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General Chemistry, 5th ed.Whitten, Davis & Peck
DefinitionsLeft click your mouse to continue.
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DIRECTIONS
• This slide show presentation is designed to function like flash cards.
• To check your answer, and/or to move on to the next slide, simply left click your mouse.
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ACTIVITY (of a component of an ideal mixture)
A dimensionless quantity whose magnitude is equal to molar
concentration in an ideal solution, equal to partial pressure (in
atmospheres) in an ideal gas mixture, and defined as 1 for pure solids or
liquids
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Created by Tara L. Moore, MGCCC
CHEMICAL EQUILIBRIUM
A state of dynamic balance in which the rates of forward and reverse
reactions are equal; there is no net change in concentrations of reactants
or products while a system is at equilibrium
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DYNAMIC EQUILIBRIUM
An equilibrium in which processes occur continuously, with no net
charge
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EQUILIBRIUM CONSTANT, K
• A quantity that indicates the extent to which a reversible reaction occurs
• Its magnitude is equal to the mass action expression at equilibrium
• K varies with temperature
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HETEROGENEOUS EQUILIBRIA
Equilibria involving species in more than one phase
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HOMOGENEOUS EQUILIBRIA
Equilibria involving only species in a single phase; all gases, all liquids, or
all solids
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LeCHATELIER’S PRINCIPLE
If a stress (change of conditions) is applied to a system at equilibrium,
the system shifts in the direction that reduces the stress
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MASS ACTION EXPRESSION
aA + bB cC + dD
Q (or K at equilibrium) = [C]c[D]d
[A]a[B]b
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REACTION QUOTIENT, Q
• The mass action expression under any set of conditions (not necessarily equilibrium)
• Its magnitude relative to K determines the direction in which reaction must occur to establish equilibrium
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REVERSIBLE REACTIONS
Reactions that do not go to completion and occur in both the
forward and reverse directions
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VAN’T HOFF EQUATION
The relationship between H0 for a reaction and its equilibrium constants at two different temperatures
ln [KT2/KT1] = (H0/R)(1/T1 – 1/T2)
Or
log [KT2/KT1] = (H0/2.303R) (1/T1 – 1/T2)
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For any reaction, the value of Kc (equilibrium constant)
• Varies only with temperature
• Is constant at a given temperature
• Is independent of the initial concentrations
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Q < K
Forward reaction predominates until equilibrium is established
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Q = K
System is at equilibrium
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Q > K
Reverse reaction predominates until equilibrium is established
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If a change of conditions (stress) is applied to a system at
equilibrium
• The system responds in the way that best tends to reduce the stress in reaching a new state of equilibrium– Concentration changes– Pressure changes (volume for gas phase)– Temperature changes– Introduction of catalysts
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When a “new equilibrium” is established
• The rates of the forward and reverse reactions are equal again
• Kc is again satisfied by the concentrations of reactants and products
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A decrease in volume (increase in pressure)
Shifts the reaction in the direction that produces the smaller number of
moles of gas
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An increase in volume (decrease in pressure)
Shifts the reaction in the direction that produces the larger number of
moles of gas
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If there is no change in the number of moles of gases in a
reaction
A volume (pressure) change does not affect the position of equilibrium
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An increase in temperature
Favors endothermic reactions
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A decrease in temperature
Favors exothermic reactions
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n = ?
n = (ngas prod) – (ngas react)
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Kp = ?
Kp = Kc(RT) n
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Kc = ?
Kc = Kp (RT)- n
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For pure solids or liquids
The activity is taken as 1, so terms for pure liquids and pure solids do not
appear in the K expression for heterogeneous equilibria
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G0 = ?
G0 = -RT ln K
Or
G0 = -2.303 RT log K
R = 8.314 J/(mol*K)
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For equilibria that involve only gases
The thermodynamic equilibrium constant (related to G0) is Kp
For those that involve species in solution, it is equal to Kc
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G0 < 0; K > 1
Products favored over reactants at equilibrium
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G0 = 0; K = 1
At equilibrium when [C]c[D]d…=[A]a[B]b…. (very rare
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G0 > 0; K < 1
Reactants favored over products at equilibrium