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Chapter 20 - Brady(1) - Introduction to General and Organic...

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1 Chapter 20 Thermodynamics Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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2 Introduction Thermodynamics is concerned with energy changes – the flow of energy from one substance to another The First Law of Thermodynamics Internal energy may be transferred as heat , q , or work , w , but cannot be created or destroyed – was originally discussed in Chapter 7
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3 The change in internal energy, E , is defined in terms of heat ( q ) and work ( w ) Internal energy is a state function which means that its value does not depend on how the change from one state to another was carried out system the by done is Work 0 system the on done is Work 0 system the by released is Heat 0 system the by absorbed is Heat 0 < < + = - = w w q q w q E E E initial final The First Law
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4 Energy Changes E is positive if energy flows into a system, and is negative if energy flows out If work is done on a system, the system gains and stores energy. If the system does work on the surroundings, the system loses some energy.
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5 There are two types of work that we are interested in: electrical (discussed in the next chapter) and P - V work which will be discussed now The work done by a system depends on the volume change and the external pressure For a reaction at constant volume, energy change must appear as heat gained or lost! V P V P w external - = - = constant) held (volume ) ( v q V P q E = - + = Work in Chemical Systems
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6 q v is called the heat at constant volume Usually reactions are carried out at fixed pressure For these reactions, enthalpy, H , is more convenient constant) held (pressure ) ( constant) held (pressure p q V P V P q V P E H PV E H = + - = + = + = Enthalpy and Enthalpy Changes
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7 q p is called the heat at constant pressure The internal energy and enthalpy changes are different whenever a volume change occurs for the system Only when the volume change is large is the difference significant Large volume changes can occur when gases are involved in the reaction V P H E - = - Comparing H with E
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8 Treating the gases as ideal RT n E H RT n P RT n P V P n n n P T P RT n V n P T P nRT V gas gas gas reactants gas products gas gas gas + = = × = - = × = = and thus , ) ( ) ( ) and constant (at so can vary, only and constant at gas) ideal any for (true
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9 H = E + n gas RT R = 8.314 J mol -1 K -1 , T in Kelvins Example : CaCO 3 (s) CaO(s) + CO 2 (g) H o = +571 kJ and E o = +569 kJ The difference is energy required to do work in pushing Back the atmosphere as the system expands!!
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10 A spontaneous change is a change the occurs by itself (without continuous outside assistance) Examples: A rock falling off a ledge and water flowing down-hill Once a spontaneous event begins, it has a tendency to continue to completion Nonspontaneous events occur at the expense of spontaneous ones Spontaneity
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