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Chapter 9 ppt - CHAPTER 9 BALANCES ON REACTIVE PROCESSES...

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CHAPTER 9 BALANCES ON REACTIVE PROCESSES
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Consider the familiar reaction in which water is formed from hydrogen and oxygen: ) ( 2 ) ( 2 ) ( 2 2 2 v g g O H O H + On the molecular level, the reaction might be depicted as follows: H Each time this reaction takes place, three chemical bonds are broken (two between hydrogen atoms and one between oxygen atoms). H2 O2
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) Four bonds are formed among the atoms of the two water molecules. As it happens, more energy is released when the water molecule bonds form than it takes to break the hydrogen and oxygen molecules bonds . For the reactor temperature to remain constant, the net energy released must be transferred away from the reactor, otherwise it can raise the reactor temperature by several thousand degrees.
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In any reaction between stable molecules, energy is required to break the reactant chemical bonds and energy is released when the product bonds form . Exothermic reactions – if the first process absorbs less energy than the second process releases: the product molecules at a given T and P have lower internal ener- gies (and hence lower enthalpies) than the reactant molecules at the same T and P . For an exothermic reaction, the net energy released the heat of reaction must be transferred from the reactor as heat or work, or else the system temperature increases.
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Endothermic reactions – if less energy is released when the product bonds form than it took to break the reactant bonds. For an endothermic reaction, energy must be added to the reactor as heat or work to keep the temperature from decreasing. An energy balance on a reactor tells the process engineer how much heating or cooling the reactor requires in order to operate at the desired conditions .
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In this chapter we show how enthalpy changes that accompany chemical reactions are determined from tabu- lated physical properties of the reactants and products and how calculated enthalpies of reaction are incorporated in energy balances on reactive processes .
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9.1 HEATS OF REACTION Consider the reaction between solid calcium carbide and liquid water to form solid calcium hydroxide and gaseous acetylene: ) ( ) ( ) ( ) ( 2 ) ( 2 2 2 2 2 g H C s OH Ca l O H s CaC + + The expression stoichiometric quantities of reactants means molar amounts of the reactants numerically equal to their stoichiometric coefficients.
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The heat of reaction (or enthalpy of reaction ), , is the enthalpy change for a process in which stoichiometric quantities of reactants at T and P react completely in a single reaction to form products at the same T and P . For example, the heat of calcium carbide reaction at 25 and 1atm is ) , ( ˆ P T H r mol kJ atm C H o r / 4 . 125 ) 1 , 25 ( ˆ - = ) ( ) ( ) ( ) ( 2 ) ( 2 2 2 2 2 g H C s OH Ca l O H s CaC + +
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) ( ) ( ) ( ) ( 2 ) ( 2 2 2 2 2 g H C s OH Ca l O H s CaC + + mol kJ atm C H o r / 4 . 125 ) 1 , 25 ( ˆ - = These two equations signify that if 1 mol of solid CaC2 reacts completely with 2 mol of liquid H2O to form 1 mol of Ca(OH)2 and 1 mol of C2H2, and the initial and final
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