4.5 & 4.6 Equilibria

Question 1

What is an dynamic equilibrium?

Answer 1

An equilibrium is where the forward and reverse reactions are occurring at the same rate. This means the amount of reactants and products stay the same.

Question 2

How is an equilibrium set up?

Answer 2

As reactants are used up, the forward reaction slows down. Meanwhile more products are formed, speeding up the reverse reaction. Eventually the forward and reverse reaction happen at the same rate.

Question 3

What is a reversible reaction?

Answer 3

This is a reaction that can go forwards and backwards, symbolised by a ⇋.

Question 4

Give an example of a physical equilibrium.

Answer 4

Liquid bromine can change to bromine gas when shaken in a closed flask.
Br2(l) ⇋ Br2(g)

Question 5

Give an example of a chemical equilibrium.

Answer 5

Hydrogen has and iodine gas forming hydrogen iodide.

H2(g) + I2(g) ⇋ 2HI(g)

Question 6

Name some conditions for a dynamic equilibrium.

Answer 6

Closed system at a constant temperature.

Question 7

What is the Contact process? Identify a reversible reaction within it.

Answer 7

It manufactures sulfuric acid (for dyes, fertilisers and batteries). The conversion of sulfur dioxide to sulfur trioxide is reversible.
2SO2(g) + O2(g) ⇋ 2SO3(g)

Question 8

What is the Haber process?

Answer 8

It manufactures ammonia for use in fertilisers.

N2(g) + 3H2(g) ⇋ 2NH3(g)

Question 9

What is the equilibrium constant?

Answer 9

Kc is calculated from the ratio of [product] to [reactant]. It is constant for a particular temperature of a reaction at equilibrium.

Question 10

How do you express Kc for aA + bB ⇋ dD + eE?

Answer 10

Kc = ([D]^d x [E]^e) / ([A]^a x [B]^b)

Question 11

How can Kc be useful?

Answer 11

It can be used to find concentrations in equilibrium mixtures.

Question 12

What is partial pressure?

Answer 12

The pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own.

Question 13

How can you find the total pressure of a gas mixture?

Answer 13

It is the sum of all the partial pressures of the individual gases.

Question 14

What is the mole fraction of a gas?

Answer 14

The proportion of a gas mixture that is a particular gas.

Question 15

How do you calculate the mole fraction of a gas in a mixture?

Answer 15

(number of moles of gas) / (total number of moles of all gasses in the mixture)

Question 16

How do you calculate partial pressure of a gas?

Answer 16

mole fraction of gas x total pressure of the mixture

Question 17

How do you calculate the equilibrium constant for the gas equilibrium aA(g) + bB(g) ⇋ dD(g) + eE(g)?

Answer 17

Kp = (p(D)^d x p(E)^e) / (p(A)^a x p(B)^b)

Question 18

How can Kp be useful?

Answer 18

It can be used to find equilibrium partial pressures.

Question 19

Which reactants do you include for Kp in heterogenous equilibria?

Answer 19

Gases only. You do not include liquids or solids.

Question 20

Write an expression for Kp for:

NH4HS(s) ⇋ NH3(g) + H2S(g).

Answer 20

Kp = p(NH3) x p(H2S)

Question 21

How is total entropy change related to the equilibrium constant, K?

Answer 21

∆S total = RlnK

Question 22

What does K tell you about the progression of a reaction?

Answer 22

A very large value of K means a very high proportion of products as well as a large change in entropy.

Question 23

What does a reaction with an equilibrium constant of less than 10^-10 tell you?

Answer 23

The reaction does not take place. (∆S total = -191)

Question 24

Give the values of K and ∆S total for a reaction with equilibrium push well to the left.

Answer 24

K = 10^-5, ∆S total = -96

Question 25

When is a reversible reaction balanced between products and reactants?

Answer 25

When K = 1 and ∆S total = 0

Question 26

Where is the equilibrium position of a reaction where K = 10^5

Answer 26

Well to the right.

Question 27

For what values of K will a reaction go to completion?

Answer 27

Greater than 10^10.

Question 28

What is Le Chatelier’s principle?

Answer 28

If there’s a change in pressure or temperature, the equilibrium will move to help counteract the change.

Question 29

How does increasing pressure affect the equilibrium position?

Answer 29

It shirts the equilibrium to the side with fewer gas molecules.

Question 30

How, if at all, does increasing the pressure affect the value of Kp?

Answer 30

Kp stays the same for any pressure.

Question 31

What affect does increasing temperate have on the equilibrium position of a reversible reaction?

Answer 31

The equilibrium shifts in the direction of the endothermic reaction to absorb the heat added.

Question 32

What affect does decreasing temperate have on the equilibrium position of a reversible reaction?

Answer 32

The equilibrium shifts in the direction of the exothermic reaction to replace the lost heat.

Question 33

If a change in equilibrium position due to temperature change means more product is formed, what happens to the value of the equilibrium constant?

Answer 33

It increases.

Question 34

If a change in equilibrium position due to temperature change less more product is formed, what happens to the value of the equilibrium constant?

Answer 34

It decreases.

Question 35

What effect do catalysts have on an equilibrium.

Answer 35

They have no effect on position or yield but equilibrium is approached faster.

Question 36

Explain three methods of increasing rate of reaction.

Answer 36

Using a catalyst. Increasing temperature as it results a higher rate of successful collisions. Increasing pressure as it increasing the rate of collisions.

Question 37

Why can temperature be a compromise in an industrial process such as the Haber process?

Answer 37

Since it’s exothermic, a lower temperature would increase yield but significantly slow down rate.

Question 38

Why can pressure be a compromise in an industrial process such as the Haber process?

Answer 38

A high pressure gives a high yield and rate however it is also increasingly expensive and dangerous.

Question 39

Why don’t industrial processes such as the Haber process reach equilibrium?

Answer 39

Because the system isn’t closed. The gas mixture continually leaves the reactor and is liquified to remove the ammonia. Reactants are recycled.

Question 40

How is atom economy calculated?

Answer 40

(mass of atoms in product) / (mass of reactants in reactants) x 100%

Question 41

Give two ways of increasing % atom economy.

Answer 41

Recycling unreacted materials. Finding an alternative route for synthesis.

Question 42

Give five factors that need to be considered when planning an industrial process.

Answer 42

Extent of reaction. Rate of reaction. Atom economy. Energy consumption. Safety procedures.