Major Concepts Calorimetry (from last time)

Transcription

1 Major Concepts Calorimetry (from last time) Heat capacity Molar heat capacity (per mole) Specific heat capacity (per mass) Standard state enthalpies: Hº Physical Changes Chemical Changes Hess's Law Balancing chemical reactions & energies Standard state enthalpies: Hº Calculating H s tables of Hº Heats of Reaction and Formation Temperature Dependence in r Hº(T) Kirkhoff s Law 9/9/10 Thermochemistry (lec7) 1

2 Calorimetry is: the science of measuring heat based on observing the temperature change when a body absorbs or discharges energy as heat Calorimeters: Calorimetry Constant-Pressure Calorimeters, e.g., the coffee cup calorimeter Differential Scanning Calorimeters (DSC) The device has an overall heat capacity, and is otherwise insulating 9/9/10 Thermochemistry (lec7) 2

3 Recall: Heat Capacity Heat Capacity, C P, is the amount of heat required to raise the temperature of a substance by one degree at constant pressure. Heat Capacity = C P = where Heat Capacity vs. Molar Heat Capacity: C p = q ΔT c p = C p n = ( units are J ) K q nδt q ΔT = heat absorbed ΔT = dh dt P C P denotes Heat Capacity at constant pressure units are J K mole ( ) ΔT = T f -T i 9/9/10 Thermochemistry (lec7) 3

4 Properties of Heat Capacity C P = dh dt P and c P = C P is always a positive number d( H ) n dt q and ΔT must be both negative or positive If q is negative, then heat is evolved or given off and the temperature decreases If q is positive, then heat is absorbed and the temperature increases Heat flows from High to Low Temperature P 9/9/10 Thermochemistry (lec7) 4

5 Specific Heat Capacity The word Specific before the name of a physical quantity very often means divided by the mass Specific Heat Capacity is the amount of heat required to raise the temperature of one gram of material by one degree Kelvin (at constant pressure) cs = C P m units are JK -1 g -1 9/9/10 Thermochemistry (lec7) 5

6 Calorimetry: Balancing Chemical Equations with Enthalpy Changes (by example) CO (g) + ½ O 2 (g) CO 2 (g) ΔH= -283kJ An exothermic reaction If Quantities doubled (x2), then double ΔH 2CO (g) + 1 O 2 (g) 2 CO 2 (g) ΔH= -566kJ If the original reaction is reversed, then change sign of H CO 2 (g) CO (g) + ½ O 2 (g) ΔH= +283kJ An endothermic reaction 9/9/10 Thermochemistry (lec7) 6

7 Skill: Calculating Heat Changes for a Reaction How much heat energy is needed to decompose 9.74g of HBr(g) (M =80.9 g/mol) into its elements? Given: H 2 (g) + Br 2 (g) 2HBr (g) ΔH = kj mol -1 Strategy: Decomposition is actually the reverse reaction ΔH to decompose 2 moles of HBr is +72.8kJ/mol 9/9/10 Thermochemistry (lec7) 7

8 Skill: Calculating Heat Chances for a Reaction How much heat energy is needed to decompose 9.74g of HBr(g) (M =80.9 g/mol) into its elements? Given: H 2 (g) + Br 2 (g) 2HBr (g) ΔH = kj mol -1 Solution : Decomposition is actually the reverse reaction 2HBr (g) H 2 (g) + Br 2 (g) ΔH = kj mol -1 q = ΔH = 72.8 kj 2 moles HBr = 4.38 kj 9.74 g HBr g HBr mole 9/9/10 Thermochemistry (lec7) 8

9 Standard State Enthalpies: Hº(T) Designated using a superscript (pronounced naught) This is needed because Enthalpy varies with conditions Is written as ΔH ΔH 25 o C or ΔH K If no temperature is indicated in the sub-script, assume 25 C. In this course, the standard pressure is 1 bar. (Warning: in some text books, the standard pressure is 1 atm.) Standard State conditions All concentrations are 1M or 1 mol/l All partial pressures are 1 atm or ΔH 9/9/10 Thermochemistry (lec7) 9

10 Refer to Slide 7.2 Standard Enthalpy Changes 9/9/10 Thermochemistry (lec7) 10

11 Solid, liquid & vapor regions Normal: Melting point Freezing point Triple point Critical point Supercritical fluid region Phase Diagram of Water 9/9/10 Thermochemistry (lec7) 11

12 Hess s Law Sometimes its difficult or even impossible to conduct some experiments in the lab. Because Enthalpy (H) is a State Property, it doesn t matter what path is taken as long as the initial reactants lead to the final product. Hess s Law is about adding or subtracting equations and enthalpies. Hess s Law: If two or more chemical equations are added to give a new equation, then adding the enthalpies of the reactions that they represent gives the enthalpy of the new reaction. 9/9/10 Thermochemistry (lec7) 12

13 Enthalpy Changes for Formation & Reaction Refer to Slide 7.3 9/9/10 Thermochemistry (lec7) 13

14 Skill: Using Hess's law (by example) Rxn #2 N 2 (g) + O 2 (g) 2 NO ΔH 2 = +180 kj Rxn #3 2 NO (g) + O 2 (g) 2 NO 2 ΔH 3 = 112 kj Rxn #1 N 2 (g) + 2 O 2 (g) 2 NO 2 ΔH 1 = +68kJ 9/9/10 Thermochemistry (lec7) 14

15 Temperature Dependence in Hº(T) Refer to Slide 7.4 9/9/10 Thermochemistry (lec7) 15