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thermo_ism_ch04

# thermo_ism_ch04 - Chapter 4 Thermochemistry Problem numbers...

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Chapter 4: Thermochemistry Problem numbers in italics indicate that the solution is included in the Student’s Solutions Manual Questions on Concepts Q4.1) Under what conditions are H and U for a reaction involving gases and/or liquids or solids identical? H = U + ( PV ). H U for reactions involving only liquids or solids because, to a good approximation, the volume does not change as a result of the reaction. If gases are involved, ( PV ) = nRT . Therefore, H U if the number of moles of reactants and products that are gases are the same. Q4.2) If f H o for the chemical compounds involved in a reaction are available at a given temperature, how can the reaction enthalpy be calculated at another temperature? The reaction enthalpy can be calculated to high accuracy at another temperature only if the heat capacities of all reactants and products are known using ( 29 298.15 298.15 . T T K P K H H C T dT = ∆ + o o If the heat capacities of reactants and products are similar, the reaction enthalpy will not vary greatly over a limited temperature range. Q4.3) Does the enthalpy of formation of compounds containing a certain element change if the enthalpy of formation of the element under standard state conditions is set equal to 100 kJ mol –1 rather than to zero? If it changes, how will it change for the compound A n B m if the formation enthalpy of element A is set equal to 100 kJ mol –1 ? Yes, because part of the enthalpy change of the reaction will be attributed to the element. For the reaction n A + m B → A n B m , ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 A B A B A B A B reaction f n m f f f n m reaction f f H H n H m H H H n H m H = ∆ - - = ∆ + + o o o o o o o o Therefore, ( 29 A B f n m H o for the compound will increase by 100 n kJ mol –1 . Q4.4) Is the enthalpy for breaking the first C–H bond in methane equal to the average C–H bond enthalpy in this molecule? Explain your answer. 4-1

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Chapter 4/Thermochemistry No. The average bond enthalpy is the average of the enthalpies of the four steps leading to complete dissociation. The enthalpy of each successive dissociation step will increase. Q4.5) Why is it valid to add the enthalpies of any sequence of reactions to obtain the enthalpy of the reaction that is the sum of the individual reactions? Because H is a state function, any path between the reactants and products, regardless of which intermediate products are involved, has the same value for H . Q4.6) The reactants in the reaction 2NO( g ) + O 2 ( g ) → 2NO 2 ( g ) are initially at 298 K. Why is the reaction enthalpy the same if the reaction is (a) constantly kept at 298 K or (b) if the reaction temperature is not controlled and the heat flow to the surroundings is measured only after the temperature of the products is returned to 298 K?
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thermo_ism_ch04 - Chapter 4 Thermochemistry Problem numbers...

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