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Calculating DH°, DS°, and DG° using Hess's Law and State Function

Learn how to calculate DH°, DS°, and DG° using Hess's Law and State Function. Understand the signs and calculations for each thermodynamic parameter.

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Calculating DH°, DS°, and DG° using Hess's Law and State Function

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  1. Ba(OH)2(s) 8H20(s) + 2 NH4SCN(s) → Ba2+(aq)+ SCN-(aq) + 2NH3(aq) + 10 H20(l) Feels very cold!!!! What is the sign on DH°? + How can you calculate DH°? Hess’s Law What is the sign on DS°? +

  2. A(s)→ A(g)DS = + Order of entropy from phase perspective…. solid < liquid < aqueous < gas

  3. A(g)→ 2B(g)DS = + Fewer gas molecules are less entropic than a larger number of gas molecules

  4. A(s)+ 2B(g)→ C(s) +2D(g)DS = + Where D is a more complex gas than B Complex gas molecules are more entropic than simple gas molecules

  5. Ba(OH)2(s) 8H20(s) + 2 NH4SCN(s) → Ba2+(aq)+ SCN-(aq) + 2NH3(aq) + 10 H20(l) What is the sign on DH°? + How can you calculate DH°? Hess’s Law What is the sign on DS°? + How can you calculate DS°? State Function…use summation of P - R What is the sign on DG°? - Because it occurs… State Function…use summation of P - R How can you calculate DG°? DG°= DH° - TDS° or

  6. G = G° + RT ln P A(g)→ 2 B(g) GB= G°B + RT ln PB GA= G°A + RT ln PA …that means DG= 2GB - GA DG= 2(G°B + RT ln PB) - (G°A + RT ln PA)

  7. DG= 2(G°B + RT ln PB) - (G°A + RT ln PA) DG= 2G°B + 2RT ln PB - (G°A + RT ln PA) DG= 2G°B – G°A + RT (2 ln PB- ln PA) DG°react. (ln PB2- ln PA) DG= DG°react. + RT ln (PB2 / PA) DG= DG°react. + RT ln Q

  8. 1.0 x 10-4 atm 1.0 x 10-2 atm 2 H2S(g) + SO2(g)↔ 3S(rhombic) + 2 H2O(g) 3.0 x 10-2 atm At 298 K Find DG° = SnDG°f(prod.) - SnDG°f(react.) = -90 kJ K (at eq.) Find DG = DG° + RT ln Q = -90 kJ + RT ln (9 x 106) 0 (at eq.) = -90 kJ + 40 kJ = -50 kJ DG° = - RT ln K (spont.!) Find K = eDG°/-RT = e36.3 = 6.1 x 1015

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