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Chemical Reaction
Equilibrium
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A Combination of Theories• Chemical reaction equilibrium
combines ideas from atomic theory, kinetic molecular theory, collision reaction theory and the concept of reversibility and dynamic equilibrium
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Hydrogen-Iodine Reaction System
• Initially, hydrogen and iodine are placed in a flask and the iodine purple is the only visible color
H2(g) + I2(g) 2HI(g)
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H2(g) + I2(g) 2HI(g)
• Early on, H2 and I2 combine to form HI faster than HI forms H2 and I2. • Overall the amount
of I2 decreases so the color lightens
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H2(g) + I2(g) 2HI(g)
• At equilibrium the flask contains all 3 substances.• The purple shows that some iodine
remains• The constancy of the color shows
equilibrium is occurring• Forward and reverse reactions are
occurring at equal rates
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Equilibrium• The state in which both reactants
and products are present at concentrations which have no further tendency to change with time. Usually this state results when the forward reaction proceeds at the same rate as the reverse reaction
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Dynamic Equilbrium
• A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants and products, but substances move between the chemicals at an equal rate, meaning there is no net change
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Static Equilibrium
• A system of particles is in static equilbrium when all the particles of the system are at rest and the total force on each particle is permanently zero
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Equilibrium Expressions• Use the general reaction:
aA + bB cC + dD
A and B are reactantsC and D are products
Lower case letters are coefficients
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Kc = [products]
[reactants]
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Kc = [C]c [D]d
[A]a [B]b
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Kc = equilibrium constant• each Kc is for 1 specific temp• all substances are assumed to
be present or their concentration will be 0• Kc may be large or small but
never equal to zero
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• The value of Kc depends on the temp and on large changes in the equilibrium concentration of reactants or products
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• The value of Kc can be used to estimate whether reactants or products are favored at equilibrium
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Quantitative Reaction• Kc is very large (Kc > 10)• Percent reaction is >99%• product concentrations are
greater than reactants
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Favors Products• Kc is large (Kc > 1.0)• Percent reaction is >50%• product concentrations are
larger than reactants
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Favors Reactants• Kc is small (Kc < 1.0)• percent reaction <50%• reactant concentrations are
larger than products
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No Favor• Kc = 1.0• equilibrium contains 50%
products• percent reaction = 50%
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General Rules of Kc• Solids are omitted because
concentrations don’t change• Liquids are omitted (if there is
one as an aqueous solution)• Water is not included because
concentration does not change
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• If there is more than one liquid, include them all in the expression• Include all other states• A complete description of
equilibrium states includes the temp, composition and concentration of all entities
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Example 1
• Write the equilibrium equation for the combustion of ethanol:
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Example 2
• For the reaction of carbon and oxygen to produce carbon dioxide, the following equilibrium concentrations were measured:
[C(g)] = 0.50M [O2(g)] = 0.80M [CO2(g)] = 0.30M