Question

(old tut te quesion askeu abuul the 18lal Bold energy, nöt bond energies of the separate molecular species. If the question had asked about bond energy changes of particular molecular species, we would have to include separate bond energies in our model. We will do this in FNT 9) Think about the general case of chemical reactions. a) When a single compound breaks up into separated atoms, what can you say with absolute certainty regarding the change in bond energy of that compound? Why? What can you say with absolute certainty regarding the change in bond energy of a compound that is formed from separated constituent atoms? Why? b) Consider this chemical reaction: the combustion of propane: 9) C3Hg+ 5 O23 CO2+4 H20 - We can model this process as if the reactant molecules are broken down into their constituent atoms, which are then re-assembled into the product compounds. Represent this process with an energy system diagram that contains four separate bond energy systems, one for each molecular species. There will be one for the CHs, one for the 5 O2, etc. Using your result from part (a) show the direction of each bond energy change with an arrow in the standard way. The indicator for each compound is the number of moles of that compound. Show the initial and final values of each of the indicators in the standard way. c) The magnitude of the bond energy changes for all the molecules involved in this process when they are separated into atoms are: CsHs : 4002 kJ/mol; O2 495 kJ/mol ; CO2: 1607 kJ/mol; H2O:925 kJ/mol. Determine the bond energy changes, the AEbond, for each of the reactants and products and write a conservation of energy equation in the standard way (AE1 + AE2+ AE3 + AE Q). Write this equation out with appropriate subscripts to clearly identify each term. Rewrite it with the numerical values being careful to get the algebraic sign correct, and calculate Q. Is the algebraic sign of Q that you calculated consistent with our sign convention for Q and with what you know about combustion? 16 DL 2 Unit 1: Applying Models to Thermal Phenomena

Answer 1

a) When a single compound breaks up into separate atoms, you can always assume that energy was "liberated" because all bonding needs energy (enthalpy of bonding)

I can say that with certainty, as all atoms are separate, if any type of bonding form between the particles, you may not assure that. You can only assume that if the particle was "broken" down into atoms, all the energy required to bond them is free, and this will be an exothermic preaction (heat goes out of the system)

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