Equilibria, Le Chatelier's Principle and Kc

Question 1

Give examples of reactions that are reversible

Answer 1

the reaction between nitrogen and hydrogen to produce ammonia

Question 2

What is dynamic equilibrium?

Answer 2

when the rates of the forward and backward reactions are the same

Question 3

Why is it called DYNAMIC equilibrium?

Answer 3

Because although the concentrations of the composition are stable the forward and backward reactions are still occurring

Question 4

Give a summary of the rate x time graph for getting to equilibrium

Answer 4

At the start: forward reaction is fast - no reverse reaction
After the start: reverse reaction speeds up as forward reaction slows down
At equilibrium: as quickly as the products are being made the products are changing back to reactants

Question 5

What is the forward reaction?

Answer 5

reactants changing into products

Question 6

What is the backward reaction?

Answer 6

products changing back into reactants

Question 7

When does chemical equilibrium occur and when is it established?

Answer 7

when the concentrations of reactants and products remain constant and when the rate of the forward reaction equals the rate of the backward reaction

Question 8

What is Le Chatelier’s Principle?

Answer 8

If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance. (if any factor is changed which affects the equilibrium mixture, the position of the equilibrium will shift so as to oppose the change)

Question 9

What does a negative enthalpy sign mean?

Answer 9

heat is given out in the forward reaction - exothermic

Question 10

If we increase the temperature of a reversible reaction at equilibrium which way will the equilibrium move?

Answer 10

moves in endothermic direction

Question 11

If we decrease the temperature of a reversible reaction at equilibrium which way will the equilibrium move?

Answer 11

moves in exothermic direction

Question 12

What is the definition of a catalyst?

Answer 12

A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

Question 13

What happens when the concentration of a reactant is increased?

Answer 13

The equilibrium will shift to the right

Question 14

What happens when the pressure on the system is increased?

Answer 14

the equilibrium will shift to minimise the pressure increase which in effect means the equilibrium moves in the direction to produce the least moles

Question 15

What is the effect of adding a catalyst to a reversible reaction at equilibrium? Why?

Answer 15

It makes no difference to the position of equilibrium because a catalyst speeds up the forward and back reaction to the same extent

Question 16

Why use a catalyst for a reversible reaction?

Answer 16

A catalyst speeds up the rate at which a reaction reaches dynamic equilibrium

Question 17

What is the temperature and pressure used at most ammonia plants?

Answer 17

200 atm and 450 degrees C

Question 18

What is significant about the conditions used at most ammonia plants?

Answer 18

it is a lower pressure and a higher temperature than would give the maximum conversion (compromise conditions)

Question 19

Why are compromise conditions necessary? (ammonia plants)

Answer 19

• higher pressures would be too dangerous/expensive
• 450 degrees C gives a moderate yield in a reasonable time (it takes too long to establish the equilibrium at 350 degrees C)

Question 20

What is the atom economy of the Haber process? What is the atom economy for the conversion to ammonia?

Answer 20

100 % but conversion to ammonia is much less

Question 21

Why is the atom economy of the conversion to ammonia much less than the Haber process?

Answer 21

1, Gases flow over the catalyst too quickly for equilibrium to be reached
2, An equilibrium process which tries to attain a moderate yield in a reasonable time

Question 22

What is a homogeneous reaction?

Answer 22

A homogeneous reaction is where both products and reactants are present in the same phase.

Question 23

What is the value of Kc unchanged by?

Answer 23

changes in concentration, pressure and use of a catalyst

Question 24

What is the value of Kc changed by?

Answer 24

only temperature

Question 25

For the reaction aA + bB (reversible reaction arrows) cC + dD, write the expression for the equilibrium constant Kc

Answer 25

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

Question 26

What do the square brackets mean in the Kc expression?

Answer 26

Concentration in moldm-3

Question 27

What is the significance of Kc?

Answer 27

It is a measure of how far a reaction goes. If Kc is exactly 1, then the position of the equilibrium must be exactly halfway between reactants and products.

Question 28

Are products on the top or on the bottom of the Kc expression?

Answer 28

on the top

Question 29

What does it mean if Kc is greater than 1?

Answer 29

then there are more products than reactants at equilibrium - the forward reaction is favoured

Question 30

What does it mean if Kc is less than 1?

Answer 30

the reverse reaction is favoured - there are more reactants than products at equilibrium

Question 31

How can you find Kc if you don’t have the concentration of reactants and products at equilibrium?

Answer 31

1, Initial amounts of each product or reactant
2, Change in each reactant
3, Equilibrium number of moles for each product and reactant
4, Equilibrium concentration of each reactant (moldm-3)
5, Kc expression (and calculation)
6, Units

Question 32

What are the conditions for dynamic equilibrium?

Answer 32

1, the rate of the forward reaction = the rate of the reverse reaction
2, concentrations of reactants and products are constant
3, closed system
4, macroscopic properties remain constant i.e. temp, pressure

Question 33

Give two features of a reaction in equilibrium

Answer 33

• the concentration of reactants and products remain constant
• the rate of the forward reaction is equal to the rate of the reverse reaction

Question 34

Why do catalysts have no effect on the position of equilibrium?

Answer 34

• catalysts increase the rate of the forward and reverse reactions
• by an equal amount

Question 35

Give the meaning of the term ‘dynamic’ in the context of ‘dynamic equilibrium’

Answer 35

The forward and reverse reaction are occurring at the same time

Question 36

Give the meaning of the term ‘dynamic equilibrium’

Answer 36

• the concentration of reactants and products remain constant
• the rate of the forward reaction is equal to the rate of the reverse reaction

Question 37

State a condition for a reaction in equilibrium

Answer 37

closed system

Question 38

State Le Chatelier’s Principle

Answer 38

A reaction in equilibrium will act to oppose any change imposed upon it

Question 39

State in general terms how a catalyst works?

Answer 39

• It provides an alternative reaction route
• With a lower activation energy

Question 40

How does an increase in pressure alter Kc?

Answer 40

It remains unchanged

Question 41

What is the only thing that affects the value of Kc?

Answer 41

temperature