10 - chemical equilibrium

Question 1

are most combustion reactions exo or endothermic?

Answer 1

exothermic and usually irreversible

Question 2

when is a reaction considered complete?

Answer 2

once 99% is completed

Question 3

how is equilibrium established?

Answer 3

once the rates of the forward and backward reactions become equal and after that point there is no further change in concentrations of reactants and products

Question 4

what two conditions must be met for dynamic equilibrium to be established?

Answer 4

• reaction must be reversible
• reaction must be in a closed container

Question 5

what 3 features define a system that is in dynamic equilibrium?

Answer 5

• both forward and backward reactions are continuously occurring
• the rate of the forward reaction is equal to the rate of the backward reaction
• the concentrations of reactants and products remain constant

Question 6

how does a change in concentration affect equilibrium?

Answer 6

increase in one of the reactants moves equilibrium right
decrease in one of the reactants shifts equilibrium left

Question 7

how does a change in pressure affect equilibrium?

Answer 7

only applies to reversible reactions involving gases
increased pressure of reactants - shifts right
decreased pressure of reactants shifts left

Question 8

how does a change in temperature affect equilibrium?

Answer 8

increase in temp shifts towards the endothermic direction
exothermic
increased - left
decreased - right
endothermic
increased - right
decreased - left

Question 9

how does a catalyst affect equilibrium?

Answer 9

the rate increases however equilibrium is not altered as the rate of the forward and backward reactions increase the same amount

Question 10

what is a limitation of qualitative predictions regarding equilibrium?

Answer 10

could be two factors altering equilibrium - we don’t know which would have the greatest impact

Question 11

what is the Kc equation?

Answer 11

aA + bB <–> cC + dD

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

usually everything is in the gaseous form, the same form or omitting the abnormal state

Question 12

what are the units for Kc?

Answer 12

replace each species in the equation with mol dm-3 and figure out the units

Question 13

what does it mean if Kc is greater than 1?

Answer 13

equilibrium lies to the right

Question 14

what does it mean if Kc is less than 1?

Answer 14

equilibrium lies to the left

Question 15

what are the two main goals in industry when making products?

Answer 15

• quick as possible
• as completely as possible

Question 16

what are the typical conditions in the Haber process?

Answer 16

450*C and 250atm
if the reaction was to reach equilibrium the yield would be 30% however it is not left in the chamber long enough therefore the yield is about 15%

Question 17

why are the conditions 450*C and 250atm used for the Haber process?

Answer 17

• needs a high temp to overcome the triple bonded nitrogen
• catalyst does not function at low temps
• if the temp was higher it would be uneconomical and potentially cause the backwards reaction to occur which is at a faster rate than the forwards reaction
• higher pressure increases yield however is expensive so 250atm is a compromise

Question 18

what catalyst is used in the Haber process in industry?

Answer 18

iron
tungsten and platinum also work however they are more expensive

Question 19

what is manufactured through the Contact process?

Answer 19

sulfuric acid
from sulfur dioxide and oxygen
450*C and 1 or 2atm
vanadium catalyst