CHEM 101 1nd Edition Lecture 20Outline of Last Lecture I. Energy and ChemistryII. Temperature and HeatIII. System and SurroundingsIV. Specific Heat CapacityV. Change of StateOutline of Current Lecture I. First Law of ThermodynamicsII. Enthalpy Current LectureI. First Law of Thermodynamicsa. Thermodynamics is the science of heat and worki. Work is done whenever a mass is moved against opposing forse1. Some for of energy is required for work to be doneii. If a system does work on its surroundings, energy must be expended by the systemiii. If work is done by the surroundings on a system, the energy of the systemwill increaseb. Heat transfers from surroundings to system in endothermic processi. CO2 (s, -78 °C) --> CO2 (g, -78 °C)1. A regular array of molecules in a solid --> gas phase molecules (sublimation)2. Gas molecules have higher kinetic energyc. Heat/Energy transfer in a physical processi. CO2 (s, -78 °C) --> CO2 (g, -78 °C)ii. Two things have happened!1. System is the CO22. Surrounding consist of objects that exchange energy with the CO2 (air, bag the CO2 is in, and book)3. Sublimation requires energya. Transferred as heat to the system from the surroundings4. System does work on the surroundings by pushing aside the These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.atmosphere and lifting the book. d. The First Law of Thermodynamicsi. ∆E = q + w1. ∆E = energy changea. Energy is often also written as ∆U for chemical systems andindicates the internal energyb. Internal energy in a chemical system is equal to the sum of the potential and kinetic energiesc. Potential energy is the attractive and repulsive forces for all of the nuclei and electrons in the system and includes energies for bonds and forces between moleculesd. Typically interested in changes in internal energy rather than the actual values 2. q = heat energy transferred3. w = work done by the systemii. Energy is conserved.II. Enthalpya. Most chemical reactions occur at constant pressure ( P), so heat transferred at content P = qpi. Where qp = ∆H ii. Where H = enthalpyiii. And so ∆U = ∆H + wp(and w is usually small)1. ∆H = heat transferred at constant P ≈ ∆U2. ∆H = change in heat content of the system3. ∆H = Hfinal- Hinitialb. ∆H = Hfinal- Hinitiali. If Hfinal > Hinitial then ∆H is positive1. Process is endothermicii. If Hfinal > Hinitial then ∆H is negative1. Process is exothermic c. Hess's Lawi. If a reaction is the sum of 2 or ore other reactions, the net ∆H is the sum of the ∆H values for the other
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