=V2T1

Gas Laws Example - A sample of oxygen gas has a volume of 150. mL when its temperature is 295 K. What will the volume of the gas be at a temperature of 309 K if the pressure is constant?

Gas Laws Givens and Unknown V1=150. mL T1=295 K V2=? T2=309 K

Gas Laws Equation  V1T2=V2T1 Solve for V2

Gas Laws Example # 2 - A sample of oxygen gas has a volume of 277 mL when its temperature is 52C. If the gas volume is increased to 344 mL, at what Celsius temperature will the gas be?

Gas Laws Givens and Unknown V1=277 mL
T1=52C - convert to K = 325 K V2=344 mL T2=?

Gas Laws Equation - V1T2=V2T1 Solve for T2 404K = 131C

Gas Laws Gay-Lussac’s law states that the pressure of a fixed amount of gas varies directly with the kelvin temperature when the volume remains constant.

Gas Laws

Gas Laws The combined gas law states the relationship among pressure, temperature, and volume of a fixed amount of gas. We can rearrange the equation to remove the fraction and get P1V1T2=P2V2T1

Gas Laws Example: A balloon is inflated to 20.0 L at 31 deg C and 89.0 kPa. Determine the new volume of the balloon if it is brought to conditions of 4 deg C and kPa. P1 = 89.0 kPa V1 = 20.0 L T1 = 31 deg C = 304 K P2 = kPa V2 = ? T2 = 4 deg C = 277 K

Gas Laws P1V1T2 = P2V2T1…solve for V2 V2 = P1V1T2/P2T1

The Ideal Gas Law The ideal gas law relates the number of particles to pressure, temperature, and volume. Avogadro’s principle states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles.

The Ideal Gas Law The molar volume of a gas is the volume 1 mol occupies at 0.00°C and 1.00 atm of pressure. 0.00°C and 1.00 atm are called standard temperature and pressure (STP). At STP, 1 mol of gas occupies 22.4 L.

The Ideal Gas Law Ideal gas particles occupy a negligible volume and are far enough apart to exert minimal attractive or repulsive forces on each other. Combined gas law to ideal gas law

The Ideal Gas Law The ideal gas constant is represented by R and is L•atm/mol•K when pressure is in atmospheres. It is expressed as L•kPa/mol•K when pressure is in kilopascals The ideal gas law describes the physical behavior of an ideal gas in terms of pressure, volume, temperature, and amount. We call the constant “R”

The Ideal Gas Law To rearrange this equation to get rid of the fraction, we get the most common expression of the law PV = nRT n = number of moles

The Ideal Gas Law Remember! The ideal gas law DOES NOT deal with changing conditions. Given three of the four variables that describe a gas, you can solve for the fourth!

The Ideal Gas Law Sample Problem - A sample of carbon dioxide with a mass of g was placed in a 350 mL container at 127 deg C. What is the pressure exerted by the gas in kPa? P = ? V = 350 mL x (1L/1000mL) = 0.35 L N = g x (1 mole/44.01g) = mol R = LkPa/(Kmol) T = 127 C = 400. K

The Ideal Gas Law PV=nRT, divide by V P = nRT/V

Ideal Gas Law You Try: An oxygen sample has a volume of 4.0 L at 37C and atm. Calculate the number of moles of oxygen in the sample.

Ideal Gas Law P = 1.086 atm V = 4.0 L N = ? R = 0.08206 Latm/(Kmol)
T=310 K PV=nRT, n=PV/RT

Real Gases Ideal gases follow the assumptions of the kinetic-molecular theory. Ideal gases experience: There are no intermolecular attractive or repulsive forces between particles or with their containers. The particles are in constant random motion. Collisions are perfectly elastic. No gas is truly ideal, but most behave as ideal gases at a wide range of temperatures and pressures.

Real Gases Real gases deviate most from ideal gases at high pressures and low temperatures. Polar molecules have larger attractive forces between particles. Polar gases do not behave as ideal gases. Large nonpolar gas particles occupy more space and deviate more from ideal gases.

Gas Stoichiometry When gases react, the coefficients in the balanced chemical equation represent both molar amounts and relative volumes. The gas laws can be applied to calculate the stoichiometry of reactions in which gases are reactants or products.

Gas Stoichiometry 2H2(g) + O2(g) → 2H2O(g)
2 mol H2 reacts with 1 mol O2 to produce 2 mol water vapor. Coefficients in a balanced equation represent volume ratios for gases.

Gas Stoichiometry Solving Volume to Volume Problems
Volume of a gas is DIRECTLY PROPORTIONAL to the number of moles. Since volume is dependant on temperature, pressure, and moles, and NOT TYPE OF GAS, then a VOLUME RATIO can be used INSTEAD of a mole ratio.

Gas Stoichiometry 3 H2 (g) + N2 (g)  2 NH3 (g)
If 2.0 L of hydrogen reacts with excess nitrogen, calculate the volume of ammonia produced in L at 20 deg C and kPa.

Gas Stoichiometry Solving other types of Gas Stoichiometry Problems
Unless Volume - Volume, you must always use a MOLE RATIO. YOU GOTTA BE IN MOLES TO USE THAT MOLE RATIO! GOT MOLES?

Gas Stoichiometry When given the volume of a gas, use the ideal gas law to find MOLES of the gas. 2 Cu2S + 3 O2 2 Cu2O + 2 SO2 What mass of copper (I) sulfide is required to react completely with 46.6 L of Oxygen at 25°C and kPa?

Gas Stoichiometry When given an amount other than volume of a gas and asked to SOLVE for volume of a gas, first solve for MOLES of the gas (using stoichiometry relationships), then use ideal gas law to solve for volume.

Gas Stoichiometry CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
What volume of oxygen gas at 29°C at 1.06 atm is needed for the combustion of 6.25 g of methane?

Gas Stoichiometry When you are at STP, you can use the molar volume of a gas and avoid using the ideal gas law altogether! 2 H2O (l)  2 H2 (g) + O2 (g) If 25.0g of water is decomposed, what volume of oxygen gas is produced at STP? Remember! Molar volume of any gas at STP = 22.4 L/mol!