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Chemical Equilibrium

GLY 4241 - Lecture 5

Fall, 2014

Chemical Reaction

• A chemical reaction may be defined as one or more substances, known as reactants, combining chemically to form one or more different substances, known as products.

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Reactants to Products

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Reaction Completeness

• How far can the reaction proceed? Almost complete

Negligible

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Speed of Reaction

• How fast does the reaction proceed?

Very slow, unless ignited This is a kinetic problem, not a thermodynamic

one

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Law of Mass Action Formulation

• First formulated by two Norwegian chemists, Cato Maximilian Guldberg & Peter Waage

• When the driving forces of the forward and back reactions become equal, equilibrium has been achieved

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Forward Reaction

• Driving force of the forward reaction = kf[NaCl]

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Back Reaction

• Driving force of the back reaction = kb[Na+][Cl-]

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Equilibrium

• kf[NaCl] = kb[Na+][Cl-]

• K is called the equilibrium constant for the equation as written

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Law of Mass Action

• The rate of a reaction is directly proportional to the concentration of each reacting substance (Konrad Krauskopf)

• Concentration is a better term than mass

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Problems

• Changing the equation requires rewriting the equilibrium expression

• Kf and kb are not physically measurable

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Disturbance of Equilibrium

• If the system is at equilibrium, and some component B is added, so that[B] increases, what happens to the system?

• The disturbance can also be a change in temperature or pressure, since K is a function of both

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Henri Louis Le Chatelier

• French chemist, 1850-1936• Formulated the principle

now named in his honor

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Le Chatelier’s Principle - 1884

• “Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause.”

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Le Chatelier’s Principle - 1888

• Le Chatelier restated his principle as “Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change”

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Formation of Water

• Introducing a spark generates free radicals

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Thermodynamic Criteria for Equilibrium

• If ΔG < 0, the reaction will take place spontaneously, although the rate may be extremely small

• If ΔG = 0, the reaction is at equilibrium

• If ΔG > 0, the reaction cannot take place without energy being supplied from outside the system

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Kinetic Barrier

• High kinetic barriers lead to metastable systems, which are not at equilibrium

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Metastable Situations in Geology

• Many high pressure minerals, formed at great depths within the earth, do not immediately revert to the lower pressure polymorphs when they reach the surface Diamond and graphite Closer to the surface, aragonite and calcite

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Generalized Equilibrium Constant

• The equilibrium constant, K, may be defined for a general system as follows:

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Formation of Water - 1

• K is specific to a reaction

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Formation of Water - 2

• Another formulation, with a new equilibrium constant

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Formation of Water - 3

• Still another formulation

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Solubility Product

• A solubility product (Ksp) is a special type of equilibrium constant• Very useful for environmental studies

• What is the concentration of a solid? It is effectively a constant, so we may set it equal to one. Then:

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Solubility • The solubility in moles/liter is equal to [Pb2+] or

[SO42-], since one ion of each is produced when the

PbSO4 molecule dissolves

• Similarly, the solubility product for galena, PbS, is 10-27.5

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Common Ion Effect

• What happens if galena (PbS) is added to a saturated solution of anglesite (PbSO4)?

• X = solubility of galena

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Using an Approximation

• This could be solved as a quadratic equation

• However, X will be less than the solubility of pure galena

• In the presence of anglesite, the solubility of galena is greatly reduced

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Check the Approximation

• With X = 2.4 x 10-24, is the above equation true?