Chapter Eight Gases, Liquids, and Solids

8/27/2015 Chapter Eight 2 Outline ►8.1 States of Matter and Their Changes ►8.2 Gases and the Kinetic–Molecular Theory ►8.3 Pressure ►8.4 Boyle’s Law: The Relation between Volume and Pressure ►8.5 Charles’s Law: The Relation between Volume and Temperature ►8.6 Gay-Lussac’s Law: The Relation between Pressure and Temperature ►8.7 The Combined Gas Law ►8.8 Avogadro’s Law: The Relation between Volume and Molar Amount ►8.9 The Ideal Gas Law ►8.10 Partial Pressure and Dalton’s Law (omit) ►8.11 Intermolecular Forces ►8.12 Liquids (only the heating curve) ►8.13 Water: A Unique Liquid (omit) ►8.14 Solids (omit) ►8.15 Changes of State

8/27/2015 Chapter Eight 3 Goals ►1.How do scientists explain the behavior of gases? Be able to state the assumptions of the kinetic–molecular theory of gases and use these assumptions to explain the behavior of gases. ►2.How do gases respond to changes in temperature, pressure, and volume? Be able to use Boyle’s law, Charles’s law, Gay-Lussac’s law, and Avogadro’s law to explain the effect on gases of a change in pressure, volume, or temperature. ►3.What is the ideal gas law? Be able to use the ideal gas law to find the pressure, volume, temperature, or molar amount of a gas sample. ►4.What is partial pressure? Be able to define partial pressure and use Dalton’s law of partial pressures.

8/27/2015 Chapter Eight 4 Goals Contd. ►5.What are the major intermolecular forces, and how do they affect the states of matter? Be able to explain dipole–dipole forces, London dispersion forces, and hydrogen bonding, and recognize which of these forces affect a given molecule. ►6.What are the various kinds of solids, and how do they differ? Be able to recognize the different kinds of solids and describe their characteristics. ►7.What factors affect a change of state? Be able to apply the concepts of heat change, equilibrium, and vapor pressure to changes of state.

8/27/2015 Chapter Eight States of Matter and Their Changes ►Matter exists in any of three phases, or states—solid, liquid, and gas, depending on the attractive forces between particles, temperature and pressure. ►In a gas, the attractive forces between particles are very weak compared to their kinetic energy, so the particles move about freely, are far apart, and have almost no influence on one another. ►In a liquid, the attractive forces between particles are stronger, pulling the particles close together but still allowing them considerable freedom to move about.

8/27/2015 Chapter Eight 6 In a solid, the attractive forces are much stronger than the kinetic energy of the particles, so the atoms, molecules, or ions are held in a specific arrangement and can only wiggle around in place.

8/27/2015 Chapter Eight 7 ►Phase change or change of state: The transformation of a substance from one state to another. ►Melting point (mp): The temperature at which solid and liquid are in equilibrium. ►Boiling point (bp): The temperature at which liquid and gas are in equilibrium. ►Sublimation: A process in which a solid changes directly to a gas. ►Melting, boiling, and sublimation all have  H>0, and  S>0. This means they are nonspontaneous below and spontaneous above a certain temperature.

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8/27/2015 Chapter Eight Gases and the Kinetic-Molecular Theory ►The behavior of gases can be explained by a group of assumptions known as the kinetic–molecular theory of gases. The following assumptions account for the observable properties of gases: ►A gas consists of many particles, either atoms or molecules, moving about at random with no attractive forces between them. Because of this random motion, different gases mix together quickly.

8/27/2015 Chapter Eight 10 ►The amount of space occupied by the gas particles themselves is much smaller than the amount of space between particles. Most of the volume taken up by gases is empty space, accounting for the ease of compression and low densities of gases. ►The average kinetic energy of gas particles is proportional to the Kelvin temperature. Thus, gas particles have more kinetic energy and move faster as the temperature increases. (In fact, gas particles move much faster than you might suspect. The average speed of a helium atom at room temperature and atmospheric pressure is approximately 1.36 km/s, or 3000 mi/hr, nearly that of a rifle bullet.)

8/27/2015 Chapter Eight 11 ►Collisions of gas particles, either with other particles or with the wall of their container, are elastic; that is, the total kinetic energy of the particles is constant. The pressure of a gas against the walls of its container is the result of collisions of the gas particles with the walls. The number and force of collisions determines the pressure. ►A gas that obeys all the assumptions of the kinetic– molecular theory is called an ideal gas. All gases behave somewhat differently than predicted by the kinetic–molecular theory at very high pressures or very low temperatures. Most real gases display nearly ideal behavior under normal conditions.

8/27/2015 Chapter Eight Pressure ►Pressure (P) is defined as a force (F) per unit area (A) pushing against a surface; P = F/A. ►A barometer measures pressure as the height of a mercury column. Atmospheric pressure presses down on mercury in a dish and pushes it up a tube. ►Pressure units: 1 atm = 760 mm Hg = 14.7 psi = 101,325 Pa 1 mm Hg = 1 torr = Pa

8/27/2015 Chapter Eight 13 Gas pressure inside a container is often measured using an open-end manometer, a simple instrument similar in principle to the mercury barometer.

8/27/2015 Chapter Eight Boyle’s Law: The Relation Between Volume and Pressure ►Boyle’s law: The volume of a gas is inversely proportional to its pressure for a fixed amount of gas at a constant temperature. That is, P times V is constant when the amount of gas n and the temperature T are kept constant. ►V  1/P or PV = k if n & T are constant ►If: P 1 V 1 = k and P 2 V 2 = k ►Then: P 1 V 1 = P 2 V 2

8/27/2015 Chapter Eight 15 The volume of a gas decreases proportionately as its pressure increases. If the pressure of a gas sample is doubled, the volume is halved.

8/27/2015 Chapter Eight Charles’ Law: The Relation Between Volume and Temperature ►Charles’s law: The volume of a gas is directly proportional to its Kelvin temperature for a fixed amount of gas at a constant pressure. That is, V divided by T is constant when n and P are held constant. ►V  T or V/T = k if n & P are constant ►If: V 1 /T 1 = k and V 2 /T 2 = k ►Then: V 1 /T 1 = V 2 /T 2

8/27/2015 Chapter Eight 17 If the Kelvin temperature of a gas is doubled, its volume doubles.

8/27/2015 Chapter Eight Gay-Lussac’s Law: The Relation Between Pressure and Temperature ►Gay-Lussac’s law: The pressure of a gas is directly proportional to its Kelvin temperature for a fixed amount of gas at a constant volume. That is, P divided by T is constant when n and V are held constant. ►P  T or P/T = k if n & V are constant ►If: P 1 /T 1 = k and P 2 /T 2 = k ►Then: P 1 /T 1 = P 2 /T 2

8/27/2015 Chapter Eight The Combined Gas Law ►Since PV, V/T, and P/T all have constant values for a fixed amount of gas, these relationships can be merged into a combined gas law for a fixed amount of gas. ►Combined gas law: PV/T = k if n constant ►P 1 V 1 /T 1 = P 2 V 2 /T 2 ►If any five of the six quantities in this equation are known, the sixth can be calculated.

8/27/2015 Chapter Eight Avogadro’s law: The Relation Between Volume and Molar Amount ►Avogadro’s law: The volume of a gas is directly proportional to its molar amount at a constant pressure and temperature. That is, V divided by n is constant when P and T are held constant. ►V  n or V/n = k if P & T are constant ►If: V 1 /n 1 = k and V 2 /n 2 = k ►Then: V 1 /n 1 = V 2 /n 2

8/27/2015 Chapter Eight 21 ►The molar amounts of any two gases with the same volume are the same at a given T & P. ►Standard temperature and pressure: (STP) = 0  C ( K) and 1 atm (760 mm Hg) ►Standard Molar Volume of a gas at STP = 22.4 L/mol

8/27/2015 Chapter Eight The Ideal Gas Law ►Ideal gas law: The relationships among the four variables P, V, T, and n for gases can be combined into a single expression called the ideal gas law. ►PV = nRT ►If the values of three of the four variables in the ideal gas law are known the fourth can be calculated. ►R = L·atm/mol·K

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8/27/2015 Chapter Eight Intermolecular Forces ►Intermolecular force: A force that acts between molecules and holds molecules close to one another. There are three major types of intermolecular forces. ►Dipole–dipole forces are weak, with strengths on the order of 1 kcal/mol ►London dispersion forces are weak, in the range 0.5– 2.5 kcal/mol. They increase with molecular weight and molecular surface area. ►Hydrogen bonds can be quite strong, with energies up to 10 kcal/mol.

8/27/2015 Chapter Eight 25 Dipole–dipole forces: The positive and negative ends of polar molecules are attracted to one another by dipole–dipole forces. As a result, polar molecules have higher boiling points than nonpolar molecules of similar size.

8/27/2015 Chapter Eight 26 ►Only polar molecules experience dipole–dipole forces, but all molecules, regardless of structure, experience London dispersion forces. ►(a) On average, the electron distribution in a nonpolar molecule is symmetrical. (b) At any instant, it may be unsymmetrical, resulting in a temporary polarity that can attract neighboring molecules.

8/27/2015 Chapter Eight 27 A hydrogen bond is an attractive interaction between an unshared electron pair on an electronegative O, N, or F atom and a positively polarized hydrogen atom bonded to another electronegative O, N, or F. Hydrogen bonds occur in both water and ammonia.

8/27/2015 Chapter Eight 28 The boiling points of NH 3, H 2 O, and HF are much higher than the boiling points of their second row neighbor CH 4 and of related third-row compounds due to hydrogen bonding.

8/27/2015 Chapter Eight Liquids ►Molecules are in constant motion in the liquid state. If a molecule happens to be near the surface of a liquid, and if it has enough energy, it can break free of the liquid and escape into a state called vapor. ►Once molecules have escaped from the liquid into the gas state, they are subject to all the gas laws. The gas molecules make their own contribution to the total pressure of the gas above the liquid according to Dalton’s law. We call this contribution the vapor pressure of the liquid.

8/27/2015 Chapter Eight 30 ►Vapor pressure rises with increasing temperature until ultimately it becomes equal to the pressure of the atmosphere. At this point, bubbles of vapor form under the surface and force their way to the top, this is called boiling. ►At a pressure of exactly 760 mm Hg, boiling occurs at what is called the normal boiling point. ►If atmospheric pressure is higher or lower than normal, the boiling point of a liquid changes accordingly. At high altitudes, for example, atmospheric pressure is lower than at sea level, and boiling points are also lower.

8/27/2015 Chapter Eight 31 At a liquid’s boiling point, its vapor pressure is equal to atmospheric pressure. Commonly reported boiling points are those at 760 mm Hg.

8/27/2015 Chapter Eight Changes of State ►When a substance changes state, energy added is used to overcome attractive forces instead of increasing kinetic energy so temperature does not change. ►Heat of fusion: The quantity of heat required to completely melt a substance once it has reached its melting point. ►Heat of vaporization: The quantity of heat required to completely vaporize a substance once it has reached its boiling point.

8/27/2015 Chapter Eight 33 A heating curve for water, showing the temperature and state changes that occur when heat is added.

8/27/2015 Chapter Eight 34 Chapter Summary ►According to the kinetic–molecular theory of gases, the behavior of gases can be explained by assuming that they consist of particles moving rapidly at random, separated from other particles by great distances, and colliding without loss of energy. ►Boyle’s law says that the volume of a fixed amount of gas at constant temperature is inversely proportional to its pressure. ► Charles’s law says that the volume of a fixed amount of gas at constant pressure is directly proportional to its Kelvin temperature.

8/27/2015 Chapter Eight 35 Chapter Summary Contd. ►Gay-Lussac’s law says that the pressure of a fixed amount of gas at constant volume is directly proportional to its Kelvin temperature. ►Avogadro’s law says that equal volumes of gases at the same temperature and pressure contain the same number of moles. ►The four gas laws together give the ideal gas law, PV = nRT, which relates the effects of temperature, pressure, volume, and molar amount. ►At 0°C and 1 atm pressure, called standard temperature and pressure (STP), 1 mol of any gas occupies a volume of 22.4 L.

8/27/2015 Chapter Eight 36 Chapter Summary Contd. ►The pressure exerted by an individual gas in a mixture is called the partial pressure. Dalton’s law: the total pressure exerted by a mixture is equal to the sum of the partial pressures of the individual gases. ►There are three major types of intermolecular forces, which act to hold molecules near one another in solids and liquids. Dipole–dipole forces occur between polar molecules. London dispersion forces occur between all molecules as a result of temporary molecular polarities. Hydrogen bonding, the strongest of the three forces, occurs between a hydrogen atom bonded to O, N, or F and a nearby O, N, or F atom.

8/27/2015 Chapter Eight 37 Chapter Summary Contd. ►Crystalline solids are those whose constituent particles have an ordered arrangement; amorphous solids lack internal order. There are several kinds of crystalline solids, ionic solids, molecular solids, covalent network solids, and metallic solids,. ►The amount of heat necessary to melt a given amount of solid at its melting point is its heat of fusion. Molecules escape from the surface of a liquid resulting in a vapor pressure of the liquid. At a liquid’s boiling point, its vapor pressure equals atmospheric pressure. The amount of heat necessary to vaporize a given amount of liquid at its boiling point is called its heat of vaporization.

8/27/2015 Chapter Eight 38 Key Words ►Amorphous solid ►Avogadro’s law ►Boiling point (bp) ►Boyle’s law ►Change of state ►Charles’s law ►Combined gas law ►Crystalline solid ►Dalton’s law ►Dipole–dipole force ►Gas constant (R) ►Gas laws ►Gay-Lussac’s law ►Heat of fusion ►Heat of vaporization

8/27/2015 Chapter Eight 39 Key Words Contd. ►Hydrogen bond ►Ideal gas ►Ideal gas law ►Intermolecular force ►Kinetic–molecular theory of gases (KMT) ►London dispersion force ►Melting point (mp) ►Normal boiling point ►Partial pressure ►Pressure (P) ►Standard temperature and pressure (STP) ►Standard molar volume ►Vapor ►Vapor pressure