Chem_340_answers_concepts_2 - Chapter 4: Thermochemistry...

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Chapter 4: Thermochemistry Q4.1) Under what conditions are H and U for a reaction involving gases and/or liquids or solids identical? H = U + (p V). H U for reactions involving liquids and solids, since, in a good approximation, the volume does not change in a chemical reaction. On the other hand, for chemical reactions involving gases, (p V) = n R T, and H U if the number of moles of reactants and products is identical. Q4.2) If the H f ° 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 accurate calculation of the reaction enthalpy at another temperature requires the knowledge of the heat capacities of all reactants and products: () T d T C H H T K 298.15 p K 298.15 T + = o o For similar heat capacities of reactants and products, the reaction enthalpy does not vary significantly 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 : () ( ) ( ) B H m A H n B A H H f f m n f reaction o o o o = () ( ) ( ) B H m A H n H B A H f f reaction m n f o o o o + + = Therefore, () m n f B A 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. No. The average bond enthalpy is the average of enthalpies for the four steps leading to a
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Chem_340_answers_concepts_2 - Chapter 4: Thermochemistry...

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