when water vapor condenses: H-bonds are formed, and energy is released; the reaction is
exothermic (ΔH is negative) and entropy decreases, since a liquid is forming from a gas (TΔS is
negative). Condensation will be spontaneous only if ΔH < TΔS. This is the case at temperatures
below 100°C; above 100°C, TΔS is more negative than ΔH, ΔG is positive, and condensation is not
spontaneous. Again, at 100°C, an equilibrium is established.
2C 6 H 6 (l) + 15O 2 (g) → 12CO 2 (g) + 6H 2 O (g) + heatIn this case, heat is released (ΔH is negative) as the benzene burns and the entropy is increased (TΔS
is positive), because two gases (18 moles total) have greater entropy than a gas and a liquid (15
moles gas and 2 liquid). ΔG is negative and the reaction is spontaneous.
THINGS TO REMEMBER
b. The combustion of C 6 H 6 (benzene)The First Law of Thermodynamics
Specific Heat and Heat Capacity
Calorimetry
Enthalpy
Hess’s Law
Entropy
The Second Law of Thermodynamics
The Third Law of Thermodynamics
Gibbs Free Energy