Chapter 11

Chapter 11 Section 1

Class Opener: Define/describe pressure. How do you measure pressure? Why would you want to measure the pressure?

Pressure Pressure (P) is defined as the force per unit area on a surface. P = F/A In SI, pressure is expressed in pascals (Pa). One pascal is a very small unit of pressure, so in many cases, it is more convenient to express pressure in kilopascals (kPa).

Relationship Between Pressure, Force, and Area

Gas Pressures Gas pressure is caused by collisions of the gas molecules with each other and with surfaces with which they come into contact. The pressure exerted by a gas depends on volume, temperature, and the number of molecules present. The greater the number of collisions of gas molecules, the higher the pressure will be. 6

Measuring Pressure A barometer is a device used to measure atmospheric pressure. Invented by Torricelli in 1643 Tube containing column of mercury supported by pressure of air. Pressure is directly related to the height of the mercury. Commonly measured as millimeters of mercury (mm Hg).

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Atmospheric Pressure Explain why the atmospheric pressure is less at the top of the mountain compared to being at sea-level.

Other Units of Pressure Atmosphere (atm) 1 atm defined to be the pressure at sea level when temperature is 0°C. 1 atm = 760 mm Hg Torr (torr) 1 torr represents the same amount of pressure as 1 mm Hg 1 atm = 760 mm Hg = 760 torr = 101.325 kPa

What Do You Think? The average atmospheric pressure in Denver, Colorado is 0.830 atm. Express this pressure in millimeters of mercury (mm Hg) kilopascals (kPa) Millipascals

Gas Mixtures and Partial Pressures First studied by John Dalton in 1803. Each gas in mixture exerts a pressure called a partial pressure. Dalton’s Law – For a mixture of gases in a container, the total pressure exerted is the sum of the pressures that each gas would exert if it were alone. PTOTAL = P1 + P2 + P3 + …

Sample Problem A mixture of three gases – He, CO2, and NH3 – is at a total pressure of 6.11 atm. The partial pressure of the He is 1.68 atm and the partial pressure of the CO2 is 3.89 atm. What is the partial pressure of gas NH3? Answer in pascals and mmHg

Outcome Sentences Sentence Starters After reflecting on today’s lesson, complete three of the sentence starters. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… 14

Chapter 16 Section 2

Class Opener: Convert the following to atm / is this pressure above or below sea level? 5.87 * 10-6 torrs 7.35 * 109 pascals Three of the primary components of air are carbon dioxide, nitrogen, and oxygen. In a sample containing a mixture of only these gases at exactly 1atm, the partial pressures of carbon dioxide and nitrogen anre given as PCO2 = 0.285mmHg and PN2 = 593.525mmhg. What is the partial pressure of oxygen in atm?

Boyle’s Law: Pressure-Volume Relationship Boyle’s Law states that the volume of a gas varies inversely with the pressure at a constant temperature. Plotting the values of volume versus pressure for a gas gives a curve like that shown at right.

Boyle’s Law: Pressure-Volume Relationship Boyle’s law can also be expressed mathematically as: P1V1 = P2V2 P1 and V1 represent original conditions, and P2 and V2 represent new set of conditions.

Sample Problem A sample of helium gas collected at 750 mm Hg occupies a volume of 250 mL. What pressure does the gas exert if the volume increases to 300 mL?

Charles’s Law: Volume-Temperature Relationship Charles’s law states that the volume of a gas varies directly with the Kelvin temperature at a constant pressure. (K = °C + 273) Liquid nitrogen on a balloon Gas volume and Kelvin temperature are directly proportional to each other as shown at right.

Charles’s Law: Volume-Temperature Relationship Charles’s law can be expressed mathematically as: V1 and T1 represent original conditions, and V2 and T2 represent new set of conditions. In order to use this equation the temperature must be in Kelvin.

Sample Problem A sample of neon gas occupies a volume of 752 mL at 25°C. What volume will the gas occupy at 50°C if the pressure remains constant?

Gay-Lussac’s Law: Pressure-Temperature Relationship Gay-Lussac’s law states that the pressure of a gas varies directly with the Kelvin temperature at a constant volume.

Gay-Lussac’s Law: Pressure-Temperature Relationship Gay-Lussac’s law can be mathematically expressed as: P1 and T1 represent original conditions, and P2 and T2 represent new set of conditions. Temperatures must be in Kelvin to use this equation.

Sample Problem The gas in a container is at a pressure of 3.00 atm at 25°C. Directions on the container warn the user not to keep it in a place where the temperature exceeds 52°C. What would the gas pressure in the container be at 52°C?

Outcome Sentences Sentence Starters After reflecting on today’s lesson, complete three of the sentence starters. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… 26

Class Opener: How are the volume and pressure of a gas at constant temperature related? Solve P1= 350.0torr, V1 = 200.0mL, P2 = 700.0torr, V2 = ? T1= -33°C, V2 = 54.0mL, T2 = 160.0°C, V1=? P1= .625atm, T1=27°C, P2= 1.125atm, T2=? A gas has a volume of 450.0mL. If the termpture is held constant, what volume would the gas occupy if the pressure were Doubled? Reduced to one fourth the original value?

The Combined Gas Law Boyle’s law, Charles’s law, and Gay-Lussac’s law can be combined into a single equation that can be used for situations in which temperature, pressure, and volume, all vary at the same time. The combined gas law can be written as:

Standard Temperature and Pressure As a reference point used for comparing gases, scientists have agreed on standard conditions. Standard Temperature – 0°C or 273 K Standard Pressure – 1 atm These conditions are commonly abbreviated as STP.

Sample Problem A helium-filled balloon has a volume of 50.0 L at 25°C and 162 kPa. What volume will it have at STP?

Relationship Between Volume and Particles Studied by Italian scientist named Amadeo Avogadro. He noted that under constant temperature and pressure, the volume of a gas is directly related to the number of particles (moles) of gas. This is known as Avogadro’s law. From his observations, he noticed that equal volumes of gases at the same temperature and pressure contain the same number of particles.

The volume occupied by one mole of gas at STP is known as the standard molar volume of a gas, which is 22.4 L.

Problems Involving Standard Molar Volume What volume does 0.0685 mol of He gas occupy at STP? What quantity of gas, in moles, is contained in 6.41 L of CO2 at STP?

Outcome Sentences Sentence Starters After reflecting on today’s lesson, complete three of the sentence starters. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… 34

Chapter 16 Section 3

Class Opener: 1. The reaction shown represents the oxidation of ammonia (NH3). 4 NH3 + 5 O2  4NO + 6 H2O How many moles of water will be formed from 34 grams of ammonia? 2. What volume, in liters, will 75.4 grams of O2 occupy at STP?

The Ideal Gas Law PV = nRT L atm Ideal gas law: mathematical relationship among pressure, volume, temperature, and number of moles of a gas. PV = nRT R is known as the ideal gas constant. L atm R = 0.0821 --------------- mol K Be sure to match the units of the known quantities and the units of R.

Sample Problems What is the pressure exerted by a 0.475 mol sample of nitrogen gas in a 10.0 L container at 25°? Calculate the mass of 675 mL of NH3 at 725 mm Hg at 27°C.

Comparing the Volumes of Reacting Gases In the early 1800s, Gay-Lussac observed that when gases reacted, they did so in simple and definite volume proportions. Example: hydrogen gas + oxygen gas → water vapor 2 L (2 volumes) 1 L (1 volume) 2 L (2 volumes) Gay-Lussac’s law of combining volumes of gases states that at constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers.

2 volumes 1 volume 2 volumes The coefficients in chemical equations of gas reactions reflect not only molar ratios, but also volume ratios. Example: 2CO(g) + O2(g) → 2CO2(g) 2 mole 1 mole 2 mol 2 volumes 1 volume 2 volumes

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) Sample Problem The complete combustion of propane, C3H8, occurs according to the following balanced equation. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) How many liters of oxygen (O2) will be required for the complete combustion of 0.350 L of propane?

Gas Stoichiometry N2(g) + 3H2(g) → 2 NH3(g) The industrial production of ammonia (NH3) proceeds according to the following equation: N2(g) + 3H2(g) → 2 NH3(g) If 20.0 mol of nitrogen (N2) is available, how many liters of NH3 can be produced at STP?

EOC Sample Question The equation represents the breakdown of potassium chlorate (KClO3). 2 KClO3  2 KCl + 3 O2 What volume of oxygen gas (O2) does 20.0 grams of potassium chlorate (KClO3) produce at STP based on the equation shown? A. 5.48 L B. 7.80 L C. 67.3 L D. 72.9 L

Outcome Sentences Sentence Starters After reflecting on today’s lesson, complete three of the sentence starters. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… 44

Chapter 16 Section 4

Class Opener: 1.

Kinetic Molecular Theory So far we have consider the behavior of gases; however, we have not explained why they behave this way. Kinetic Molecular Theory – model used to explain the properties of solids, liquids, and gases in terms of the energy of particles and the forces that act between them. The KMT is based on the idea that particles of matter are always in motion. Important to realize this is an idea and can never be proved absolutely true.

The kinetic-molecular theory of gases is based on the five postulates: Gases consist of large numbers of tiny particles that are far apart relative to their size. Most of the volume occupied by a gas is empty space Collisions between gas particles and between particles and container walls are elastic collisions. An elastic collision is one in which there is no net loss of total kinetic energy.

Gas particles are in continuous, rapid, random motion Gas particles are in continuous, rapid, random motion. They therefore possess kinetic energy. There are no forces of attraction between gas particles. The temperature of a gas depends on the average kinetic energy of the particles of the gas. The kinetic energy of any moving object is given by the following equation:

All gases at the same temperature have the same average kinetic energy. At the same temperature, lighter gas particles, have higher average speeds than do heavier gas particles. Hydrogen molecules will have a higher speed than oxygen molecules; however, both possess the same kinetic energy

Properties of Gases Expansion Gases completely fill any container in which they are enclosed. Gas particles move rapidly in all directions without significant attraction between them. Fluidity Because liquids and gases flow, they are both referred to as fluids. Because the attractive forces between gas particles are insignificant, gas particles glide easily past one another.

Low Density The density of a gaseous substance is about 1/1000 the density of the same substance in the liquid or solid state. The reason is that the particles are so much farther apart in the gaseous state. Compressibility During compression, the gas particles, which are initially very far apart, are crowded closer together.

Diffusion Gases spread out and mix with one another, even without being stirred. Such spontaneous mixing of the particles of two substances caused by their random motion is called diffusion. The random and continuous motion of the gas molecules carries them throughout the available space.

Deviations of Real Gases from Ideal Behavior An ideal gas is a hypothetical gas that perfectly fits all the assumptions of the kinetic-molecular theory. A real gas is a gas that does not behave completely according to the assumptions of the kinetic-molecular theory. Under most normal conditions, real gases behave very similar to those of ideal gases. However, at very high pressures and low temperatures, real gases deviate significantly from behavior of ideal gases.

Outcome Sentences Sentence Starters After reflecting on today’s lesson, complete three of the sentence starters on your note card and hand it to me as you leave today. Sentence Starters I’ve learned… I was surprised… I’m beginning to wonder… I would conclude… I now realize that… 55