Ch. 6: Equilibrium (Complete) Flashcards
defn: irreversible reaction
the reaction proceeds in one direction only, goes to completion, and the max amount of product formed is determined by the amount of limiting reagent initially present
defn: reversible reaction
those in which the reaction can proceed in one of two ways: forward (toward the products/right) and reverse (toward the reactants/left)
why do reversible reactions not usually proceed to completion?
the products can react together to reform the reactants
defn: dynamic equilibrium
the system eventually settles into a state in which the forward reaction rate equals the reverse reaction rate and the concentrations of the products and reactants remain constant
this occurs when the reaction system is closed and no reactants or products are added or removed
what is the difference between dynamic and static equilibrium?
dynamic: forward and reverse reactions are still occurring, just at the same rate
static: forward and reverse reactions have stopped
defn: entropy
the measure of the distribution of energy throughout a system or between a system and its environment
for a reversible reaction at a given temperature, the reaction will reach equilibrium when WHAT is true about the system’s entropy? and WHAT is true about the Gibbs free energy?
when the entropy is at maximum
Gibbs free energy is at a minimum
defn: law of mass action
if the system is at equilibrium at a constant ratio, then the following ratio is constant
Keq = [C]^c [D]^d / [A]^a [B]^b
for a generic reversible reaction aA + bB <–> cC + dD
defn: Kc
equilibrium constant
c indicates that it is in terms of concentration
defn: Kp
equilibrium constant when dealing with gases
p indicates that it is in terms of pressure
how do you find the equilibrium constant for an overall reaction when a reaction occurs in more than one step?
multiply together the equilibrium constants for each step
when this is done, the equilibrium constant for the overall reaction is equal to the concentrations of the products divided by the concentrations of the reactants in the overall reaction with each concentration term raised to the stoichiometric coefficient for the respective species
func + defn: reaction quotient, Q
serves as a timer to indicate how far the reaction has proceeded toward equilibrium
can calculate at any time during a reaction
explain the meaning of Q < Keq
Q = Keq
Q > Keq
Q < Keq: the forward reaction has not yet reached equilibrium
- there is a greater concentration of reactants (and smaller concentration of products) than at equilibrium
- the forward rate of reaction is increased to restore equilibrium
- deltaG < 0
Q = Keq: the reaction is in dynamic equilibrium
- the reactants and products are present in equilibrium proportions
- the forward and reverse rates of reaction are equal
Q > Keq: the forward reaction has exceeded equilibrium
- there is a greater concentration of products (and smaller concentration of reactants) than at equilibrium
- the reverse rate of reaction is increased to restore equilibrium
- deltaG > 0
once a reaction is at equilibrium, what motions of the reaction would be classified as NONspontaneous? (2)
- any further movement in the forward direction (resulting in an increase in products)
- any further movement in the reverse direction (resulting in the reformation of reactants)
What are the 4 main characteristics of the law of mass action and equilibrium constant expressions to keep in mind?
- the concentrations of pure solids and pure liquids do NOT appear in the equilibrium constant expression
- Keq is characteristic of a particular reaction at a given temperature (Keq is temperature-dependent)
- the larger the value of Keq, the farther to the right the equilibrium position
- if the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reverse reaction is 1/Keq
what does it mean physically if Keq > 1?
Keq < 1?
Keq > 1 –> concentration of products is greater than the concentration of reactants
Keq < 1 –> concentration of reactants is greater than that of products
what information do the exponents in Keq give us?
what does a large, positive exponent tell us?
what does a large, negative exponent tell us?
the sign and scale of these exponents gives us info about the relative quantities of reactants and products
large positive exponent: strongly favors products, less reactant will be present at equilibrium, the reaction goes almost to completion
large negative exponent: strongly favors reactants at equilibrium, only a small amount of reactant is converted to product
what is a shortcut we can take when performing equilibrium calculations with a Keq with a large negative exponent?
the amount that has reacted can be considered negligible compared to the amount of reactant that remains
defn: Le Chatelier’s principle
if a stress is applied to a system, the system shifts to relieve that applied stress
regardless of the form the stress takes, the reaction is temporarily moved out of its equilibrium state
the reaction then responds by reacting in whichever direction will result in a reestablishment of the equilibrium state
the system will always react in the direction away from the added species or toward the removed species
what are the three types of stresses that will enact Le Chatelier’s principle?
- concentration OR 2. partial pressures of the system are no longer in the equilibrium ratio
- the equilibrium ratio itself has changed as a result of a change in the temperature of the system
for changes in concentration: what happens if reactants are added? products are removed? reactants are removed? products are added?
reactants added = products removed = Qc < Keq = reaction will spontaneously react forward increasing Qc until Qc = Keq
reactants removed = products added = Qc > Keq = reaction will spontaneously react reverse decreasing Qc until Qc = Keq
in what circumstance do we take advantage of Le Chatelier’s principle? what are 2 ways we tangibly do that?
to improve the yield of chemical reactions
- products are removed as they are formed to prevent reactions from reaching equilibrium
- start with high concentrations of reactants which will lead to an absolute quantity of products formed, although the reaction will still reach equilibrium
discuss Le Chatelier’s principle in terms of changes in pressure and volume. what happens when a system is compressed?
which direction does the reaction proceed?
- its volume decreases
- its total pressure increases
- the increase in total pressure is associated with an increase in the partial pressures of each gas in the system
- thus the system is no longer in the equilibrium state
- the system will move forward or in reverse, always toward whichever side has the LOWER total number of moles of gas
discuss Le Chatelier’s principle in terms of changes in pressure and volume. what happens when a system’s volume is expanded?
which direction does the reaction proceed?
- volume increases
- total pressure and partial pressures decrease
- system no longer in equilibrium
- system will react in whichever direction has the side with the GREATER number of moles of gas in order to restore the pressure
where does Le Chatelier’s principle as applied to changes in pressure and volume come from?
the ideal gas law: there is a direct relationship between the number of moles of gas and the pressure of the gas
discuss Le Chatelier’s principle in terms of changes in pressure. What happens if the pressure of the system is increased? decreased?
pressure increased = system will react in the direction that produces FEWER moles of gas
pressure decreased = system will react in the direction that produces more moles of gas
how is the result of changes in pressure, volume, and concentration different from the result of changes in temperature?
changes in pressure, volume, and concentration affect the reaction quotient Qc or Qp
changes in temperature affect the equilibrium constant Keq
what direction does the system move when there is a change in temperature? how is that determined?
whichever direction allows it to reach its new equilibrium state at the new temperature
determined through the enthalpy of the reaction
if rxn endothermic –> heat functions as a reactant
if rxn exothermic –> heat functions as a product
then apply the same processes
defn: kinetic vs. thermodynamic product
which requires more free energy? which form faster? which are more stable?
kinetic product is formed at lower temperatures with smaller heat transfer –> free energy that must be added is lower, form faster
thermodynamic product is formed at higher temperatures with larger heat transfer –> more stable, more negative delta G
