Chemical Equilibria

Question 1

When is a system in equilibrium?

Answer 1

When the rate of the forward reaction is equal to the rate of the reverse reaction

Question 2

When does a reaction system remain in equilibrium?

Answer 2

as long as the system is located (or closed)

Question 3

What is a dynamic system?

Answer 3

one where reactants are being used up to form products and products are breaking down to form reactants but no change is observed in the reaction mixture; macroscopic properties (temperature and pressure) remain constant.

Question 4

What is the position of equilibrium?

Answer 4

extent of a reaction when equilibrium is established

Question 5

What are the features of a system at equilibrium?

Answer 5

-Rate of the forward reaction is equal to the rate of the backward reaction
-Concentration of reactants and products remains constant (*but is constantly moving)
-Systems are isolated
-Macroscopic properties remain constant (temp and pressure)

Question 6

what are the two different types of equilibrium system?

Answer 6

Homogenous and Heterogeneous

Question 7

What is heterogeneous equilibrium?

Answer 7

reactants and products are in different phases (states) (e.g. thermal decomposition)

Question 8

What is homogenous equilibrium?

Answer 8

reactants and products are in the same phase (state)

Question 9

What is [A]?

Answer 9

concentration of chemical A in
moldm-3

Question 10

What is Kc?

Answer 10

the equilibrium constant-product of the concentrations of products over the product of the concentrations of reactants gives a ratio and shows how far towards the right or left a reaction lies when a reaction is at equilibrium;

Question 11

What are stoichiometric ratios used to give the the kc equations?

Answer 11

the power?

Question 12

what type of equilibria doe kc deal
with?

Answer 12

homogeneous

Question 13

what does the larger value for the Kc mean?

Answer 13

The more the reaction has gone to completion

Question 14

what does a large kc greater than one mean?

Answer 14

products are “favoured”; equibrium lies towards the right.

Question 15

what does a small Kc less than one mean?

Answer 15

reactants are “favoured”; equilibrium lies towards the left.

Question 16

when will an equilibrium system only remain in equilibrium?

Answer 16

When it is isolated

Question 17

What happens once the conditions of equilibrium are changed?

Answer 17

The system will no longer be in equilibrium

Question 18

What are the conditions that affect equilibrium?

Answer 18

-temp
-pressure
-conc

Question 19

What is Le Chatelier’s principal?

Answer 19

When a system under dynamic equilibrium is subjected to a change, the equilibrium will shift so as to oppose the change

Question 20

what happens if the concentration of the reactants are increased? (in dynamic equilibrium)

Answer 20

They equilibrium shifts to the right hand side The yield of the product will increase

Question 21

what happens when the concentration of products are decreased? (in dynamic equilibrium)

Answer 21

-The equilibrium shifts to the right hand side
-The yield of the product will increase

Question 22

What are affected by change in pressure?

Answer 22

gases

Question 23

what does an increase in pressure of an equilibrium mixture lead to?

Answer 23

the equilibrium moving so as to decrease the pressure.
-does this by decreasing the number of molecules. (Fewer moles = lower pressure so equilibrium shifts to the side with the least gas moles).

Question 24

what does a decrease in pressure of the equilibrium mixture lead to?

Answer 24

the equilibrium mixture leads to the equilibrium moving so as to increase the pressure. It does this by increasing the number of molecules. (more moles = higher pressure so equilibrium shifts to the side with the most gas moles)

Question 25

What will the temperature increased favour?

Answer 25

The endothermic reaction

Question 26

what will the temperature reduced favour?

Answer 26

the exothermic reaction

Question 27

what do catalysts do?

Answer 27

-reduce the activation energy of a chemical reaction and therefore cause reactions to be speeded up.

Question 28

What does a catalyst do to the position of equilibrium?

Answer 28

does not affect the position of equilibrium however; it will affect the rate at which equilibrium is established

Question 29

In industry why are theoretical conditions not always used and what are often used instead?

Answer 29

-these can prove to be uneconomical or lead to a slow rate of reaction
-compromise set of conditions are used to run the plant at a profit, whilst obtaining the best yield and rate.
-so theoretical conditions are modified

Question 30

what does a low temperature lead to and why? (modification)

Answer 30

-very slow rate of attainment of equilibrium
-because the reaction has a high activation energy due to the strength of the triple bond which has to be broken before the nitrogen can react.
However a high temperature would lead to a poor yield as the position of equilibrium would move to the left

Question 31

however what would a high temperature lead to? (composition)

Answer 31

a poor yield as the position of equilibrium would move to the left

Question 32

what is the compromise condition for pressure?

Answer 32

-Very high pressure will increase the rate and also the yield but there is a significant increase in the cost of the plant.
-High specification pumps are required to maintain high pressure and maintenance of these is costly

Question 33

how is the problem of low rate of reaction overcome?

Answer 33

by using a catalyst, which enables the reaction to occur, by a different route of lower activation energy

Question 34

What else can increase the rate?

Answer 34

increasing the temperature and sacicing a high percentage yield

Question 35

what happens on the formation of ammonia?

Answer 35

-ammonia is removed so that the system does not reach equilibrium
-increases rate of reaction

Question 36

what are the conditions used in common ammonia plant?

Answer 36

200atm
450°C

Question 37

what does 80% of ammonia production go into?

Answer 37

making fertiser such as ammonium sulfate

Question 38

what are smaller proportions of ammonia used for?

Answer 38

The production of nitric acid which in turn is used to make explosives and polymers such as nylon which is a polyamide

Question 39

what problems are associated with low/high pressure? (optimum theoretical conditions)

Answer 39

Low- Slow rate of reaction
High -Uses a large amount of energy/fuel, provides a safety risk

Question 40

what problems are associated with low/high pressure? (optimum theoretical conditions)

Answer 40

Low- Slow rate of reaction
High- Uses a large amount of energy/fuel, Provides a safety risk, Uses a large amount of energy/fuel