3.2.3

Dynamic equilibrium

(a)→dynamic equilibrium: forward reaction rate same as backward reaction rate in a closed system; conc of reactants and products remain the same

(b)→Le Chatelier’s principle: if a change is made to a system in dynamic equilibrium the position of the equilibrium moves to counteract this change

→effects of conc:

• increased: equilibrium shifts to the right [forward reaction]; counteract increased reactant concentration

• decreased: shifts to left [backward reaction]; counteract decreased reactant concentration

→effects of pressure:

• increased: equilibrium shifts to side w smaller no of molecules; counteract to decrease pressure again

• decreased: shifts to side w larger no of molecules; counteract to increase pressure again

→effects of temp:

• increased: equilibrium moves in endothermic direction; counteract change [as takes in heat energy]

• decreased: shifts in exothermic direction: counteract change [to give heat energy]

(c)→catalysts on equilibrium: increases rates of forward and backward reactions equally; only causes reaction to reach equilibrium faster so position is unchanged

(d)→RP: equilibrium changes w conc:

→RP: equilibrium changes w temp:

(e)→compromise w chemical equilibrium and reaction rate:

Haber Process

• increased pressure: equilibrium shifts to fewest molecule side [forward] so equilibrium yield increases

• particles closer together so more successful collisions so higher reaction rate

• but expensive so compromised at 200atm

• low temp: reaction exothermic and decreasing temp means equilibrium shifts to exothermic way [forward] so more equilibrium yield

• but gases wont have enough kinetic energy to collide and react so compromise of 400/450C

• iron catalyst: speeds rate of reaction

Equilibrium Constant Kc

(f)→equilibrium constant expression: links equil constant K to Concentrations of reactants and products at equilibrium; aA + bB ⇌ cC + dD [image] ; Kc is specific anf only changes if temp changes

→concentration mold dm^-3 = moles / volume dm³ ; so to find Kc you must work out from moles of reactant and products at equilibrium and volume of reaction mixture

(g)→estimating position of equilibrium:

• if Kc very large: equilibrium lies on forward reaction so reaction mixture mostly products

• very small: lies on backward reaction as mixture mostly reactants

• close to 1: similar concentration of both r and p