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 .
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.
Are the following reactions endothermic or exothermic? Favorable or unfavorable?
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.
Predict whether DS is likely to be positive or negative for each of the following reactions.
Spontaneous process:Decrease in enthalpy (–DH).Increase in entropy (+DS).
Nonspontaneous:Increase in enthalpy (+DH).Decrease in entropy (–DS).
DG = DH – TDS
Which of the following reactions are spontaneous under standard conditions at 25°C?
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
2 NH3(g) N2(g) + 3 H2(g)
DH = 92.0 kJ DS = 199 J/K
Is this reaction spontaneous @ 25C? If not, at what temperature does this reaction become spontaneous?