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Chapter 6 Chemical Reactivity and Mechanisms.docx

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Harald Stover

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Chapter 6 Chemical Reactivity and MechanismsBernard HoNovember 24 201161 EnthalpyRecall that electrons achieve a lower energy state when they occupy a bonding molecular orbitalThus breaking a bond requires an input of energyFor a bond to break electrons in bonding MO must receive energy from their surroundingsoSpecifically surrounding molecules must transfer some kinetic energy to system bond being brokenEnthalpy is used to measure this exchange of energyoHq at constant pressureoChange in enthalpy for any process is defined as exchange of kinetic energy also called heat q between a system and its surroundings under conditions of constant pressureoFor a bondbreaking reaction H is primarily determined by amount of energy necessary to break bond homolyticallyHomolytic bond cleavage generates two uncharged species called radicals each of which bears an unpaired electronHeterolytic bond cleavage involves a twoheaded curved arrow generating ionsEnergy required to break a covalent bond via homolytic bond cleavage is called the bond dissociation energyoH refers to bond dissociation energy when measured under standard conditions where pressure is 1 atm and compound is in its standard stateMost reactions involve breaking and forming of several bondsoIn these cases must take into account each bond being broken or formedooTotal change in enthalpy H for reaction is referred to as heat of reactionooSign of Hfor a reaction indicates direction in which energy is exchangedand is determined from perspective of systemooA positive H indicates that system increased in energy received energy from surroundings endothermic processooA negative H indicates that system decreased in energy gave energy to surroundings exothermic process62 EntropyInformally defined as measure of disorder associated with systemMore accurately described in terms of probabilitiesspontaneousA process that involves an increase in entropy is said to be oProcess can and will occur given enough timeFor chemical reactions must take into account change in entropy of system as well as change in entropy of surroundingsoSSStotsyssurrIn order for a process to be spontaneous total entropy must increaseoEntropy of system reaction can actually decrease as long as entropy of surroundings increases by an amount that offsets decreased entropy of systemoAs long as S is positive reaction will be spontaneoustotValue of S s affected by a number of factorssys
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