R2.3 - how far? the extent of chemical change

Question 1

What is dynamic equilibrium?

Answer 1

When the rate of a forward reaction is equal to the rate of the backward reaction in a closed system where the concentration of products and reactants are equal.

Question 2

What happens when Br2 is stored in a sealed container at STP?

Answer 2

As Br2 is volatile, and has a boiling point close to STP, most molecules will evaporate, but as it it a sealed container, concentration increases and some molecules collide with the walls of the container, losing energy and changing states.

Question 3

When is there equilibrium in a physical system?

Answer 3

Rate of evaporation = Rate of condensation
- Rate doesn’t stop, but just becomes equal.
- Crystals may also change shape.

Question 4

What happens when you place HI in a sealed container?

Answer 4

It starts off colourless, and becomes more and more purple until the concentration and colour remains constant.
2HI (g) colourless <–> H2 (g) colourless + I2 (purple)

Question 5

What are the 5 characteristics of an equilibrium state?

Answer 5

1) Equilibrium is dynamic - occur at the same rate, meaning that the reaction doesn’t stop.
2) Equilibrium is achieved in a closed system - there is no exchange of matter, so there is equilibrium when products and reactants recombine with each other.
3) Concentration of products and reactants remains constant at equilibrium - being produced and destroyed at the same rate.
4) No change in macroscopic properties at equilibrium - observable properties are dependent on concentration.
5) Equilibrium can be reached in either direction for a reversible reaction.

Question 6

What is the equilibrium position?

Answer 6

Measure of the proportion of product to reactant.

Question 7

What is the equilibrium law?

Answer 7

Describes how the equilibrium constant k, can be determined from the stoichometry of a reaction

Question 8

What is the equilibrium constant?

Answer 8

Magnitude of the equilibrium constant indicates the extent of a rate of reaction at equilibrium; it is temperature dependent.
0 - mostly reactants
1 - mostly products

Question 9

What does the magnitude of K tell us?

Answer 9

K ≤ 1 - reaction hardly proceeds
K < 1 - equilibrium lies towards the reactants
K = 1 - significant amounts of both products and reactants
K > 1 - equilibrium lies towards the products
K ≥ 1 - reactions almost goes to completion

Question 10

How do you calculate the equilibrium constant?

Answer 10

K = (concentration of products to the power of co-efficient) / (concentration of reactants to the power of co-efficient)

Question 11

How do you work out K for the reverse reaction?

Answer 11

Find the reciprocal

Question 12

What factors can affect the equilibrium constant?

Answer 12

Temperature

Question 13

What is Le Chatelier’s Principle?

Answer 13

A system at equilibrium, when subject to a change will respond in such a way to minimize the effects of the change.

Question 14

What happens to equilibrium if concentration changes?

Answer 14

Increase in reactant / decrease in product concentration - equilibrium moves to the right
Decrease in reactant / increase in product concentration - equilibrium moves to the left
- Doesn’t affect k, but will affect the position of equilbrium

Question 15

What happens to equilibrium if pressure changes?

Answer 15

Increase pressure - moves to the side with the fewest amount of particles
Decrease pressure - moves to the side with the most amount of particles
- Doesn’t affect k, but will affect the position of equilbrium

Question 16

What happens to equilibrium if temperature changes?

Answer 16

Increase temperature - endothermic direction (increase in the value of k)
Increase temperature - exothermic reaction (decrease in the value of k)

Question 17

What happens to equilibrium if a catalyst is added?

Answer 17

It speeds up the rate of reaction in both directions and doesn’t affect k or the position of equilibrium.
- Enables the equilibrium mixture to be achieved more quickly, but doesn’t change yield of a single reaction.

Question 18

What is the Haber Process?

Answer 18

N2 (g) + 3H2 (g) <–> 2NH3 (g)

Question 19

Why is the Haber Process important?

Answer 19

Important for the production of fertilizers to increase yield of food production.

Question 20

What are the conditions of the Haber Process?

Answer 20

High pressure (200 atm), compromised temperature (450) and an iron based catalyst

Question 21

Why are unconverted reactants in industrial processes recycled and removed?

Answer 21

To recycle unreacted reactants and increase products formed.

Question 22

What is the contact process?

Answer 22

2SO2 (g) + O2 (g) <–> 2SO3 (aq)

Question 23

Why is the contact process important?

Answer 23

Important for the production of sulfuric acid for fertilizers, water treatment, steel and a measure of industrial strength for countries.

Question 24

What are the conditions of the contact process?

Answer 24

200 atm, 450 degrees and V2O5 catalyst

Question 25

What is the methanol production process?

Answer 25

CO (g) + 2H2 (g) –> CH3OH (g)

Question 26

Why is the methanol production process important?

Answer 26

Used for chemical feedstocks, solvents and biofuels

Question 27

HiWhat are the conditions for the production of methanol?

Answer 27

High pressure and moderate temperature.

Question 28

Why is a moderate temperature used in methanol production?

Answer 28

Compromise between yield and rate of reaction.

Question 29

Why is a high pressure used in methanol production?

Answer 29

High pressure to increase the rate of reactions and move equilbrium to the right, so more products are being made.

Question 30

What does RECCCS stand for?

Answer 30

Rate Equal Concentration Constant Closed System

Question 31

What is the reaction quotient (Q)?

Answer 31

Measure of the relative amounts of reactants and products present in a reaction at a particular time.

Question 32

How do you calculate Q?

Answer 32

Same way as K is calculated.

Question 33

What is the difference between Q and K?

Answer 33

Q = direction of reaction at any other value than equilibrium.
K = direction of reaction at equilibrium.

Question 34

What direction is the reaction of Q=K?

Answer 34

The reaction is at equilibrium and no net reaction occurs.

Question 35

What direction is the reaction if Q>K?

Answer 35

The reaction proceeds to the left in favour of the reactants.

Question 36

What direction is the reaction if Q<K?

Answer 36

The reaction proceeds to the right in favour of products.

Question 37

What is ICE?

Answer 37

Initial
Change
Equilibrium

Question 38

How do you quantify the composition of equilibrium using the ICE method?

Answer 38

1) Write out a balanced equation.
2) Draw out a table with rows labelled initial, change and equilibrium.
3) Add all the information given to a table.
4) Work out what the change is for one reactant or product. This will be the same for the other reactants or products. Remember to take the stoichometry into account.
5) Work out the equilibrium concentrations for all reactants and products.
6) Calculate K.

Question 39

When is a reaction endothermic?

Answer 39

If an increase in temperature causes an increase in the value of K.

Question 40

When is a reaction exothermic?

Answer 40

If an increase in temperature causes an decrease in the value of K.