化學平衡
相關概念:
平衡(equilibrium), 動態(dynamic);
正逆反應, 速率, 平衡常數;
濃度(concentration): 體積, 重量, 分率;
標準狀態;
Gibbs 能量;
氣相, 液相, 固相, 均勻, 不均勻;
理想, 不理想, 活性, 活性係數;
飽和溶液, 溶解度, 溶解度積.
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Chemical equilibrium
---Dynamical equilibrium (2.1.1, 2.1.2)
In 1851, Alexander William Williamson
(1824-1904, British chemist)
after a study of esterifications
argued: at equilibrium, reaction is still
occurring in forward and reverse directions, the
rates being the same in the two directions.
In 1862, Pierre Eugene Michellin Berthelot and
Pean de St. Gilles (French chemists)
carried out the first quantitative work on
chemical equilibrium
CH3COOH + C2H5OH = CH3COOC2H5 + H2O
Kc = [CH3COOC2H5][H2O]
/[CH3COOH][C2H5OH]
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During 1874-1878, Willard Gibbs
American physicist
gave the first thermodynamic treatment of
equilibrium in chemical system
In 1887, Jacobus Henricus van't Hoff
Dutch chemist
[1874 tetrahedral carbon atom]
presented a derivation of the equilibrium
constant.
[1901 the first Nobel Prize in chemistry
because of his discovery of
the laws of chemical dynamics and of
the osmotic pressure in solutions]
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---Chemical equilibrium involving ideal gases
For a gas-phase reaction (2.1.3, 2.1.5)
aA + bB = yY + zZ
--------------------------------------------------G = nRT ln (V1/V2) = nRT ln (P2/P1)
P1 = 1 bar
G = nRT ln P2u (u = unitless)
Go = the Gibbs energy at 1 bar
Gm = Gmo + RT ln Pu
--------------------------------------------------G = yGmY + zGmZ - aGmA - bGmB
= yGmYo + zGmZo - aGmAo - bGmBo
+ RT ln (PYyPZz/PAaPBb)u
= Go + RT ln (PYyPZz/PAaPBb)u
If PA, PB, PY and PZ correspond to equilibrium
pressures, then G = 0.
Go = -RT ln (PYyPZz/PAaPBb)equ
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= -RT ln KPo
KPo = the thermodynamic equilibrium constant
G = -RT ln KPo
+ RT ln (PYyPZz/PAaPBb)u
In concentration units (2.1.4)
[standard state: 1 mol L-1]
P = nRT/V = cRT
KPo = (cYycZz/cAacBb)eq (RT)y+z-a-b
= Kc (RT)y+z-a-b
Go = -RT ln Kco
G = Go + RT ln ([Y]y[Z]z/[A]a[B]b)u
In mole fractions (2.1.4)
[standard state: unit mole fraction]
PA = xAP
KPo = (xYyxZz/xAaxBb)eq Py+z-a-b
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= Kxo Py+z-a-b
Go = -RT ln Kxo
G = Go + RT ln (xYyxZz/xAaxBb)
---Chemical equilibrium involving nonideal gases
(2.2.13)
Go = -RT ln (aYyaZz/aAaaBb)equ
= -RT ln Kao
a : the activity
G = -RT ln Kao
+ RT ln (aYyaZz/aAaaBb)u
---Chemical equilibrium in solution (2.2.13)
G = Go + RT ln au
= Go + RT ln mu
 : the activity coefficient
m : mol kg-1
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Kao = (aYyaZz/aAaaBb)equ
= (mYymZz/mAamBb)u(YyZz/AaBb)u
For uncharged species in solution, the behavior
is often close to ideal, and equilibrium constants
are expressed in terms of m, c and x.
For ions, activities must be used.
For solvent species,
G = Go + RT ln x1f1
---Heterogeneous equilibrium (2.2.6-8)
CaCO3(s) = CaO(s) + CO2(g)
[CaO][CO2]/[CaCO3] = Kc'
[CaO] = constant
[CaCO3] = constant
Incorporating concentrations for pure solids
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and liquids into the equilibrium constant
[CO2] = Kc
a(CO2) = Kc more exactly
AgCl(s) = Ag+(aq) + Cl-(aq)
[Ag+][Cl-]/[AgCl] = Kc'
[AgCl] = constant
[Ag+][Cl-] = Ksp
the solubility product
a(Ag+)a(Cl-) = Ksp
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