1. Renee Y. Becker Valencia Community College CHM 1046
Thermochemistry
Renee Y. Becker
Valencia Community College
CHM 1046

2. Enthalpies of Chemical Change: Often called heats of reaction (DH) units kJ
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.

3. Are the following reactions endothermic or exothermic
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

4. Entropy
The measure of molecular disorder in a system is called the system’s entropy; this is denoted S.
Entropy has units of J/K (Joules per Kelvin).
DS = Sfinal – Sinitial
Positive value of DS indicates increased disorder.
Negative value of DS indicates decreased disorder.

6. Entropy
Second Law of Thermodynamics: Reactions proceed in the direction that increases the entropy of the system plus surroundings.
A spontaneous process is one that proceeds on its own without any continuous external influence.
A nonspontaneous process takes place only in the presence of a continuous external influence.

7. Predict whether DS is likely to be positive or negative for each of the following reactions.
a. 2 CO(g) + O2(g)  2 CO2(g)
b. 2 NaHCO3(s)  Na2CO3(s) + H2O(l) + CO2(g)
c. C2H4(g) + Br2(g)  CH2BrCH2Br(l)
d. 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g)

8. To decide whether a process is spontaneous, both enthalpy and entropy changes must be considered:
Spontaneous process: Decrease in enthalpy (–DH). Increase in entropy (+DS).
Nonspontaneous: Increase in enthalpy (+DH). Decrease in entropy (–DS).

9. Free Energy
Gibbs Free Energy Change (DG): Weighs the relative contributions of enthalpy and entropy to the overall spontaneity of a process.
DG = DH – TDS
DG < 0 Process is spontaneous
DG = 0 Process is at equilibrium
DG > 0 Process is nonspontaneous

10. 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

11. Equilibrium (DG = 0): Equilibrium is the point where DG = DH – TDS = 0
Estimate of the temperature at which a reaction becomes spontaneous (set DG =0)
T = DH / DS

12. 2 NH3(g)  N2(g) + 3 H2(g) DH = 92.0 kJ DS = 199 J/K
Is this reaction 25C? If not, at what temperature does this reaction become spontaneous?