CHMB16H3 Chapter Notes - Chapter 6: Enthalpy, Gas Constant, Sodium Hydroxide

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30 Oct 2011
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Chapter 6: Chemical Equilibrium
The Equilibrium Constant
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant K = [C]c[D]d/[A]a[B]b
A reaction is favored whenever K > 1.
When you evaluate an equilibrium constant,
oConcentrations of solutes should be expressed as moles per liter.
oConcentrations of gases should be expressed in bars.
oConcentrations of pure solids, pure liquids and solvents are omitted because
they are unity.
If the direction of a reaction is reversed, the new value of K is simply the reciprocal of
the original value of K; K1 = 1/K1.
If two reactions are added, the new K is the product of the two individual values; K3 =
K1K2.
If n reactions are added, the overall equilibrium constant is the product of n
individual equilibrium constants.
Equilibrium and Thermodynamics
Enthalpy Change (∆H): the heat absorbed or released when the reaction takes place
under constant applied pressure.
oStandard Enthalpy Change (∆Hᵒ): the heat absorbed when all reactants and
products are in their standard states.
o∆H is positive, heat is absorbed (endothermic).
o∆H is negative, heat is liberated (exothermic).
Entropy (S): a measure of the substance’s disorder.
o∆S is positive, products are more disordered than reactants.
o∆S is negative, products are less disordered than reactants.
oIn general, a gas is more disordered than a liquid, which is more disordered
than a solid.
o∆Sᵒ is the change in entropy when all species are in their standard states.
Gibbs Free Energy (∆G): ∆G = ∆H - T∆S
oA reaction is favored if ∆G is negative (spontaneous) or when K > 1.
oWhen ∆H is negative and ∆S is positive, the reaction is clearly favored.
oWhen ∆H is positive and ∆S is negative, the reaction is clearly disfavored.
oFree Energy and Equilibrium: K = e-∆Gᵒ/RT, where R is the gas constant (8.314
J/K mol) and T is temperature.
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Document Summary

The equilibrium constant: a reaction is favored whenever k > 1, when you evaluate an equilibrium constant, If the direction of a reaction is reversed, the new value of k is simply the reciprocal of the original value of k; k". If two reactions are added, the new k is the product of the two individual values; k3 = If n reactions are added, the overall equilibrium constant is the product of n individual equilibrium constants. Entropy (s): a measure of the substance"s disorder: s is positive, products are more disordered than reactants, s is negative, products are less disordered than reactants. o. J/k mol) and t is temperature: the more negative the g , the larger the equilibrium constant. If q > k, the reaction must go to the left to decrease the numerator and increase the denominator until q = k. If a reaction is at equilibrium and products are added, the reaction goes to the left.

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