chem equilibria Flashcards
what is a reversible reaction?
a reversible reaction is one which can occur in both directions
what is dynamic equilibrium?
a dynamic equilibrium refers to a reversible reaction in which the forward and reverse reactions are both taking place at the same rate resulting in no overall changes in concentrations
why can equilibrium only be achieved in a closed system?
an open system allows substances to escape or to enter, and equilibrium cannot be established.
do we include pure solids & liquids in writing equilibrium constant expressions?
no. don’t include them for heterogeneous equilibria, but if it’s homogeneous equilibria (eg whole reaction only have liquid reactants & products), then include them
bc concentrations of pure solids and liquids are considered a constant
what is the position of equilibrium?
the position of equilibrium refers to the composition of an equilibrium mixture, ie the relative proportion of products to reactants
POE on left = more reactants
POE on right = more products
what does the magnitude of Kc/Kp say about the position of equilibrium?
small K = extent of conversion of reactants to products is small, so proportion of products to reactants is low = POE on left (more reactants)
large K = extent of conversion of reactants to products is large, so proportion of products to reactants is high = POE on right (more products)
what is le chatelier’s principle?
le chatelier’s principle states that if the conditions of a system at equilibrium are changed, the position of equilibrium shifts so as to reduce that change
what is the effect of adding a reactant to the equilibrium mixture?
- concentration of the reactant increases
- by LCP, POE shifts right (towards products) to consume some reactant so decrease conc
- conc of other reactant also decreases while conc of product increases
Kc remains the same
what is the effect of decreasing the volume of the container on the position of equilibrium?
- vol of container decreased = increased pressure
- system will try to reduce pressure by reducing total number of gas molecules
- POE will shift left/right (depending on equation) towards the side with fewer moles of gas
Kc remains the same
if no. of moles on both sides of equation are the same, no. of moles don’t change when pressure changes = changes in pressure have no effect on POE
what is the effect of changes in temperature on the position of equilibrium?
- temp increase = system tries to absorb more heat = favours endo reaction
- temp decrease = system tries to product more heat = favours exo reaction
Kc changes
what is the effect of catalyst on the position of equilibrium?
- catalyst lowers Ea of both forward & reverse reactions by same amount = both forward & reverse reaction rates increase by same extent
- time to reach equilibrium is shortened
- POE not affected
- Kc not affected
what is Qc?
Qc can take on any value while Kc has a constant value. when Qc = Kc, the system is at equilibrium
- Qc < Kc = reaction proceeds in forward direction to consume more reactants and form more products until Qc = Kc
- Qc > Kc = reaction proceeds in backward direction to form more reactants and consume more products until Qc = Kc
what are the optimal conditions used in the haber process?
temp: 450C
pressure: 250 atm
catalyst: finely divided iron
why are such conditions used?
- 450C used bc low temp would favour forward exo rxn but rxn would be too slow to be economical
- 250atm usede bc high pressure favours forward rxn as it reduces no. of moles of gases but cost & maintenance of equipment would be too high
- catalyst used to increase SOR & increase efficiency without affecting yield
what is the link between 𝝙G and POE?
negative 𝝙G (spontaneous) = POE on right = forward reaction favoured = more products = K >1
positive 𝝙G (NOT spontaneous) = POE on left = reverse reaction favoured = more reactants = K < 1
