Upload
martina-newman
View
222
Download
1
Embed Size (px)
Citation preview
The melting of ice is disfavored by enthalpy (+ΔH) but favored by entropy (+ ΔS). The freezing of water is favored by enthalpy (- ΔH) but disfavored by entropy (- ΔS). Below 0°C, the enthalpy term ΔH dominates the entropy term TΔS in the Gibbs free-energy equation, so freezing is spontaneous. Above 0°C, the entropy term dominates the enthalpy term, so melting is spontaneous. At 0°C, the entropy and enthalpy terms are in balance.
What are the signs of each of the thermodynamic terms for the non-
spontaneous process represented below?
Answer
• ΔH = —• ΔS = —• ΔG= ΔH - T ΔS
• Therefore when T is small (low temperature) the reaction is spontaneous.
At what temperature is the following spontaneous?
• Br2 (l) Br2 (g)
• ΔH°= 31.0 kJ/mol
• ΔS°= 93.0 J/(Kmol)
• What is the normal boiling point of bromide?
Well?
• At ΔG° = - all rxn’s are spontaneous.
• While ΔS° favors the rxn, ΔH° favors the opposite rxn, (why?) Where these opposite tendencies balance is the boiling point of bromide.
• To find this balance and the boiling point, set ΔG° to zero, equilibrium, and solve.
Answer
• ΔG°= ΔH° - T ΔS°
• 0= ΔH° - T ΔS°
• ΔH° = T ΔS° T= ΔH° / ΔS°
• T= 3.10 x 104 J/mol = 333K
93.0 J/(Kmol)
Summary
• At T > 333 K : ΔS° controls the rxn
• At T<333 K : ΔH° controls the rxn. The reaction is spontaneous in the direction that is exothermic.
• At T = 333 K: the reaction is at equilibrium, vapor and liquid coexist.
The Haber-Bosch process
• This is the reaction of nitrogen and hydrogen to produce ammonia.
• The nitrogen (N) and hydrogen (H) are reacted over an Fe catalyst under conditions of 200 atm, 450-500°C; resulting in a yield of 10-20%:
N2(g) +3H2(g) 2NH⇌ 3(g)
ΔH -92.4kJ/mol
Haber Process
• This process reduces the entropy of the system. ΔS = -
• This process is exothermic, but it has a high activation energy.
Third Law of Thermodynamics
• states that the entropy of all crystalline solids approaches zero as their temperature approaches absolute zero. In other words, all substances lose their energy at absolute zero.