3.2.3 Chemical equilibrium
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when does a dynamic equilibrium exist
• in a closed system where the rate of forward reaction is equal to rate of backward reaction + concentration of reactants and products don’t change
what is a closed system
• a closed system is isolated from its surroundings > so temp, pressure, conc of reactants + products are unaffected by outside influences
what happens to equilibrium position when pressure increases
• position of equilibrium will shift to the side with fewer gaseous molecules
what happens to the equilibrium position when temp increases + decreases
• if temp inc, equilibrium will shift in the endothermic direction, to take in the extra heat energy • if temp dec, equilibrium shifts in the exothermic reaction, to release extra heat energy
what happens to the equilibrium position in the presence of a catalyst
• no effect on position of equilibrium > speeds up rate of forward + reverse reactions equally > will increase the rate at which equilibrium is established but no effect on position/yields
in industrial conditions, why must there be a compromise when choosing optimal conditions for a reversible reaction
• compromise needed between rate + yield > high temp = faster rete but low yield > high pressure = faster rate but low yield > can also be safety + cost concerns with high pressure
what does le Chatelier’s principle state
• for a closed system at equilibrium, if a change is made, the position of equilibrium will shift in the direction which will oppose the change
what does the equilibrium constant, Kc provide
• the actual position of equilibrium as a number > indicates whether there are more reactants / products at equilibrium
what is the expression for Kc for this general reversible reaction: aA + bB = cC + dD
Kc = [C]c x [D]d / [A]a x [B]b Kc = [products] / [reactants]
what does it mean if the the value of Kc = 1
• indicates position of eqm halfway between reactants + products
what does it mean if the value of Kc > 1
• indicates position of eqm toward the products
what does it mean if the value of Kc < 1
• indicates the position of eqm towards the reactants
what is the units for Kc
• moldm-3
what is the only thing that affects Kc
• changing temp changes value of Kc > depends on whether reaction if exothermic or endothermic