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Chapter 16. Spontaneous processes Occur without assistance Fast or slow Thermodynamics predicts if reactions will occur but only focuses on the initial and final states Entropy (S) Randomness of a system. S(gas) >> S(liq.) > S(solid) 1 st law of thermodynamics - PowerPoint PPT Presentation
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Chapter 16
• Spontaneous processes– Occur without assistance– Fast or slow
– Thermodynamics predicts if reactions will occur but only focuses on the initial and final states
• Entropy (S)– Randomness of a system
S(gas) >> S(liq.) > S(solid)
• 1st law of thermodynamics– Energy of the universe is conserved
• 2nd law of thermodynamics– Entropy of the universe increases for any
spontaneous process Suniv = Ssys + Ssurr
Suniv = + spont. Suniv = - spont. In opposite direction
• Effect of temperature– H2O (l) H2O (g) more entropy as gas Ssys = + Ssurr = - Ssurr depends on the H and T
Ssurr = - H / T T must be in Kelvin
• Free Energy (G)
G = H - TS all are system
G = - spont. = + rev. spont. = 0 equil.
G / T = - H / T + S
- G / T = Ssurr + S = Suniv
– So G has to be neg. to give a pos. Suniv
– From the equation we can predict when reactions will be spontaneous given H and S
G = H - TS
H S G
+ + - at high T
+ - +
- - - at low T
- + -
• Entropy in chemical reactions– We can predict entropy for reactions based on
the states of the substances and/or the amount of gas particles present.
– Na2CO3(s) Na2O(s) + CO2(g) + S– 2H2(g) + O2(g) 2H2O(g) - S
• 3rd law of thermodynamics– Entropy of a perfect crystal at 0K is zero
• We can calculate S by using the following equationSo =npSoprod - nrSoreact
Unlike H all chemicals will have a value
Pg 800
• Free energy and chemical reactions– Standard Free Energy Change Go
Go = Ho - TSo
What’s the free energy for an exothermic reaction that releases 45 kJ of energy and has an increase in entropy of 250 J at 300K?
Go = -45000 J – (300K)(250J) = -120000 J
– “Hess’s Law” method -- add reactions together to get an overall reaction and add the Go to get an overall Go
– Calculate the Go for the following reaction
– C diamond + O2 (g) CO2 (g) Go = -397 kJ– CO2 (g) C graphite + O2 (g) Go = 394 kJ
– C diamond C graphite
Go = -397 kJ + 394 kJ = -3 kJ
– Standard free energy of formation Gof
Gof for elements is zero
Go = np Gof prod - nr Gof react
2CH3OH (g) + 3O2 (g) 2CO2 (g) + 4H2O (g)Go = ?
Go = 2n(-394kJ/n)+4n(-229kJ/n)-3n(0)-2n(-163kJ/n)
Go = -1378 kJ
• Dependence on pressure– Since pressure effects entropy it also effects
free energy– G = Go + RT ln(P) R = 8.3145 J/nK
G = Go + RT ln(Q) Q = (Ppa / Prb)
• Meaning of Free Energy G tells us which side of a reaction is favored
but it doesn’t tell us that the reaction will go to completion.
– The lowest G is at the equilibrium point
• Free energy and Equilibrium G = Go + RT ln(Q)– At equilibrium G = 0 and Q = K
– 0 = Go + RT ln(K) Go = - RT ln(K)
Go = 0 then K = 1 and no shift will occur Go = - then K > 1 and the reaction shifts right Go = + then K < 1 and the reaction shifts left
Free energy and work
wmax = G