Thermochemistry Heats of Reaction

Transcription

1 hermchemistry Heats f Reactin aa + bb cc + dd hermchemical Semantics q V = Heat f Rxn at [V] = U = Energy (change) f Rxn q P = Heat f Rxn at [P] = H = Enthalpy (change) f Rxn Exthermic rxns: q < 0 Endthermic rxns: q > 0 Rxns can ccur with nearly infinite pssible cnditins. S, we must use a REFERENCE SAE a SANDARD SAE And relate all reactins t this state. Prf. Zvi C. Kren.07.0

2 Bmb Calrimeter: Calculatin f Mlar Heat f Cmbustin Cmbustin: Sample + O (g) Prducts + Heat Fr a pure substance: q = m C, [V,C] Fr a calrimeter apparatus: q cal = C cal, [V,C]. Calculate Calrimeter Cnstant fr the apparatus t be used: Use a standard reference sample with kwn q rxn. Measure. C cal = q rxn /. Cmbust an unknwn sample: q rxn = C cal 3. Calculate q rxn per mle f sample. 4. U = q V. 5. dh = du + d(pv) H = U + V P rxn (PV) g R(n) g. q P q V First Law Prblems: calrimeter: Prf. Zvi C. Kren.07.0

3 Relatinship Between H rxn and U rxn (als shwn befre) Recall, H U + PV H rxn U rxn + (PV) rxn H rxn U rxn + R n g Fr example, cnsider the fllwing rxn.: A(s) + B(l) + 4C(g) D(g) + E(g) H rxn U rxn + R n g H rxn U rxn + R (3-4) H rxn U rxn - R (PV) rxn = (PV) prducts - (PV) reactants = (PV) prducts(s,l,g) - (PV) reactants(s,l,g) But, fr a given quantity, V gas >> V slid,liquid PV gas >> PV slid,liquid (PV) prducts(g) - (PV) reactants(g) Assume ideal gases: PV = nr, (nr) prducts(g) - (nr) reactants(g) = R n g H = q P U = q V 3 Prf. Zvi C. Kren.07.0

4 Standard State f a Substance & Reference State f an Element he standard state f a substance at a specified temperature is its pure frm at bar f pressure. Nte: N specific temp is given; a standard state can be defined at each In lder bks, atm was used: atm =.035 bar = 0.35 kpa he reference state f an element is its mst stable state at the specified temperature and bar. EXAMPLES: he reference states fr nitrgen are the fllwing stable frms: At 5 C: N (diatmic) and a gas; At,000 C: N (mnatmic) and a gas; At -70 C: crystalline (slid) At ther temps., between mp and bp, the liquid is mst stable he reference states fr carbn are the fllwing stable frms: At 5 C: graphite (slid); At atm, graphite is mre stable than diamnd. At,000 C: C (mnatmic) and a gas; 4 Prf. Zvi C. Kren.07.0

5 Fr a physical change: H O(l) H O(g), Standard Enthalpy/Energy Changes " H vap (373K)" r " vap H (373K)" Standard enthalpy f vaprizatin f water at 00 C = the enthalpy change per mle when pure liquid water at bar vaprizes t a gas at bar Fr a chemical change: Hrxn,98.5 (CH 4) r Hcmb,98.5 (CH 4) refers t: CH 4 (g, bar) + O (g, bar) CO (g, bar) + H O(l, bar) 5 Prf. Zvi C. Kren.07.0

6 Hess s Law f Heat Summatin: Calculating Heat f Reactin, H rxn If a rxn is made up f ther rxns, then the heats are summed. Why? Because H (like U) is a state functin. Fr example: H A Reactants Prducts Prblem: H B H C H D H E () () (3) H A = H B + H C + H D + H E (Als crrect fr U, S, G,...) [Recall Brn-Haber Cycle] Find H fr rxn (), () A + D C, H =? Frm the fllwing data: () A + B 5C, H = 50 kj (3) B C + D, H 3 = -75 kj Slutin: Because rxn() = rxn() rxn(3), H = H H 3 = 50 (-75) = 00 kj. By the way, f curse it s the same fr U: U = U - U 3 But recall: K = K / K 3 6 Prf. Zvi C. Kren.07.0

7 Standard Enthalpy f Frmatin: Fr any rxn, frm Hess s Law: Elements H rxn = H RE + H EP = H E H R + H P H E = (H P H E ) (H R H E ) = H f (P) H f (R) H H rxn P H f R H f Reactants Standard Enthalpy f Frmatin f a Substance: the standard reactin enthalpy fr the frmatin f ONE mle f the cmpund frm its elements in their reference (mst stable) states. Prducts H f (element) 0 7 Prf. Zvi C. Kren.07.0

8 Calculating Heat f Reactin, H rxn frm Heats f Frmatin, H f (cntinued) Fr example, frmatin f CH 4 : C(s,gr) + H (g) CH 4 (g), H f (CH 4 ) H f = standard heat (r enthalpy) f frmatin H rxn = H f - H f Cnsider the fllwing cmbustin reactin: C 6 H 6 (l) O (g) 6 CO (g) + 3H O(l) P H cmb = 6 H f (CO ) + 3 H f (H O) - H f (C 6 H 6 ) H f (O ) he reference state fr enthalpy values f any cmpund is its elements: H f (any element) 0. R 8 First Law Prblems: thermchemistry: Prf. Zvi C. Kren.07.0

9 Gustav Rbert Kirchhff 84 (Königsberg, Prussia; nw Kaliningrad, Russia) 887 (Berlin, Germany) Kirchhff s Law: H rxn = f() Prducts H H rxn ( ) Reactants H rxn ( ) 9 Prf. Zvi C. Kren.07.0

10 Kirchhff s Law: emperature-dependence f H rxn d( Hrxn)? r H H? d Methd : Differentials. Methd : Hess s Law MEHOD : Differentials. Begin with : H H d Hrxn) d(hprducts) d(h d d d d Hrxn) d d( H ) C (rxn) d ) rxn Prducts ( Reactants CP(Prds) CP rxn Indefinite Integratin: P ( CP H Reactants (Reacts) (rxn) Kirchhff s Law d ( H rxn ) C P (rxn) d Hrxn CP (rxn) d I I cnst. f integratin Definite Integratin: H H C (rxn) d Fr example: N + 3H NH 3, and, e.g., C (rxn) CP (rxn) CP(NH 3) CP(N ) 3CP(H 0 P A B C (cnt d) Prf. Zvi C. Kren.07.0 P )

11 MEHOD : Hess s Law. aa + bb H cc + dd H (A,B) H (C, D) H aa + bb cc + dd ΔH ΔH (A, B) ΔH P,B a C A bc d ΔH cc P,C dc P,D ΔH (C, D) P, d ΔH ΔH cc C dc P,D ac P,A bc P,B P, d ΔC P d rxn (as befre with definite integratin) First Law Prblems: Kirchhff: Prf. Zvi C. Kren.07.0

12 emp. Heat Assciated with Phase Changes (Physical Prcesses) Heating Curve (Cling Curve) bp (Nte the latent heats f fusin and vaprizatin) l Vaprizatin: l v H vap (H O: 60 J/g) Cndensatin: v l, H cnd =? g mp s Fusin: s l H fus (H O: 333 J/g) Fr H O: C (s) =.06 J/g deg C (l) = 4.84 J/g deg =.0 cal/ g deg =.88 J/g deg C (g) Slidificatin (r Crystallizatin): l s, H slid (r cryst) = time Fr example: Calculate the heat ( thermal energy ) required fr the fllwing prcess: 0.0 g, (ice, -0 C) (steam, 0 C) Prf. Zvi C. Kren.07.0