04CHM114_0204

04CHM114_0204 - CHEM 114A Spring 2004 Mon. Feb 2‘“1...

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Unformatted text preview: CHEM 114A Spring 2004 Mon. Feb 2‘“1 2004 Lecture N0. 7 website wwwpublic.asu.edu/~caangell Chapter.5 Thermochemistry(cont’d) Energy as heat and work, and the first law of thermodynamics The “law of conservation of energy”, and the relation between heat and work....Engineers need to know it!! Work = pressure x change in volume = -PAV if system expands (system does work on the surroundings, or PAV (positive) if system is compressed (work done on, system) FIRST LAW Of thermo summarizes the experience that a system's energy can be changed by either "flowing heat into” Or "dong work on” the system, and that the change in internal energy is always the sum 'ofthetwo AE = q + W 5". W0?“ “and. “N. ‘H 0n- 1 W Important distinction between heat and work... can measure the work done. EXPLAIN: river-rain analogy 9‘“; (Always... “hr “‘3” k"! 0 Lt “kl-r a‘~* “III-'1 ‘n I'LIH I: Mu. w" k F' ‘1'“ Measurements of energy changes. L“. M‘oJew‘ "km-Fa... CalorimEtFy: measurement of heat ' change, q (Ch. 5.5) @0115 of calorimetry . performed at constant volume, then W (= PAV) must be zero E = EM ‘Emw: Q: —I 2. at constant pressure (work may be donfiepending on AV) ..__..1 £1- : Hfinal " Hinitial = qp using the definition Ll H = E + PV .u ‘ AH = AE +A(PV) H~""’. "‘ E’ ‘ new“ for constant pressure ‘5”3) AH = AE + PAH Q a nu- Mostly we measure AH in chemical processes. ENDOTHERMIC VS EXOTHERMIC PROCESSES 4' X. 3.3.“; Reyna" Hem- HtA-r (I,le .Ek-M) Application to the boiling of water: How much of the AH of boiling is due to work done in creating the volume of the gaseous state? Remember Vm(gas) = 2000 mefliq) at STP e.g. water AHb = q = 44,000J/m01 at 373K p H) o ‘ L = AE + PAV System expands so system does work on surroundings, so the work is negative. w = -P atm x 22.4 l/mol (x 373/273) .2; .3 v. Tl 16": = -30.6 litre-atm/mol This has to be converted to joules Conversion factor 1 l.-atm = 101.3J # so, w = - 3,100 J so AE — q w = 4 000 (“3,100) Jlmol = m J/mol The change in energy AB is 1% than the change in enthalpy AH How about combustion of carbon to give C02? 'C(s)+ 02(g) = C02(g) there is one mole of gas on each side of the equation AV = 0, therefore no work is done and AE = AH How about a couple of others? like. the Born-Haber process for ammonia production? N494: 3km» 2 NH; (5) LI- mu H 1MB! 5“ 2. formation 0 carbon monoxi e. e. «Bl/,0, -—I Q0 M- 8" A“ P" ' _ 9. 1A 5 A“ - u. ’ Yhel. ‘3 Note that in a combustion process that releases heat and increases volume, the AE will be larger than the AH. Note also that the work component is generally small, and will be negligible in reactions NOT involving gases. STANDARD HEATS OF FORMATION The heat released or absorbed when a compound is formed from its elements in their standard states. (use a superscript 0. . ...AH°) standard states? are the stable states at one atm pressure. e.g. water, not steam. 7-1: C “R- Q examples: M a K ‘Pu I 0H!» H43» ‘40,,“3—p H;b(5) “WINK” C’s Mfi “HR «.4- um: Swans: H1- rc scams-mu qr (ah-e". {s u “Fw- IE2 pug-.3: H» (,9) 4- 7... 0.. (a) —9 Hv° (U ~—~\. Hess's Law (Sec. 5.6) ...
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04CHM114_0204 - CHEM 114A Spring 2004 Mon. Feb 2‘“1...

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