Equilibrium Flashcards
define chemical equilibrium
the state reached when the concentrations of reactants and products remain constant over time
- forward and reverse reactions occur at the same rate
- reactions are still occurring, but there is no net reaction
- chemical equilibria are dynamic and reversible
Kc =
aA + bB <–> cC + dD
Kc = [C]^c[D]^d/[A]^a[B]^b
concentrations at equilibrium
Kc when you reverse the reaction
1/Kc
Kc when you multiply the reaction by 2
Kc^2
Kc when you add two reactions
Kc1 x Kc2
units of the equilibrium constant
equilibrium constants are defined as unites quantities
homogeneous equilibria
all species are present in the same phase.
heterogeneous equilibria
system’s state of equilibrium contains components from multiple phases
why are pure solids and liquids excluded from equilibrium expressions?
they are always in their standard state, where a=1: their concentrations don’t change
equilibrium constant Kp
for gases, we can use pressures instead of concentrations
Kp = Kc(RT)^Δn
Kp = Pc^c x Pd^d/Pa^a x Pb^b
Δn is the number of moles of gaseous products minus the number of moles of gaseous reactants
Kc > 10^3
reaction proceeds nearly to completion, products favoured
Kc < 10^-3
reaction proceeds hardly at all, reactants favoured
10^-3<Kc<10^3
appreciable concentrations of both reactants and products
reaction quotient, Qc
defined similarly to an equilibrium constant Kc except that the []’s in Qc can have ant values, not necessarily equilibrium values
if Qc = Acc
no net reaction occurs, already at equilibrium
if Qc < Kc
net reaction goes from left to right
if Qc > Kc
net reaction goes from right to left
le Chatelier’s principle
if a stress is applied to a reaction mixture at equilibrium, a net reaction then occurs in the direction that relieves the stress
in general, when an equilibrium is disturbed by the addition or removal of any reactant or product, Le Chateliers principle predicts that
- the concentration stress of an added reactant or product is relieved by net reaction in the direction that consumes the added substance
- the concentration stress of a removed reactant or product is relieved by net reaction in the direction that replenishes the removed substance
in general, when an equilibrium is disturbed by a change in volume that results in a corresponding change in pressure, Le Chateliers principle predicts that
- reduction in volume increases the partial pressures of all gases, which provokes a net reaction in the direction that decreases the total moles of gas
- enlargement in volume decreases the partial pressures of all gases, which provokes a net reaction in the direction that increases the total moles of gas
adding inert gas
no effect, because the partial pressures (molar concentrations) don’t change
temperature can alter the equilibrium concentrations, but for a different reason:
it changes the value of Kc
in general, when an equilibrium is disturbed by a change in temperature,
Kc for an exothermic reaction (-ΔH) decreases as the temperature increases
Kc for an endothermic reaction (+ΔH) increases as the temperature increases
linking chemical equilibrium and kinetics
A + B <-(Kf)-(Kr)-> C + D
rate forward = Kf[A][B]
rate backward = Kr[C][D]
at equilibrium: Kf[A][B]=Kr[C][D]
Kf/Kr = [C][D]/[A][B] = Kc
Kc = kf/kr
if Kc is really large
if Kc is nearly unity
if Kc is really small
Kf»_space; Kr and the reaction goes almost to completion
Kf = Kr and both reactants and products exist at equilibrium
Kr»_space; Kf and the reaction mixture consists mostly of reactants
effect of a catalyst on equilibrium
- because the forward and reverse reactions pass through the same transition state, the catalyst lowers the activation energy barrier by the same amount
- the catalyst accelerates the forward and reverse reactions by the same factor, so the composition of the equilibrium mixture is unchanged
- Kc is not affected by the catalyst
- we reach the same equilibrium, only faster
Haber proces
N2 + 3H2 <-> 2NH3
exothermic
reaction conditions:
- catalyst: iron mixed with metal oxides increases the rate
- temperature: 400-500’C
- pressure: yield improved by running the reaction at high pressures (130-300atm)
