Chapter 5 - CHAPTER 5 THERMOCHEMISTRY THE CORE IDEA...

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Unformatted text preview: CHAPTER 5: THERMOCHEMISTRY THE CORE IDEA: Buckminster Fuller: “… the sunlight unwinding …” Some basic concepts: A thermochemical equation: heat is a stoichiometric quantity H 2 (g) + ½ O 2 (g) H 2 O(l) + heat ENERGY = the capacity to transfer heat or to do work THERMODYNAMICS == heat in motion ... the transfer of energy in chemical reactions THERMOCHEMISTRY == measurement and prediction of the effects of heat // the study of the relation between chemical reactions and changes in heat energy HEAT TRANSFER and THERMAL EQUILIBRIUM: Potential energy … energy because of position … a STATE FUNCTION PE = mgh (we don‟t care how they got to h) Kinetic energy … energy of an object in motion KE = ½ mu 2 Conservation of energy: energy neither created nor destroyed, but it can be converted from one form to another How does all this play out for molecules? Same thing, except gravitational forces play virtually no role at the atomic- molecular level … What does matter? TEMPERATURE AND CHARGE How? Temp goes up … average KE goes up … kinetic energy associated with random motion of molecules = THERMAL ENERGY Interactions between particles (charge, bonds) … potential energy stored in arrangement of particles (back to „sunlight unwinding‟) Electrostatic potential energy = coulombic interaction E el Columbic interactions determine PE at the atomic-molecular level More language of thermochem … Systems, Surroundings, and the Flow of Energy Endothermic and Exothermic processes How the chemist views the world … Molecular motions contribute to overall internal energy -- An idea … INTERNAL ENERGY (E) = the sum of all the kinetic and potential energies of all the components of the system of interest … E is tough to measure … but E (changes in internal energy) are relatively easy to measure … E = E final – E initial Value depends only on initial and final state – how the change between the two occurs doesn‟t matter BASIC PRINCIPLE: law of conservation of energy applies … energy change in system must be balanced by energy change in surroundings … first law of thermodynamics energy changes in system and surroundings EQUAL IN MAGNITUDE BUT OPPOSITE IN SIGN … their SUM IS ZERO … ENERGY IS CONSERVED Units of energy: calorie … kilocalorie … dietary Calorie … joule Doing work: Idea – you do work on a system, you increase the system‟s internal energy Volume of gas changes – work is done by system on surroundings piston compresses mixture of fuel and oxygen … work done on system by surroundings and temp of gas rises fuel ignites … gases expand, push piston down … work done on surroundings by system E = q + w = heat flowing into system + work done on system Classic example … pressure-volume work done by hot-air balloon against surrounding atmosphere … system = air in the balloon work is done on surroundings (the atmosphere) as the balloon expands work = P V = pressure-volume work (P = atmospheric pressure = constant) change in internal energy of system: E = q + (-P V) = q - P V WATCH SIGNS!!! This is the...
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This note was uploaded on 02/22/2011 for the course CHEM 25 taught by Professor X during the Spring '06 term at Lehigh University .

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Chapter 5 - CHAPTER 5 THERMOCHEMISTRY THE CORE IDEA...

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