Lecture10

Properties of Gases (from Chapter 4) : 

Properties of Gases (from Chapter 4) Gases may be compressed. Gases expand to fill their containers uniformly. All gases have low density. Gases may be mixed. A confined gas exerts constant pressure on the walls of its container uniformly in all directions.

Dalton’s Law of Partial Pressures : 

Dalton’s Law of Partial Pressures The total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases in the mixture. The partial pressure of one gas in a mixture is the pressure that gas would exert if it alone occupied the same volume at the same temperatures. PT = p1 + p2 + p3 + p4 +……

Properties of Gases vs. Properties of Liquids : 

Properties of Gases vs. Properties of Liquids Gases may be compressed. Gases expand to fill their containers Gases have low densities Gases may be mixed in a fixed volume Liquids cannot be compressed Liquids do not expand to fill their containers Liquids have relatively high densities. Volume of a liquid expands when mixed.

Properties of Liquids and Intermolecular Attractions : 

Vapor pressure: Due to evaporation, the open space above any liquid contains particles in the vapor state. The partial pressure exerted by these gaseous particles is called vapor pressure. Vapor pressure is inversely related to the intermolecular attractions. Properties of Liquids and Intermolecular Attractions

Slide 6: 

Fig 15.3, p. 423

Properties of Liquids and Intermolecular Attractions : 

Properties of Liquids and Intermolecular Attractions Molar heat of vaporization: “The energy required to change one mole of a liquid to a gas, while at constant temperature and pressure.” Molar heat of vaporization is directly related to the intermolecular attractions.

Slide 8: 

Energy must be added to a system to overcome the attractive forces that are exerted among liquid molecules. When an equal quantity of vapor condenses to a liquid, an equal amount of energy is released. Fig 15.4, p. 424

Properties of Liquids and Intermolecular Attractions : 

Boiling Point: (a) def: “Temperature at which the vapor pressure of the liquid is equal to the pressure above it’s surface.” (b) This point is reached when “the average kinetic energy of the liquid particles is high enough to overcome the forces of attraction that hold the particles in the liquid state.” The normal boiling point is the boiling temperature at one atmosphere – the temperature at which the vapor pressure is equal to one atmosphere. The boiling point is directly related to the intermolecular attractions. Properties of Liquids and Intermolecular Attractions

Slide 11: 

Viscosity: “the internal resistance to flow.” Viscosity is based partially on the intermolecular attractions. When comparing particles of approx. the same size, generally the higher the intermolecular attractions, the higher the viscosity.

Properties of Liquids and Intermolecular Attractions : 

Surface Tension: “The tendency of a liquid to achieve a minimum surface area (sphere).” Surface area is directly related to the intermolecular attractions. Properties of Liquids and Intermolecular Attractions

Intermolecular Forces : 

Induced Dipole Forces: (aka London dispersion forces, LDF) An instantaneous dipole is created within the atom or molecule via the instantaneous movement of the electrons around the nucleus. All molecules have LDF. The strength of these forces depends on how easily the electron distribution can be distorted (polarized). Large molecules are more polarizable than small molecules Intermolecular Forces

Slide 16: 

Fig. 15-10, p. 427

Intermolecular Forces : 

Intermolecular Forces Dipole-Dipole Forces: (aka Polar molecules) The attractive force between molecules due to the existence of an overall dipole moment. Polar molecules have stronger d-d forces than nonpolar molecules because polar molecules have stronger intermolecular attractions than nonpolar molecules.

Intermolecular Forces : 

Intermolecular Forces Hydrogen Bonding (strongest): The attractive force between a highly electronegative atom of one molecule with the hydrogen on another molecule also containing a very electronegative atom. N, O, F are the electronegative atoms.

Water : 

Water Boiling point Hydrogen bonding High surface tension High heat of vaporization Low vapor pressure Viscous Universal solvent Liquid at room temperature ↑ density until 4°C then density ↓ 9% ↑ in volume as it freezes

Energy & Change of State : 

Energy & Change of State SI unit of energy = Joule (J) The energy required to vaporize a substance, q, is proportional to the amount of substance. This energy is called the heat of vaporization, D Hvap. The process is endothermic, therefore D Hvap is positive. D Hvap = q/m → q = D Hvap x m When vapor condenses into a liquid, the reverse energy change, heat energy is removed from the substance. Known as the heat of condensation. This process is exothermic, therefore D Hcon is negative.

Energy & Change of State : 

To melt a solid, energy must be added to overcome the forces that hold the crystal structure together. This is known as the heat of fusion, D Hfus. The process is endothermic, therefore D Hfus is positive. D Hfus = q/m → q = D Hfus x m D Hfus of a substance is the energy required to melt one gram of that substance. The heat of solidification is the reverse of heat of fusion and is exothermic and D Hsol is negative. Energy & Change of State

Energy & Change of Temperature: Specific Heat : 

Energy & Change of Temperature: Specific Heat The specific heat is the heat flow required to change the temperature of one gram of a substance by one degree Celsius, c. q = m x c x DT Specific heat of water = 4.18 J/g °C = 1 cal/g °C 1 cal = 4.184 J

Slide 26: 

p. 447

Change in Temperature plus Change of State : 

Change in Temperature plus Change of State If you were to take some ice from a freezer, place it in a flask and apply steady heat to it, five things would happen: The ice would warm to its melting point. The ice would melt at the melting point. The water would warm to its boiling point. The water would boil at the boiling point. The steam would become hotter.

Slide 28: 

Fig. 15-32, p. 448

Slide 29: 

How to calculate the total heat flow for a change in temperature plus a change of state Step 1 : Sketch a graph having the shape shown on the next slide. Be sure to mark all relevant starting and ending points. Step 2: Calculate the heat flow, q, for each parts of the graph between all of the starting and ending points. Step 3: Add the heat flows. Be sure the units are the same, either kilojoules or joules, for all numbers being added.

Sample problem for a change in temperature plus a change of state : 

Sample problem for a change in temperature plus a change of state Problem: Calculate the total heat flow when 19.6 grams of ice, initially at -12°C, is heated to steam at 115°C. Required information: Given: m = 19.6 g Ti = -12°C Tf = 115°C Look up the following: Tm = 0°C Tb = 100ºC DHfus = 335 J/g DHvap = 2.26 kJ/g

Slide 31: 

p. 448 1 2 3 4 5

Set up for answers : 

Answer: q1 = 0.485 kJ q2 = 6.53 kJ q3 = 8.19 kJ q4 = 44.3 kJ q5 = 0.59 kJ Sq = q1 + q2 + q3 + q4 + q5 = 60.1 kJ Set up for answers q1 = m x c x DT q2 = m x DHfus q3 = m x c x DT q4 = m x DHvap q5 = m x c x DT

Energy & Change of Temperature: Specific Heat : 

Energy & Change of Temperature: Specific Heat qH2O = -qmetal DT = Tf – Ti mH2O cH2O DTH2O = - mmetal cmetal DTmetal A student places 138 g of an unknown metal at 99.6 C into 60.50 g of water at 22.1C. The entire system reaches a uniform temperature of 31.6 C. Calculate the specific heat of the metal. A student places a 124 g of iron at 104 C into 87.9 mL of water at 23.8 C. What is the final temperature of the system? Heat capacity of Fe = 0.45 J/g °C 0.256 J/g °C 34.4 °C