Ch. 14 pt 1 - Chemical Equilibrium: Equal but Opposite Reaction Rates
how to alter the partial pressures of reactants and products (LeChatelier)
change the VOLUME of the reaction mixture while keeping the temperature CONSTANT (this is an ISOTHERMAL process)
law of mass action
chemical law stating that the rate of a reaction is proportional to the concentration of the reacting substances the relationship between the balanced chemical equation and the expression of the equilibrium constant the principle relating the balanced chemical equation of a reversible reaction to its mass action expression
mass action expression
equivalent to the equilibrium constant expression, but applied to reaction mixtures that may or may not be at equilibrium
studies of equilibrium reveal only the extent to which a reaction proceeds, NOT:
how rapidly it proceeds to equilibrium
in heterogeneous equilibria, what states do we include/not include in our equilibrium expression?
include concentrations of gases/aqueous materials do NOT include concentrations of liquids/solids
how do we derive the equilibrium constant K?
reactant ⇄ product K comes from k(f)/k(r) where k(f) is the RATE constant of the FORWARD reaction (reactant → product) and k(r) is the RATE constant of the REVERSE reaction (reactant ← product)
why is chemical equilibrium a dynamic process?
reactants form products and products form reactants at equal rates
K(c) of reverse equilibrium ex: K(c) = 100 for A + B ⇄ 2C; what is K(c) for 2C ⇄ A + B
reciprocal: K(forward) = 1/K(reverse) in this example, K(c) would be 1/100
LeChatelier - adding a product
shift equilibrium AWAY FROM products
LeChatelier - adding a reactant
shift equilibrium AWAY FROM reactants
LeChatelier - increasing temperature
shift equilibrium AWAY from the side that is increased if ENDOthermic: shift to form more product if EXOthermic: shift to form more reactant
LeChatelier - removing a product
shift equilibrium TOWARD products
LeChatelier - removing a reactant
shift equilibrium TOWARD reactants
LeChatelier - decreasing temperature
shift equilibrium TOWARD the side that is increased if ENDOthermic: shift to form more reactant (shift away from product) if EXOthermic: shift to form more product (shift away from reactant)
why can we use balanced chemical equation to write the equilibrium constant expression? why is this different from rate laws?
the composition of an equilibrium mixture of reactants and products does not depend on the terms and exponents of the forward and reverse rate laws or on how rapidly the reaction proceeds — the value of an equilibrium constant K says nothing about how rapidly a reaction proceeds toward equilibrium; it tells us only the extent to which the reaction proceeds before it reaches equilibrium This is in contrast to rate law expressions, in which the exponents are often not the same as the coefficients in the balanced equation, because rate law exponents reflect the stoichiometry of the rate-determining step, not necessarily the stoichiometry of the overall reaction
equilibrium constant for overall reaction
the overall equilibrium constant for a sum of two or more reactions is the product of the equilibrium constants of the individual reactions *K(overall) = K1 × K2 × ⋯ × Kn* K1: equilibrium constant for 1st elementary step K2: equilibrium constant for 2nd elementary step ⋮ Kn: equilibrium constant for nth elementary step K(overall): equilibrium constant for overall rxn
Le Chatelier's Principle
the principle that a system at equilibrium responds to stress in such a way that it relieves that stress states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress a system at equilibrium adjusts to MINIMIZE any stress applied to it
what information does the reaction quotient value provide us with
the value of Q provides us with a kind of status report on how a reaction is proceeding
equilibrium constant
(K) the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation the value of the ratio of concentration (or partial pressure) terms in the equilibrium constant expression AT A SPECIFIC TEMPERATURE
reaction quotient
(Q) numerical value of the mass action expression for any values of concentrations (or partial pressures) of the reactants and products Ratio of the concentrations of the products to the concentrations of the reactants at any point during the reaction aside from equilibrium, where each reactant and product in the expression is raised to the power of its stoichiometric coefficient. Commonly denoted by Q.
LeChatelier - compressing the reaction mixture
(decreasing volume, increasing pressure ON ENTIRE SYSTEM) shift equilibrium in the direction that REDUCES total MOLES of gas (to reduce P) → shift equilibrium right
how to use a RICE table after setting it up
(for notes on SET UP, go to card "how to set up a RICE table") 1. substitute the terms from the E row into the K expression for the reaction 2. cross multiply to get rid of the fraction 3. after distributing out terms/combining like terms, should get an equation that generally fits the form of a quadratic ax^2 + bx + c = 0 4. use quadratic formula to solve for x: x = [-b ± √(b^2 - 4ac)] / 2a 5. plug in possible values of x back into E rows of table → use the x that yields ONLY POSITIVE results all across table (partial pressure/conc can't be negative) 6. *IF |x| < 0.05(initial conc) → ignore ±x in the overall K equation* → rewrite equation to NOT include the ±x terms → solve for x again
LeChatelier - expanding the reaction mixture
(increasing volume, decreasing pressure ON ENTIRE SYSTEM) shift equilibrium in the direction that INCREASES total MOLES of gas (to reduce P) → shift equilibrium left
LeChatelier - increasing partial pressure on reactant or product
(same direction as concentration shifts) shift AWAY FROM side that is increased shift the equilibrium so that more of the substance is consumed in the reaction
LeChatelier - decreasing partial pressure on reactant or product
(same direction as concentration shifts) shift TOWARD side that is decreased shift the equilibrium toward the production of more of that substance
LeChatelier - increasing concentration of reactant or product
(same direction as partial pressure shifts) shift AWAY FROM side that is increased shift the equilibrium so that more of the substance is consumed in the reaction
LeChatelier - decreasing concentration of reactant or product
(same direction as partial pressure shifts) shift TOWARD side that is decreased shift the equilibrium toward the production of more of that substance
Q vs K
(specifically referring to Q(c) and K(c) but same applies to partial pressures) Q < K: smaller concentrations of products and larger concentrations of reactants are in the initial reaction mixture than will be present at equilibrium → to achieve equilibrium, some of the reactants must form products, increasing the value of Q until it matches K → reaction as written proceeds in FORWARD direction Q = K: no change in concentrations of reactants and products because currently at equilibrium Q > K: larger concentrations of products and smaller concentrations of reactants are in the initial reaction mixture than will be present at equilibrium → to achieve equilibrium, some of the reactants must form reactants, decreasing the value of Q until it matches K → reaction as written proceeds in REVERSE direction
how to set up a RICE table
*R:* reaction ex: N2(g) + O2(g) ⇄ 2 NO(g) *I:* initial concentration (or partial pressure) - write the given inital conc/pressure for reactants; assum 0 M (or atm) for product ex: partial pressures in atm • N2: 0.79 (given) • O2: 0.21 (given) • NO: 0 (ASSUMED) *C:* (reaction will proceed in FWD direction from I row bc Q = 0 < K) use algebra to fill in C row - use balanced chemical eq to define change in reactant vs. product conc (include SIGNS: reactant conc DEcreases and product conc INcreases → direction of change for reactant is NEGATIVE; direction of change for product is POSITIVE) ex: rxn stoic is 1:1:2 (N2:O2:NO) so write -x for N2, -x for O2, +2x for NO in C row *E:* add the terms from the I and C rows ex: N2: 0.79-x; O2: 0.21-x; NO: 2x
LeChatelier - effect of catalyst
A catalyst increases the rate of the reaction by changing the reaction mechanism and, as a result, decreasing the height of the activation energy barrier. However, the barrier height is reduced by the same amount whether the reaction as written proceeds in the forward or reverse direction, so the increase in reaction rate produced by the catalyst is the same in both directions. Therefore, a catalyst has no effect on the equilibrium constant of a reaction or on the composition of an equilibrium reaction mixture. A catalyst does, however, decrease the amount of time needed for a reaction to reach equilibrium.
equilibrium constant expression in terms of partial pressures for generic equation aA + bB ⇄ cC + dD
K = P(products) / P(reactants) THUS: K(P) = (C^c D^d) / (A^a B^b) *NOTES:* —K(p) means equilib constant in terms of partial pressures —NO BRACKETS [ ] used bc this K value is in terms of PRESSURES, NOT concentrations —ALL reactants/products must be in GAS PHASE to use this eq
equilibrium constant expression in terms of concentrations for generic equation aA + bB ⇄ cC + dD
K = [products] / [reactants] THUS: K(c) = ([C]^c [D]^d) / ([A]^a [B]^b) *K(c) means equilib constant in terms of concentrations
range of K
K > 0
K(c) of reaction multiplied by a factor x ex: K(c) = 100 for A + B ⇄ 2C; what is K(c) for 0.5 A + 0.5 B ⇄ C (x = 0.5 = 1/2)
K becomes (K(c))^x, where x is the factor multiplied throughout the reaction in this example, K(c) = K(c,original)^0.5 = 100^0.5 = 10
When two reactions are added together, how does the expression for the equilibrium constant of the sum reaction (K3) relate to the expression for the equilibrium constants of the original reactions (K1 and K2)?
K3 = K1 × K2 (helpful for when we have individual K's of each elementary step in multistep rxn but need overall K of rxn → multiply each elementary step K out to get overall K)
why do we not include liquids and solids in equilibrium constant expression
The concentrations of pure solids, pure liquids, and solvents are omitted from equilibrium constant expressions because they do not change significantly during reactions when enough is present to reach equilibrium
when can we ignore "x" compared to initial amount? (ignoring ±x in equilibrium statement)
When the K is very small (10^-4 or less)
equilibrium constant expression
[products]/[reactants] the ratio of the equilibrium concentrations or partial pressures of products to reactants, each term raised to a power equal to the coefficient of that substance in the balanced chemical equation for the reaction
chemical equilibrium
a dynamic process in which the concentrations of reactants and products remain constant over time and the rate of a reaction in the forward direction matches its rate in the reverse direction
RICE table
a table of reactant and product concentration (or partial pressure) values used to handle concentration calculations where we know the value of K and the starting concentrations/partial pressures of reactants/products and we want to calculate their equilib concentrations or pressures reaction, initial (concentration), change (in concentration), equilibrium (concentration)
heterogeneous equilibria
an equilibrium involving reactants and/or products in more than one phase
homogeneous equilibria
an equilibrium system where all reactants and products are in the same phase