Chemistry 123: Physical and Organic Chemistry Topic 2: Thermochemistry

Transcription

1 Topic 2: Introduction, Topic 2: Thermochemistry Text: Chapter 7 and 19 (~ 3 weeks) 2.0 Introduction, terminology and scope 2.1 Enthalapy and Energy Change in a chemical process; 1st law of Thermodynamics 2.2 Enthalapy of Formation, Calorimetry, Heat capacity, Bond Energies, and Standard State 2.3 Entropy; 2nd and 3rd laws of thermodynamics 2.4 Free Energy; Spontaneity and Equilibrium. 2.5 Relationship between dh,ds, dg and K 2.6 Change of dg with Temperature adn Concentration; Effect of Temperature on K (van't Hoff) Winter 2009 Page Introduction, terminology and scope A System is the part of the universe under study. The Surroundings is anything outside of the System. An open system allows for exchange of both material and energy A closed system can exchange only energy An isolated system, does not permit the transfer of mass or Energy. Energy is the ability to do work Work occurs when a force acts through a distance. Kinetic Energy is the energy of a moving object. Thermal Energy is the kinetic energy associated with random molecular motion Winter 2009 Page 2 1

2 Kinetic Energy Work e k = w = Fd 1 2 mv 2 [e k ] = kg m s [w ] = kg m s 2 m 2 = J = J Winter 2009 Page 3 Understanding Calorie (cal): The quantity of heat required to change the temperature of one gram of water by one degree Celsius. Joule (J): International System of Units (SI) unit for heat 1 cal = J James Joule ( ) Winter 2009 Page 4 2

3 Heat Capacity The quantity of heat (q) required to change the temperature of a system by one degree. Molar heat capacity. System is one mole of substance. Specific heat capacity, c. System is one gram of substance Heat capacity Mass times c = C specific heat. Water: c = g j -1 C -1 q = m c ΔT q = C ΔT Winter 2009 Page 5 Winter 2009 Page 6 3

4 Conservation of Energy In interactions between a system and its surroundings the total energy remains constant energy is neither created nor destroyed. q system + q surroundings = 0 q system = -q surroundings Winter 2009 Page 7 Determination of Specific Heat Winter 2009 Page 8 4

5 EXAMPLE 7-2 Determining Specific Heat from Experimental Data. Using the data presented on the last slide to calculate the specific heat of lead. Water is the surroundings q lead = -q water q water = mcδt = (50.0 g)(4.184 J/g C)( ) C q water = J q lead = J = mcδt = (150.0 g)(c lead )( ) C c lead = 0.13 Jg -1 C -1 Winter 2009 Page 9 Heats of Reaction and Calorimetry Chemical energy. Contributes to the internal energy of a system. Heat of reaction, q rxn. The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature. Winter 2009 Page 10 5

6 Heats of Reaction and Calorimetry Exothermic reactions. Produces heat, q rxn < 0. Endothermic reactions. Consumes heat, q rxn > 0. Calorimeter A device for measuring quantities of heat. Add water Winter 2009 Page 11 Winter 2009 q rxn < 0 q rxn > 0 Page 12 6

7 q rxn = -q cal q cal = q bomb + q water + q wires + Define the heat capacity of the calorimeter: q cal = Σm i c i ΔT = CΔT heat all i C = heat capacity of the calorimeter Winter 2009 Page 13 Using Bomb Calorimetry Data to Determine a Heat of Reaction. The combustion of g sucrose, in a bomb calorimeter, causes the temperature to rise from to C. The heat capacity of the calorimeter assembly is 4.90 kj/ C. (a) What is the heat of combustion of sucrose, expressed in kj/ mol C 12 H 22 O 11? (b) Verify the claim of sugar producers that one teaspoon of sugar (about 4.8 g) contains only 19 calories. 1 food Calorie ( 1 Calorie with a capital C) is actually 1000 cal or Kcal Winter 2009 Page 14 7

8 Calculate q calorimeter : q cal = CΔT = (4.90 kj/ C)( ) C = (4.90)(3.41) kj = 16.7 kj Calculate q rxn : q rxn = -q cal = kj per g kj q rxn = -q cal = = kj/g g q rxn = kj/g g 1.00 mol = kj/mol (a) Calculate q rxn for one teaspoon: q rxn = (-16.5 kj/g)( 4.8 g 1 tsp 1.00 cal )( )= -19 kcal/tsp J (b) Winter 2009 Page 15 Coffee cup Calorimeter A simple calorimeter. Well insulated and therefore isolated. Measure temperature change. q rxn = -q cal Winter 2009 Page 16 8

9 Winter 2009 Page 17 Winter 2009 Page 18 9

10 Winter 2009 Page 19 Pressure- Volume Work of a reaction In addition to heat effects chemical reactions may also do work. Gas formed pushes against the atmosphere. Volume changes. Pressure-volume work. Winter 2009 Page 20 10

11 Pressure- Volume Work of a reaction Winter 2009 Page 21 Pressure- Volume Work of a reaction Assume an ideal gas and calculate the volume change: V i = nrt/p = (0.100 mol)( L atm mol -1 K -1 )(298K)/(2.40 atm) = 1.02 L V f = 2.04 L ΔV = 2.04 L-1.02 L = 1.02 L Calculating Pressure-Volume Work. Suppose the gas in the previous figure is mol He at 298 K and the each mass in the figure corresponds to an external pressure of 1.20 atm. How much work, in Joules, is associated with its expansion at constant pressure. w = -PΔV = -(1.20 atm)(1.02 L)( = J -101 J ) 1 L atm Winter 2009 Page 22 11