3.2.3 Chemical equilibrium

Question 1

when does a dynamic equilibrium exist

Answer 1

• 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

Question 2

what is a closed system

Answer 2

• a closed system is isolated from its surroundings
  > so temp, pressure, conc of reactants + products are unaffected by outside influences

Question 3

what happens to equilibrium position when pressure increases

Answer 3

• position of equilibrium will shift to the side with fewer gaseous molecules

Question 4

what happens to the equilibrium position when temp increases + decreases

Answer 4

• 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

Question 5

what happens to the equilibrium position in the presence of a catalyst

Answer 5

• 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

Question 6

in industrial conditions, why must there be a compromise when choosing optimal conditions for a reversible reaction

Answer 6

• 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

Question 7

what does le Chatelier’s principle state

Answer 7

• 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

Question 8

what does the equilibrium constant, Kc provide

Answer 8

• the actual position of equilibrium as a number
  > indicates whether there are more reactants / products at equilibrium

Question 9

what is the expression for Kc for this general reversible reaction:
aA + bB = cC + dD

Answer 9

Kc = [C]c x [D]d / [A]a x [B]b
Kc = [products] / [reactants]

Question 10

what does it mean if the the value of Kc = 1

Answer 10

• indicates position of eqm halfway between reactants + products

Question 11

what does it mean if the value of Kc > 1

Answer 11

• indicates position of eqm toward the products

Question 12

what does it mean if the value of Kc < 1

Answer 12

• indicates the position of eqm towards the reactants

Question 13

what is the units for Kc

Answer 13

• moldm-3

Question 14

what is the only thing that affects Kc

Answer 14

• changing temp changes value of Kc
  > depends on whether reaction if exothermic or endothermic