Lecture Notes B: Thermodynamics I (cont)

Transcription

1 Lecture Notes B: Thermodynamics I (cont) 1) Energy flow in chemical systems: Overview and outline General approach In the heat transfer problems of Lecture Notes A: Iron poker in water q iron = -q water Scotch and water q scotch = -q water In both cases, the heat is due to, or causes, a change in temperature of the substance without a chemical transformation. This process is described by q = m C v T. We are next going to discuss processes that involve chemical transformations: Chemical reactions: burning wood or fuel You heat a pot of water over a campfire Phase transitions: melt ice, boil water You use ice to cool down your drinking water Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 1 of 7

2 Some applications Hydrogen economy Hydrogen economy: How much H 2 does it take to replace a gallon of gas? Bio-energetics The bombardier beetle hunts by spraying boiling water: how does it make it? How does your body store energy in ATP? The tools of thermochemistry How do we measure and quantify heat given off or absorbed by a reaction (these notes)? How do we predict the amount of heat that will be given off (next set of notes)? What is the correct way to summarize the results of experiments so we can predict new situations? How can we predict it for reactions we haven t seen before? 2) How do we measure and quantify heat given off or absorbed by a reaction? Heat transfer at constant volume: Bomb Calorimeter If you seal some chemical species in a constant volume bomb, and measure the heat produced or consumed in a chemical process, you have measured E. Heat transfer at constant pressure: Definition of enthalpy If you measure the heat produced or consumed in a chemical process at constant pressure, you have measured H, where H is the enthalpy. Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 2 of 7

3 Relation to C p and C v Difference between constant volume and constant pressure is only big for gasses Constant volume (C v ) As you heat, all energy goes to the internal motion of molecules,thereby increasing T Constant pressure (C p ) As you heat, the volume increases (gas is doing work). So your heat is being used both to increase T and to do work. Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 3 of 7

4 3) How do we quantify the heat given off by a reaction? Consider burning acetylene: C 2 H 2 (g) + 3/2 O 2 (g) CO 2 (g) + H 2 O(l) H= kj Exothermic : give off heat reactants products + heat H negative Endothermic: absorbs heat reactants + heat products H negative Problem: Calculate the change in enthalpy when 5.00 grams of acetylene is burned. Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 4 of 7

5 4) Phase changes H 2 O (s) H 2 O (l) H fus = kj H 2 O (l) H 2 O (g) H vap = 40.7 kj Concept What are the correct signs for enthalpy s of the above reactions? a) H fus = kj, H vap = 40.7 kj b) H fus = kj, H vap = kj c) H fus = kj, H vap = 40.7 kj d) H fus = kj, H vap = kj Problem How many grams of ice (at 0 o C) is needed to cool 1 liter of water from 25 o C to 0 o C? Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 5 of 7

6 5) Hess s law Consider burning acetylene: C 2 H 2 (g) + 3/2 O 2 (g) CO 2 (g) + H 2 O(l) H= kj Concept What is H for the reaction: 2CO 2 (g) + 2H 2 O(l) 2C 2 H 2 (g) + 3 O 2 (g) (a) H= kj (b) H= kj (c) H= kj (d) H= kj A + B C H 1 C D + E H 2 A+ B D+E H = H 1 + H 2 Reverse of a reaction: A + B C H 1 C A + B - H 1 Multiple of a reaction: A + B C H 1 2A +2B 2C 2 H 1 Problem Two gaseous pollutants that form in auto exhaust are CO and NO. An elegant way to eliminate these is through the reaction: CO (g) + NO (g) CO 2 (g) + 1/2 N 2 (g) Calculate H for the above reaction given the following (all values are for 25 o C), CO (g) + 1/2 O 2(g) CO 2 (g) H = kj N 2 (g) + O 2(g) 2 NO (g) H = kj Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 6 of 7

7 What is E for the above reaction? CO (g) + NO (g) CO 2 (g) + 1/2 N 2 (g) Lecture Notes B: Thermodynamics I(cont) Distributed on: Monday, January 17, 2005 Page 7 of 7