General chemistry Chapter 3_may 9_2023.pptx

General chemistry Chapter 3

1. 1 Adama Science and Technology University Department of Applied Chemistry School of Applied Natural Sciences General Chemistry (Chem1101 ) Chapter 3: Kinetic Molecular Description of the state of Matter

2. 2 A gas is a substance that is normally in the gaseous state at ordinary temperatures and pressures; A vapor is the gaseous form of any substance that is a liquid or a solid at normal temperatures and pressures. Thus , at 25°C and 1 atm pressure, we speak of water vapor and oxygen gas. Substances That Exist as Gases We live at the bottom of an ocean of air whose composition by volume is roughly 78 percent N 2 , 21 percent O 2 , and 1 percent other gases, including CO 2 . Only 11 elements are gases under normal atmospheric conditions . Of the gases listed in Table 5.1, only O 2 is essential for our survival. Hydrogen cyanide (HCN) is a deadly poison. Carbon monoxide (CO), hydrogen sulfide (H 2 S), nitrogen dioxide (NO 2 ), O 3 , and sulfur dioxide (SO 2 ) are somewhat less toxic. The gases He and Ne are chemically inert; that is, they do not react with any other substance. Most gases are colorless. Exceptions are F 2 , Cl 2 , and NO 2 . The dark-brown color of NO 2 is sometimes visible in polluted air. NO 2 gas

3. 3 All gases have the following physical characteristics : • Gases assume the volume and shape of their containers . • Gases are the most compressible of the states of matter . • Gases will mix evenly and completely when confined to the same container . • Gases have much lower densities than liquids and solids

4. 4 Pressure of a Gas Gases exert pressure on any surface with which they come in contact, because gas molecules are constantly in motion . Pressure is one of the most readily measurable properties of a gas. To understand how we measure the pressure of a gas, it is helpful to know how the units of measurement are derived . Finally, we define pressure as force applied per unit area : The SI unit of pressure is the pascal (Pa), defined as one newton per square meter: Atmospheric Pressure It is the pressure exerted by Earth’s atmosphere (Figure 5.1 ). The actual value of atmospheric pressure depends on location, temperature, and weather conditions. How is atmospheric pressure measured? The barometer is probably the most familiar instrument for measuring atmospheric pressure. A simple barometer consists of a long glass tube, closed at one end and filled with mercury (see Figure 5.2). 1 torr = 1 mmHg 1 atm = 760 mmHg = 760 torr = 101,325 Pa Q) The pressure outside a jet plane flying at high altitude falls considerably below standard atmospheric pressure. Therefore, the air inside the cabin must be pressurized to protect the passengers. What is the pressure in atmospheres in the cabin if the barometer reading is 672 mmHg? Q) Practice Exercise: Convert 749 mmHg to atmospheres.

5. 5 A manometer is a device used to measure the pressure of gases other than the atmosphere. The principle of operation of a manometer is similar to that of a barometer. There are two types of manometers , The closed-tube manometer ( Figure 5.3(a )]) The open-tube manometer (Figure 5.3(b)) Figure 5.3 Two types of manometers used to measure gas pressures. ( a) Gas pressure is less than atmospheric pressure . ( b) Gas pressure is greater than atmospheric pressure .

6. 6 The Gas Laws The Pressure-Volume Relationship: Boyle’s Law In Figure 5.4(a) the pressure exerted on the gas by the mercury added to the tube is equal to atmospheric pressure . In Figure 5.4(b) an increase in pressure due to the addition of more mercury results in a decrease in the volume of the gas and in unequal levels of mercury in the tube. The mathematical expression showing an inverse relationship between pressure and volume is

7. 7

8. 8 As T increases V increases The Temperature-Volume Relationship: Charles’s Law

9. 9 Q) A sample of chlorine gas occupies a volume of 946 mL at a pressure of 726 mmHg. What is the pressure of the gas (in mmHg) if the volume is reduced at constant temperature to 154 mL? P 1 x V 1 = P 2 x V 2 P 1 = 726 mmHg V 1 = 946 mL P 2 = ? V 2 = 154 mL P 2 = P 1 x V 1 V 2 726 mmHg x 946 mL 154 mL = = 4460 mmHg P x V = constant

10. 10 Variation of Gas Volume with Temperature at Constant Pressure V a T V = constant x T V 1 / T 1 = V 2 / T 2 T (K) = T ( C) + 273.15 Charles’ Law Temperature must be in Kelvin

11. 11 Q) A sample of carbon monoxide gas occupies 3.20 L at 125 C. At what temperature will the gas occupy a volume of 1.54 L if the pressure remains constant? V 1 = 3.20 L T 1 = 398.15 K V 2 = 1.54 L T 2 = ? T 2 = V 2 x T 1 V 1 1.54 L x 398.15 K 3.20 L = = 192 K V 1 / T 1 = V 2 / T 2 T 1 = 125 ( C) + 273.15 (K) = 398.15 K

12. 12 Avogadro’s Law V a number of moles ( n ) V = constant x n V 1 / n 1 = V 2 / n 2 Constant temperature Constant pressure

13. 13 Q) Ammonia burns in oxygen to form nitric oxide (NO) and water vapor. How many volumes of NO are obtained from one volume of ammonia at the same temperature and pressure? 4NH 3 + 5O 2 4NO + 6H 2 O 1 mole NH 3 1 mole NO At constant T and P 1 volume NH 3 1 volume NO

14. 14 Ideal Gas Equation Charles’ law: V a T (at constant n and P ) Avogadro’s law: V a n (at constant P and T ) Boyle’s law: V a ( at constant n and T ) 1 P V α nT P V = constant x = R nT P nT P R is the gas constant PV = nRT

15. 15 The conditions 0 C and 1 atm are called standard temperature and pressure (STP). PV = nRT R = PV nT = (1 atm)(22.414L) (1 mol )(273.15 K) R = 0.082057 L • atm / (mol • K) Experiments show that at STP, 1 mole of an ideal gas occupies 22.414 L.

16. 16 Q) What is the volume (in liters) occupied by 49.8 g of HCl at STP? PV = nRT V = nRT P T = 0 C = 273.15 K P = 1 atm n = 49.8 g x 1 mol HCl 36.45 g HCl = 1.37 mol V = 1 atm 1.37 mol x 0.0821 x 273.15 K L • atm mol • K V = 30.7 L

17. 17 Q) Argon is an inert gas used in lightbulbs to retard the vaporization of the filament. A certain lightbulb containing argon at 1.20 atm and 18 C is heated to 85 C at constant volume. What is the final pressure of argon in the lightbulb (in atm )? PV = nRT n, V and R are constant nR V = P T = constant P 1 T 1 P 2 T 2 = P 1 = 1.20 atm T 1 = 291 K P 2 = ? T 2 = 358 K P 2 = P 1 x T 2 T 1 = 1.20 atm x 358 K 291 K = 1.48 atm

18. 18 Density ( d ) Calculations d = m V = PM RT m is the mass of the gas in g M is the molar mass of the gas Molar Mass ( M ) of a Gaseous Substance dRT P M = d is the density of the gas in g/L

19. 19 Q) A 2.10-L vessel contains 4.65 g of a gas at 1.00 atm and 27.0 C. What is the molar mass of the gas? dRT P M = d = m V 4.65 g 2.10 L = = 2.21 g L M = 2.21 g L 1 atm x 0.0821 x 300.15 K L • atm mol • K M = 54.5 g/mol

20. 20 Gas Stoichiometry Q) What is the volume of CO 2 produced at 37 C and 1.00 atm when 5.60 g of glucose are used up in the reaction: C 6 H 12 O 6 ( s ) + 6O 2 ( g ) 6CO 2 ( g ) + 6H 2 O ( l ) g C 6 H 12 O 6 mol C 6 H 12 O 6 mol CO 2 V CO 2 5.60 g C 6 H 12 O 6 1 mol C 6 H 12 O 6 180 g C 6 H 12 O 6 x 6 mol CO 2 1 mol C 6 H 12 O 6 x = 0.187 mol CO 2 V = nRT P 0.187 mol x 0.0821 x 310.15 K L • atm mol • K 1.00 atm = = 4.76 L

21. 21 Dalton’s Law of Partial Pressures V and T are constant P 1 P 2 P total = P 1 + P 2

22. 22 Consider a case in which two gases, A and B , are in a container of volume V. P A = n A RT V P B = n B RT V n A is the number of moles of A n B is the number of moles of B P T = P A + P B X A = n A n A + n B X B = n B n A + n B P A = X A P T P B = X B P T P i = X i P T mole fraction (X i ) = n i n T

23. 23 Q) A sample of natural gas contains 8.24 moles of CH 4 , 0.421 moles of C 2 H 6 , and 0.116 moles of C 3 H 8 . If the total pressure of the gases is 1.37 atm , what is the partial pressure of propane (C 3 H 8 )? P i = X i P T X propane = 0.116 8.24 + 0.421 + 0.116 P T = 1.37 atm = 0.0132 P propane = 0.0132 x 1.37 atm = 0.0181 atm

24. 24 Kinetic Molecular Theory of Gases A gas is composed of molecules that are separated from each other by distances far greater than their own dimensions. The molecules can be considered to be points ; that is, they possess mass but have negligible volume. Gas molecules are in constant motion in random directions, and they frequently collide with one another. Collisions among molecules are perfectly elastic. Gas molecules exert neither attractive nor repulsive forces on one another. The average kinetic energy of the molecules is proportional to the temperature of the gas in kelvins. Any two gases at the same temperature will have the same average kinetic energy KE = ½ mu 2

25. 25 Gas diffusion is the gradual mixing of molecules of one gas with molecules of another by virtue of their kinetic properties. NH 3 17 g/mol HCl 36 g/mol NH 4 Cl r 1 r 2 M 2 M 1  = molecular path

26. 26 Deviations from Ideal Behavior 1 mole of ideal gas PV = nRT n = PV RT = 1.0 Repulsive Forces Attractive Forces

27. 27 Van der Waals equation non-ideal gas P + ( V – nb ) = nRT an 2 V 2 ) } corrected pressure } corrected volume Effect of intermolecular forces on the pressure exerted by a gas.

28. 28 Liquids and Solids Liquids and solids are quite a different story. Molecules in a liquid are held together by one or more types of attractive forces , A liquid also has a definite volume , A liquid can flow , can be poured, and assumes the shape of its container. In a solid, molecules are held rigidly in position with virtually no freedom of motion. The molecules are arranged in regular configurations in three dimensions. There is even less empty space in a solid than in a liquid. Solids are almost incompressible and possess definite shape and volume.

29. 29 Intermolecular Forces Intermolecular forces are attractive forces between molecules. They exert even more influence in the condensed phases of matter-liquids and solids In contrast to intermolecular forces, intramolecular forces hold atoms together in a molecule. Generally, intermolecular forces are much weaker than intramolecular forces . Molecules that are held together by stronger intermolecular forces, then its boiling point is higher. Types of intermolecular forces 1) Dipole-Dipole Forces Dipole-dipole forces are attractive forces between polar molecules, that is, between molecules that possess dipole moments . 2) Ion-Dipole Forces Coulomb’s law also explains ion-dipole forces, which attract an ion (either a cation or an anion) and a polar molecule to each other 3) Dispersion Forces Dispersion forces, which are also called London forces What attractive interaction occurs in nonpolar substances? 4) The Hydrogen Bond (NH 3 , H 2 O, and HF)

30. 30 Properties of Liquids (a) Surface Tension A measure of the elastic force in the surface of a liquid is surface tension. The surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area (for example, by 1 cm 2 ). Liquids that have strong intermolecular forces also have high surface tensions. Thus, because of hydrogen bonding, water has a considerably greater surface tension than most other liquids. Intermolecular forces acting on a molecule in the surface layer of a liquid and in the interior region of the liquid. Another example of surface tension is capillary action. Two types of forces bring about capillary action. One is cohesion , which is the intermolecular attraction between like molecules ( in this case, the water molecules). The second force, called adhesion , is an attraction between unlike molecules, such as those in water and in the sides of a glass tube. Surface tension enables the water strider to “walk” on water.

31. 31 (a) Surface Tension ( a) When adhesion is greater than cohesion, the liquid ( for example , water) rises in the capillary tube. (b) When cohesion is greater than adhesion, as it is for mercury, a depression of the liquid in the capillary tube results. Note that the meniscus in the tube of water is concave, or rounded downward, whereas that in the tube of mercury is convex, or rounded upward. (a) (b)

32. 32 (b) Viscosity Viscosity is a measure of a fluid’s resistance to flow. The greater the viscosity, the more slowly the liquid flows . The viscosity of a liquid usually decreases as temperature increases; thus, hot molasses flows much faster than cold molasses. Liquids that have strong intermolecular forces have higher viscosities than those that have weak intermolecular forces Water beads on an apple , which has a waxy surface.

33. 33 Crystal Structure Solids can be divided into two categories: crystalline and amorphous. The structure and properties of crystalline solids, such as melting point, density, and hardness , are determined by the attractive forces that hold the particles together.

34. 34 Phase Changes Phase changes, transformations from one phase to another , occur when energy (usually in the form of heat) is added or removed from a substance. Phase changes are physical changes that are characterized by changes in molecular order; molecules in the solid state have the most order, and those in the gas phase have the greatest randomness.

35. 35 Phase Diagrams The overall relationships among the solid, liquid, and vapor phases are best represented in a single graph known as a phase diagram. A phase diagram summarizes the conditions under which a substance exists as a solid, liquid, or gas. The phase diagram of water . Each solid line between two phases specifies the conditions of pressure and temperature under which the two phases can exist in equilibrium. The point at which all three phases can exist in equilibrium (0.006 atm and 0.01°C) is called the triple point .

36. 36 Solutions Solutions There are many types of solutions; the most common is the liquid solution in which the solvent is a liquid and the solute is a solid or a liquid. Molecules that possess similar types of intermolecular forces readily mix with each other. Solubility is a quantitative measure of the amount of a solute dissolved in a solvent at a specific temperature (like-dissolve-like is Miscible ) Solvation is the process in which an ion or a molecule is surrounded by solvent molecules arranged in a specific manner

37. 37 Solutions A solution that contains the maximum amount of a solute in a given solvent, at a specific temperature , is called a saturated solution . Before the saturation point is reached, the solution is said to be unsaturated ; it contains less solute than it has the capacity to dissolve . A third type, a supersaturated solution , contains more solute than is present in a saturated solution. Supersaturated solutions are not very stable. In time, some of the solute will come out of a supersaturated solution as crystals. The process in which dissolved solute comes out of solution and forms crystals is called crystallization . In a supersaturated sodium acetate solution (left), sodium acetate crystals rapidly form when a small seed crystal is added .

38. 38 A Molecular View of the Solution Process In liquids and solids , molecules are held together by intermolecular attractions . These forces also play a central role in the formation of solutions. Three types of interactions : Solvent-solvent interaction Solute-solute interaction Solvent-solute interaction separation of solvent molecules separation of solute molecules Step 1 & 2 require energy input to break attractive intermolecular forces; therefore, They are endothermic . In step 3 the solvent and solute molecules mix. This step may be exothermic or endothermic. The heat of solution ΔH soln is given by If the solute-solvent attraction is stronger than the solvent-solvent and solute-solute attractions, the solution process is favorable; that is, it is exothermic (ΔH soln < 0). If the solute-solvent interaction is weaker than the solvent-solvent and solute-solute interactions, the solution process is endothermic ( ΔH soln >0 ).

39. 39 Types of Concentration Units End of Chapter 3