topic 10

Question 1

dynamic equilibrium

Answer 1

when forward and backward reactions are occurring at equal rates. Concentrations of reactants and products stays constant and reaction is continuous

Question 2

forward reaction ?

Answer 2

exothermic

Question 3

backward reaction ?

Answer 3

endothermic

Question 4

Le Chatelier’s principle

Answer 4

if an external condition is changed the equilibrium will shift to oppose the change

Question 5

effect of increased temperature on equilibrium

Answer 5

equilibrium will shift to oppose this and move in the endothermic direction to try and reduce the temp by absorbing heat

Question 6

effect of decreasing temperature on equilibrium

Answer 6

equilibrium will shift to oppose this and move in the exothermic direction to try and increase the temp by giving out heat

Question 7

what temp gives higher yield

Answer 7

lower temps have a higher yield but result in slow rates of reaction

Question 8

effect of increased pressure on equilibrium

Answer 8

equilibrium will shift to the side with fewer moles of gas to oppose the change and therefore reduce the pressure

Question 9

effect of decreases pressure on equilibrium

Answer 9

equilibrium will shift to the side with more moles of gas to oppose the change and increase the pressure

Question 10

effect of change in pressure if there are equal amounts of moles on both sides

Answer 10

no effect

Question 11

lower or higher pressure for a high product yield

Answer 11

higher pressure gives higher yield of product and produces faster rate

Question 12

cons of high pressures

Answer 12

expensive to produce - high energy costs for pumping
equipment is expensive

Question 13

effect of concentration equilibrium

I2 + 2OH- ⇌ I- + IO- + H2O

Answer 13

increasing conc of OH- ions causes equilibrium to shift to oppose this and move in the forward direction to remove the OH- ions. Position of eqb shift towards the right giving a higher yield of I- and IO-.

goes from brown to colourless

Question 14

What causes Kc to change?


Answer 14

Kc ONLY changes with TEMPERATURE. It does not change if pressure or concentration is altered.

Question 15

What is said when Kc is small?


Answer 15

the equilibrium favours the reactants

Question 16

What is the effect of changing conditions on value of Kc?


Answer 16

the larger the Kc the greater the amount of products.

Question 17

What is the effect of catalysts on equilibrium?


Answer 17

No effect, but it speeds up the rate at which equilibrium is achieved

Question 18

Equilibrium constant, Kc formula?



Answer 18

K = [C]p [D]q
[A]m [B]n

Question 19

importance of equilibrium to industrial processes - haber process

pressure
temp
equation

Answer 19

N2 + 3H2 ⇌ 2NH3

T = 450
P = 200 - 1000 atm
catalyst = iron

Low temp gives good yield but slow rate - compromise temp used
High pressures gives a good yield and high rate - too high a pressure would lead too high energy costs for pumps to produce the pressure

Question 20

importance of equilibrium to industrial processes - contact process

Answer 20

stage 1: S + O2 —-> SO2
stage 2: SO2 + 0.5 O2 ⇌ SO3

T = 450
P = 1 to 2 atm
catalyst V2O5

low temp gives good yield but slow rate : compromise moderate temp used
High pressure gives slightly better yield and high rate : too high pressure would lead to too high energy costs for pumps to produce the pressure

Question 21

importance of equilibrium to industrial processes -production of methanol from CO

Answer 21

CO + 2H2 ⇌ CH3OH

T = 400
P = 50 atm
catalyst = chromium and zinc oxides

low temp gives good yield but slow rate : compromise temp used
High pressure gives slightly better yield and high rate : too high pressure would lead to too high energy costs for pumps to produce the pressure

Question 22

importance of equilibrium to industrial processes - hydration of ethene to produce ethanol

Answer 22

CH2=CH2 + H2O ⇌ CH3CH2OH

T = 300
P = 70 atm
catalyst = concentrated H3PO4

Low temp gives good yield but slow rate : compromise temp used
High pressure gives slightly better yield and high rate : too high pressure would lead to too high energy costs for pumps to produce the pressure
High pressure also leads to unwanted polymerisation of ethene to polyethene

Question 23

using a catalyst

Answer 23

in all cases they speed up the rate allowing lower temps to be used (lower energy costs) but have no effect on equilibrium

Question 24

using high pressures - disadv

Answer 24

too high energy costs for pumps to produce the pressure and too high equipment costs

Question 25

advantage of recycling unreacted reactants

Answer 25

improves overall yields of the processes