Chemical equilibrium

Question 1

What’s dynamic equilibria?

Answer 1

Is a system which involves a reversible reaction

Question 2

what is a reversible reaction?

Answer 2

one which may proceed in another direction

Question 3

What occurs during a reversible reaction (rate of reaction)?

Answer 3

• the rate of the forward reaction is fastest at the start
• the rate of the backwards reaction increases as the reaction progresses

Question 4

What occurs during a reversible reaction (forward and reverse reactions)?

Answer 4

eventually, the rates of the forward and reverse reactions will be equal. A dynamic equilibrium is reached.

Question 5

What are the main features of a dynamic equilibrium?

Answer 5

• both forward and backward reactions occur at equal rates
• a closed system is required.
• concentrations of reactants and products are constant

Question 6

what does a closed system do?

Answer 6

this prevents the loss or gain of reagents that could disrupt the equilibrium

Question 7

What conditions can affect the position of equilibrium?

Answer 7

• concentration
• pressure
• temperature
• amount of reactants and products

Question 8

What is Le Chatelier’s principle?

Answer 8

When any factor which affects a dynamic equilibrium is changed, the position of equilibrium will shift. In order to minimise/oppose that change

Question 9

How do you answer equilibria questions?

Answer 9

-which reaction is favoured

-why?
- conc: to minimise change
- temp: endo or exothermic reaction
- pressure: identify the side with the least moles

-which way did the equilibrium move
-composition: + or - reactants/products

Question 10

What does the direction of equilibrium depend on in terms of concentration?

Answer 10

depends on whether the concentration of a reactant or product is changed.
The equilibrium will move to minimise what has changed.

Question 11

How does increased reactant/ reduced product move the equilibrium position?

Answer 11

to the right

Question 12

When does changing total pressure apply?

Answer 12

when gases are present (check the state symbols)

Question 13

what does the direction of equilibrium depend on in total pressure changes?

Answer 13

depends on the number of gaseous moles on either side of the equation

Question 14

what does the change of total pressure favour (increase/decrease) ?

Answer 14

• an increase in pressure favours the side with the least no. of moles
• a decrease in pressure favours the side with the most no. of moles

Question 15

What does the changing of temperature depend on?

Answer 15

the sign of the forward reaction.

Question 16

What happens to equilibrium when the temperature is increased?

Answer 16

the equilibrium will move to cool the reaction down

Question 17

if the reverse endothermic reaction is favoured…?

Answer 17

the equilibrium position moves to the left

Question 18

increasing temperature will favour

Answer 18

ENDOTHERMIC REACTION

Question 19

decreasing temperature will favour

Answer 19

EXOTHERMIC REACTION

Question 20

What is the Haber process?

Answer 20

used in the manufacture of ammonia, which is used to make fertilisers, explosives, nylon and nitric acid etc.

Question 21

What is the equation for the haber process?

Answer 21

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Question 22

what position is needed for maximum yield (haber process)?

Answer 22

equilibrium must be shifted as far to the right hand side as possible

Question 23

What temperature is needed for maximum yield and why (haber process)?

Answer 23

Low temp
- favours the forward exothermic reaction
- equilibrium moves to the right

Question 24

What pressure is needed for maximum yield and why (haber process)?

Answer 24

High pressure
- less gaseous moles of products/less gaseous moles on the right
- equilibrium moves to the right

Question 25

what other considerations should be taken into account in terms of rate of reaction (haber process)?

Answer 25

• using low temp will reduce the rate significantly so equilibrium will take many hours to achieve
• increased pressure will increase rate
• energy cost must also be considered, high pressure is expensive and can also be dangerous

Question 26

What conditions are compromised to achieve an acceptable rate?

Answer 26

• 200 atm pressure
• 450 degree temp
• iron catalysts are used. Reduction of energy costs.

Question 27

The bigger the equilibrium constant…

Answer 27

the greatest the amount of products present (and less reactants) in the mixture

Question 28

What is the equilibrium equation?

Answer 28

aA + bB –> cC + dD

Question 29

What is the equation to work out equilibrium constant Kc?

Answer 29

kc = [C]^c[D]^d
———————-
[A]^a[B]^b

Question 30

what do the [] square brackets indicate in the equilibrium constant Kc equation?

Answer 30

concentration

Question 31

Equilibrium constant Kc is NOT affected by

Answer 31

changes in conc., pressure or use of a catalyst.

Question 32

What is equilibrium constant Kc affected by?

Answer 32

Temperature

Question 33

What is the equation for equilibrium constant Kp?

Answer 33

Kp = (pC)^c.(pD)^d
————————
(pA)^a.(pB)^b

Question 34

What are the () brackets meaning?

Answer 34

partial pressures of the gases

Question 35

What is the equation for the mole fraction?

Answer 35

mole fraction = mole of gas/total moles

Question 36

what is the equation for the partial pressures?

Answer 36

partial pressures = mole fraction x total pressures

Question 37

what must mole fractions add up to?

Answer 37

1