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CH 301 Exam 3 Question Type Notes.docx

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CH 301
Dave A.Laude

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Exam #3 Question Type Notes Color coordination: Laude’s Notes: in blue Travis’s Notes: in green Course Packet Notes: in black 1. First law of thermodynamics You need to know the first law in its various forms and apply it. ∆Euniverse0 = q + w Energy is conserved; there can be no perpetual motion machine, etc. ∆E = ∆U = 0 [universe is the isolated system] isolated system Internal energy deals with the first law of thermodynamics  The sum of heat (q) and work (w)  A measure of the change in heat of a system at constant volume 2. Definition: enthalpy You will be given a variety of statements about enthalpy (∆H). Some are true, some are not. Be able to list 4 or 5 ways you know ∆H. Ex: ∆H = q at constant pressure ∆H = ∆E + P∆V, which is a correction for PV work at atmospheric pressure *hint: if you see ∆H in an equation on the help sheet, it is fair game that you remember it here ∆G = ∆H + T∆S  Enthalpy is the measure of a system’s ability to change entropy of surroundings ∆E = q in a constant volume bomb calorimeter ∆H = q in a constant pressure Styrofoam cup container  When no gas is present, PV is negligible o so ∆H ~ ∆E (values of these two state functions get close)  When gas is present at constant pressure: ∆H = ∆E + P∆V = ∆E + ∆nRT 3. Signs for thermodynamics quantities I’ll give you a chemical or physical process and you tell me ∆S, ∆H, ∆G, w and q. You know the drill, “Be the system”. Sign Negative (-) Positive (+) ∆H Exothermic Endothermic q Heat leaves Heat enters w Work on surroundings (a bomb) Work on system (“arming a bomb” ∆S Increasing order Decreasing order ∆G Reaction happens (spontaneous) Reaction doesn’t happen (nonspontaneous) 4. Definitions: State functions A gift. You have only been told of two path dependent values, q and w. All the capital letters are state functions and independent. Independent of path: internal energy, temperature, pressure, volume, entropy, enthalpy, G, n Path dependent: q (heat) and w (work) Definition of state function: its value depended solely on its final quantity minus the initial quantity. 5. Definition: Heats of formation I’ll give you a bunch of chemical reactions. You have to pick which ones are ∆H f Remember:  Must be written as a formation  Must involve elements in standard state on the left o H2(g), 2 (g),graphite, Hg(l), etc.  Must make a mole of product Left side of chemical reaction  right side of chemical reaction Elements in standard state  1 mol of 1 product Exam #3 Question Type Notes 6. Definition: Heat capacity Be able to apply the definition of heat capacity C = q/∆T to a variety of systems.  Note that C is per gram or mole, so the more stuff, the more heat  Note the inverse relationship of C and ∆T; as C gets large, ∆T changes little o Ex: water Heat capacity of water is 4.184 J/g*K C and ∆T are inversely related (at constant q) *q = mC∆T+ 7. Calculation: Bomb calorimeter Classic bomb calorimetric problem ∆H system -∆H surrounding-mC∆T – C ∆cal  -mC∆T  water  Ccal  bomb calorimeter Make sure to remember signs. The system is a combustion reaction and is negative. Remember units. C is given in Joules usually but amounts of energy produced are kJ. This is plug and chug, don’t make it hard. Remember to have right amount for answer. ∆U = mC∆T + C ∆ca;  m = 1g/mL  ∆T = T finalTinital  C will be provided cal Make sure units are correct (asking per gram!) 8. Calculation: Hess’ law and heats of formation Classic plug and chug. ∆H °= ∑∆H °- ∑∆Hf ° rxn fproduct reaction This can be especially easy. Remember to multiply by coefficients and double check signs. Methane is CH 4 ° In this method, the molecules become elements in standard states (∆H ). f 9. Calculation: Hess’ law and combined reaction enthalpies ° One of the harder questions on the test. Do at the end. I’ll give you a collection of ∆H rxnand reactions. Combine them by flipping and multiplying through to cancel and yield ∆H rxn°of interest. *hint: tragically, fractions are found in the coefficients you need to use To make reaction backwards, multiply ∆H by -1. 10. Calculation: statistical mechanics determination of internal energy I’ll give you a number of molecules. You tell me the kinetic energy on number of modes of motion for translational, rotational, vibrational. *hint: apply to a single molecule first Translation = 3  3/2kT Rotation = 2  kT (linear) = 3  3/2kT (nonlinear) Vibration = 3N – 5 (linear = 3N – 6 (nonlinear) After you find the energy for one molecule, multiply through by number of molecules in problem. E = 1/2kT  Number of modes per molecule = 3N  K is for individual particles  R is for moles Ex: SF6  3 modes * 7 molecules = 21modes  Translational = 3 (nonlinear)  Rotational = 3 (nonlinear)  Vibrational = 3(7) – 6 = 15 Exam #3 Question Type Notes The average energy per motion is E = 1/3kT  Temperature is in Kelvin  K is Boltzmann constant 11. Calculation: Bond enthalpies Classic calculation following Hess’ law. ∆H rxn°= ∑∆BE reaction∑∆BEfproduct  Make sure you draw out the structures so you don’t make a mistake on the number of bonds in a molecule  Make sure you do stuff on left – stuff on right As always, easy problems with lots of math to mess it up. Walk it carefully and neatly. In this method, the molecules become gas atoms [only for gases]. 12. Calculation: Work calculation Classic work calculation W = -P∆V = -∆nRT I’ll give you a chemical reaction. You determine ∆n = nqright – q left, then multiply by RT. Remember  RT = 2.5 kJ at room temperature  If ∆n grows, work done on surrounding is negative  If ∆n shrinks, work done on system is positive Work done by system is negative, work done on system positive. If Pext 0, no work is done [can happen in an open container with 1 atm of external pressure]. 13. Definition: Interna
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