Thermochemistry. Renee Y. Becker Valencia Community College CHM 1046. Enthalpies of Chemical Change: Often called heats of reaction ( D H ) units kJ Endothermic: Heat flows into the system from the surroundings and D H has a positive sign .
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Renee Y. Becker
Valencia Community College
Endothermic:Heat flows into the system from the surroundings and DH has a positive sign.
Exothermic:Heat flows out of the system into the surroundings and DH has a negative sign.
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) DH = –2219 kJ
CH3Cl(g) + HCl(g) CH4(g) + Cl2(g) DH = 98.3 kJ
DS = Sfinal – Sinitial
Positive value of DS indicates increased disorder.
Negative value of DS indicates decreased disorder.
Spontaneous process: Decrease in enthalpy (–DH). Increase in entropy (+DS).
Nonspontaneous: Increase in enthalpy (+DH). Decrease in entropy (–DS).
DG = DH – TDS
a. AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) DG° = –55.7 kJ
b. 2 C(s) + 2 H2(g) C2H4(g)DG° = 68.1 kJ
c. N2(g) + 3 H2(g) 2 NH3(g) DH° = –92 kJ; DS° = –199 J/K
Equilibrium is the point where DG = DH – TDS = 0
T = DH / DS
DH = 92.0 kJ DS = 199 J/K
Is this reaction spontaneous @ 25C? If not, at what temperature does this reaction become spontaneous?