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Chapter 6

CHEM UA 125- Chapter 6 Notes

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John Halpin

if matter undergoes a change the quantity of energy that the matter contains also changesthermodynamics the study of energy and its transformationsthermochemistry the branch of thermodynamics that deals with heat in chemical and physical change61potential energy can be converted to kinetic energy which appears as work and heatwhen energy is transferred from one object to another it appears as work andor heatwe will examine this idea in terms of the release or absorption of energy during a chemical or physical changesystem the part of the universe we are focusing onneed to define the systemsurroundings everything else besides the systeminternal energy E the sum of all the potential and kinetic energieswhen reactants in a chemical system change to products the E changeschange in EE finalE initialE productsE reactants a change in the energy of the system must be accompanied by an equal and opposite change in the energy of the surroundingsenergy diagram the final and initial states are horizontal lines along a vertical energy axis with change in E as the difference in the heights of the linesa system can chagne its internal energy byreleasing energy in a transfer to the surroundingsE finalE initial change in E0absorbing some energy in a transfer from the surroundingsE finalE initial change in E0energy is transferred from system to surroundings in two formsHeat thermal energy qenergy transferred as a result of a difference in temperature between the system and the surroundingsWorkthe energy transferred when an object is moved by a forcechange in energy qwwe define the sign of the energy change from the systems perspectiveenergy transferred into the system system ends up with more energyenergy transferred out from the system the system ends up with less energyIf a system transfers energy only as heat no work is donechange in Eq0heat flowing out from a system heat is released so q is negativeheat is flowing into a systemheat is absorbed so q is positiveIf a system transfers energy as work only no heatchange in E0wenergy is transferred as work done by the system on the surroundingssystem releases energy as work w is negativework is done by the surroundings on the systemenergy is transferred as work done on the system by the surroundings w is positivelaw of conservation of energy first law of thermodynamics the total energy of the system plus the surroundings remains constantchange in E universechange E systemchange E of surroundingsSI unit of energyjoule JJ kgm2s2also calorie cal british thermal unit Btu
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