Equilibria, Le chateliers, Kc

Question 1

How is a dynamic equilibrium reached?

Answer 1

• At the start of the reaction the forward reaction is fast and the backward reaction is slow
• The backward reaction then speeds up as the forward reaction slows down
• The rate of the forward reaction becomes the same as the rate of the backward reaction

Question 2

Dynamic equilibrium definition

Answer 2

Dynamic means: the forward and backward reactions proceed at equal rates
Equilibrium means: the concentration of the reactants and products are constant

Question 3

When is dynamic equilibrium carried out

Answer 3

only occurs if reversible reaction carried out in a sealed container, no products escaping = closed system

Question 4

State Le Chatelier’s principle

Answer 4

The position of equilibrium will shift to oppose the change made to it

Question 5

Changes in concentration

Answer 5

CO + H2O ⇢ CO2 + H2. (↑CO)
-equilibrium will shift to the right
-to oppose addition of CO
-yield of CO2 and H2 increases

Question 6

Changes in pressure

Answer 6

2NO2 ⇢ N2O4

Pressure increase:
-fewer moles of right
-equilibrium shifts to right
-to oppose increase in pressure
yield of N2O4 increases

• equal number of moles= no affect on position of equilibrium

Question 7

Changes in volume

Answer 7

Increase in volume , decreases pressure
Decrease in volume, increases pressure

CH4 + H2O ⇢ CO + 3H2

Vol increases:
-decrease in pressure
-more moles on right
-equilibrium shifts to right
-oppose decrease in pressure
-yield of H2 increases

Question 8

Changes in temperature

Answer 8

Temp ↑ endo
Temp ↓ exo

CH4 + H2O ⇢ CO + 3H2 ΔH= -206

-forward reaction exothermic
-equilibrium shifts to right
-oppose decrease in temp
-yield of CO and H2 increases

Question 9

Effect of catalyst on equilibrium

Answer 9

• the rate of both the forward and reverse reactions increases equally
• there is no change to the position of the equilibrium
• dynamic equilibrium is reached faster

Question 10

What is Kc

Answer 10

Equilibrium constant

Question 11

Kc equation

Answer 11

products / reactants

A + 2B ⇢ 2AB

Kc= [AB]2 / [A] [B]2

units= (2) - (2+1) = -1
mol-1 dm3

*no units don’t need volume

Question 12

Concentration equation

Answer 12

conc= moles / volume
(mol dm-3) = (mol)/ (dm-3)

cm-3 = divide by 1000

Question 13

Kc scale

Answer 13

Kc< 1:
very small
lots of reactants
equilibrium shifts to left

Kc>1:
very large
lots of products
equilibrium shifts to right

Question 14

What changes Kc value?

Answer 14

Only temperature

Question 15

Reverse Kc reaction

Answer 15

1 / ans
(ans = previous Kc number)

Question 16

Compromised Conditions

Answer 16

pressure of 400 atmospheres and temperature 350℃ would give highest yield

compromise- p= 200 atmospheres, temp=450℃

• higher p= too exprensice
• 450 gives good yield in fast time
• The catalyst does not affect the position of equilibrium but reduces the cost of the reaction, as less fossil fuel is burned to generate heat energy. 

Question 17

Compromised Conditions example

Answer 17

CH4 + H20 ⇢ CO + 3H2. ΔH= +206
Reaction - temp 800℃, low P 300kPa, Ni catalyst

Temperature:
* forward reaction endothermic, equilibrium shifts to right to oppose increase in temp
* Increases yield
*Increased temp= higher cost
compromised used

Pressure:
* less moles on left equilibrium shifts to right to increase yield
* lower pressure increases yield
* Iow pressure - low cost

catalyst:
* no effect on equilibrium
* allows low temp to be used
* lowers cost