Chapter 17 - Chemical Equilibrium

Question 1

Reversible reaction

Answer 1

One in which the products react to give back the reactants, i.e, the reaction goes in both directions

Question 2

Static equilibrium

Answer 2

The whole system is at rest, e.g, a meter stick in balance

Question 3

Dynamic equilibrium

Answer 3

Reactants are continuously forming products and the products are continuously forming reactants; and the rate of the forwards reaction is equal to the rate of the backwards reaction

Question 4

Chemical equilibrium

Answer 4

State of dynamic balance in a reversible reaction where the rate of the forward reaction is the same as the rate of the backwards reaction

Question 5

Once the equilibrium has been reached there is no further change in the __________ of either reactants or products

Answer 5

Once the equilibrium has been reached there is no further change in the CONCENTRATION of either reactants or products

Question 6

Scientists want the position of equilibrium to be ‘_______________’ as possible in order to ___________ the amount of products produced

Answer 6

Scientists want the position of equilibrium to be ‘AS FAR TO THE RIGHT’ as possible in order to MAXIMISE the amount of products produced

Question 7

Le Chatelier’s Principle

Answer 7

If a stress (different conditions) is applied to a system at equilibrium, the system readjusts to relieve the stress applied

Question 8

What are the ‘stresses’ that can be applied to a system at equilibrium?

Answer 8

Temperature
Concentration
Pressure
Catalysts

Question 9

What happens if the temperature of a system at equilibrium is decreased?

Answer 9

The reaction that adds heat to the reaction (the exothermic one) will be favoured, i.e, goes in the exo direction

Question 10

What happens if the temperature of a system at equilibrium is increased?

Answer 10

The reaction that absorbs heat (the endothermic one) will be favoured, i.e, goes in the endo direction

Question 11

What is important to note about changing the pressure of a system at equilibrium?

Answer 11

Changing the pressure of a system at equilibrium only applies to reactions involving gases. Also only applies when there are a different number of molecules in the forward and reverse reactions

Question 12

What happens if the pressure of a system at equilibrium is reduced?

Answer 12

The system favours/goes to the side with the greater number of molecules (take up more room)

Question 13

What happens if the pressure of a system at equilibrium is increased?

Answer 13

The system favours/goes to the side with the smaller number of molecules (i.e brings a reduction in volume)

Question 14

What happens to a system at equilibrium when the concentration of ‘X’ increases?

Answer 14

The system goes in the direction that removes ‘X’

Question 15

What happens to a system at equilibrium when the concentration of ‘X’ decreases?

Answer 15

The system goes in the direction that forms ‘X’

Question 16

What are the three ‘conditions’ of the effect of catalysts on reactions in equilibrium?

Answer 16

○ It must be a positive catalyst for both the forward and backwards reactions
○ It will not change the amount of each reactant and product at the point of equilibrium
○ The catalyst speeds up the rate at which the position of equilibrium is reached, but does not actually change that position

Question 17

Ammonia can be manufactured by combining…

Answer 17

…nitrogen with hydrogen using an iron catalyst

Question 18

What is the effect of pressure on the Haber Process?

Answer 18

At high pressure, the reaction is forced to the right and so increases the yield of ammonia.

However, this is expensive to generate and is dangerous, so a compromise pressure of about 200 atmospheres is used

Question 19

What is the effect of temperature on the Haber Process?

Answer 19

The forward reaction is exothermic, so if we lower the temperature the forward reaction will increase.

However, lower temperatures decrease the rate of reaction and the process is uneconomically slow, so a compromise temperature of about 500°C is used

Question 20

According to Le Chatelier’s Principle, what area the best conditions to maximise the yield of ammonia in the Haber Process?

Answer 20

High pressure and low temperature

Question 21

What is the catalyst used in the reaction to make sulfuric acid?

Answer 21

vanadium pentoxide

Question 22

What is the effect of pressure on the Contact Process?

Answer 22

High pressure forces the reaction to the right and so gives an increased yield of SO₃.

However, this is costly and the sulfur dioxide will liquefy at high pressure, so a pressure of just above atmospheric pressure is used

Question 23

What is the effect of temperature on the Contact Process?

Answer 23

The forward reaction is exothermic so lowering the temperature will favour the forward reaction.

However, if we lower the temperature, the rate of reaction will decrease so 450°C is used

Question 24

What does a larger value of Kc indicate?

Answer 24

The further the reaction has gone to completion, i.e, how far the equilibrium is to the right

Question 25

What does a lower value of Kc indicate?

Answer 25

Only a small fraction of the reactants have been converted into products i.e the equilibrium lies on the left

Question 26

Once equilibrium has been established for a given reaction, the value of Kc…..

Answer 26

….is constant for that given reaction

Question 27

What is the only factor that can change the value of Kc for a reaction that has established equilibrium?

Answer 27

A change in temperature