HW_wk4_solutions_06 - 2R03 HW for weeks 4 Oct 6 2006...

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2R03 HW for weeks 4 Oct 6 2006 STUDENT NAME ______________ _____________ STUDENT NUMBER ___________ Credit: 1.5% if submitted with fully explained solutions on or before the due date of Oct 18, 2006 ( drop boxes ##28-30 , which are located in ABB 1 st floor in the beginning of the hallway behind the blue doors opposite the main office ABB-156). Please attend the assigned tutorials if you have any question on these exercises . Note: All gases are assumed to be ideal and all thermochemical data are for 298.15K unless otherwise specified 1. Classify as endothermic or exothermic (a) a combustion reaction for which Δ r H ø =-2020 kJmol -1 , (b) a dissolution for which ΔH ø = +4.0 kJ mol -1 , (c) condensation, (d) freezing (e) vapor deposition (f) the standard formation reaction of gaseous CO 2 Δ f H ø =-394 kJmol -1 Solution: (a) exothermic, Δ r H ø negative (b) endo, Δ r H ø positive (c) (opposite of vaporization) exo, Δ r H ø negative (d) (opposite of fusion) exo, Δ r H ø negative (e) (opposite of sublimation) exo, Δ r H ø negative (f) exo, Δ r H ø negative 2. Which of the following statements is true and which is false? Explain. (A) Fusion, vaporization and sublimation are always endothermic processes. (B) The bond enthalpy doesn’t depend only on the two bonded atoms. (C) The enthalpy change of any reaction can be computed using the formation enthalpies. (D) All formation reactions are exothermic. Solution: (A) is true because you always need to provide heat to go to from a condensed phase to a less condensed phase as inter-molecular interactions must be broken. (B) is also true because the bond enthalpy depends on many other factors such as the bond order and the nature of the two molecular fragments that are bonded. (C) True because it is always possible to devise a path from reactants to products using formation reactions or their reverse. (D) False because there are some formation reactions that are endothermic ( e.g. liquid benzene) 3. Which of the following statements is true and which is false? Explain. (A) The only way to compute a reaction enthalpy is by using formation enthalpies. (B) Formation reactions are defined as the reactions that form one mole of a given compound in a specified state from stoichiometric amounts of the elements in their gas states. (C) Kirchoff’s Law is used to compute the variation of reaction enthalpies using heat capacities at constant volume. Solution: (A) is false because reaction enthalpies can be computes also starting from non-formation enthalpies. For instance we saw how bond enthalpies or combustion enthalpies can used to compute the enthalpies of other reactions. (B) Is false because formation reactions are defined as the reactions that
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