Thermodynamics and Chemical Kinetics: Formulas and Calculations - Prof. Stephen P. Mccord, Exams of Chemistry

Download Thermodynamics and Chemical Kinetics: Formulas and Calculations - Prof. Stephen P. Mccord and more Exams Chemistry in PDF only on Docsity! Exam 2 - Formulas Chapter 15 – Thermodynamics Hess’s Law : Hrxn = Ha + Hb + Hc + ... There are many times when you can simply calculate a “delta” value by looking up the appropriate Table values – like Appendix K in your book. Note the 2 ways you can get H here, one with standard enthalpies of formation and one using bond energies. Hrxn 0 = n Hf products 0 n Hf reactants 0 Hrxn 0 = B.E.reactants B.E.products Note the similar equations for calculating G and S from standard table values Grxn 0 = n Gf products 0 n Gf reactants 0 Srxn 0 = n Sf products 0 n Sf reactants 0 CALORIMETRY: Although the equations are identical, you DO get different thermodynamic functions when you do Coffe-Cup calorimetry (constant pressure) verses Bomb Calorimetry (constant volume). Remember also that you will get ZERO for the work term in constant volume calorimetry. No volume change means no work. Also, you are measuring the heat in/out of the surroundings (qsurroundings)which means the sign for q is the opposite of what it is for the system (qsystem)which is what we are interested in. Sign convension for heat (q) and work (w): +q = heat flows INTO system (endothermic – heat absorbed) -q = heat flows OUT of system (exothermic – heat released) +w = work done ON the system -w = work done BY the system E = q + w w = P V = ( n)RT E = qp P V E = H P V E = H ( n)RT That last equation (underlined)is the most straight forward way to convert H to E. You DO need a balanced chemical equation AND must know n which is the CHANGE in the number of GAS moles when going from reactants to products. n = (#moles of GAS products) – (#moles of GAS reactants) No, don’t count liquids, solids, or even aqueous species — only the GASES. Coffee Cup Calorimetry (constant pressure) qp,surroundings = (SHwater mwater T )+ (HCcalorimeter T H sys = qp,sytem = qp,surroundings Bomb Calorimetry (constant volume) qsurroundings = (SHwater mwater T )+ (HCcalorimeter T ) Esys = qv,sytem = qv,surroundings