Module 3.2.3 - Chemical equilibrium

Question 1

What is a reversible reaction?

Answer 1

Both reactions are occurring at the same time
(A reaction made up of forward and backward reaction, at the same time)

Question 2

What is dynamic equilibrium?

Answer 2

Exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction

Question 3

What are characteristics of a dynamic equilibrium? (4)

Answer 3

• Rate of forward reaction is equal to the rate of the reverse reaction
• The system is closed
• The concentration of reactants/products do not change
• Macroscopic properties (temperature, pressure and concentration of reactants and products do not change)

Question 4

What factors affect dynamic equilibrium? (4)

Answer 4

• Concentration of reactants/products
• Pressure (gases)
• Temperature
• Catalysts

Question 5

What 4 points must you consider when answering an exam question relating to le chatelier’s principle?

Answer 5

1. Where (equilibrium shifts to to oppose change)
2. Why (what change is being opposed)
3. Composition (equilibrium position)
4. Is there a colour change?

Question 6

What is gas pressure?

Answer 6

The force exerted by gas particles/molecules when they collide with their container

Question 7

How do you calculate which side of the equilibrium has more moles of gas?

Answer 7

Add up each molecule for reactants and products
e.g. N2(g) + 3H2 (g) -> 2NH3 (g)
4 moles on left, 2 on right so higher pressure on left

Question 8

How do you determine what effect temperature change will have on dynamic equilibrium?

Answer 8

Given enthalpy change of the forward reaction (- is exothermic, + is endothermic)
Exo gives heat out to surroundings
Endo absorbs heat from surroundings

Question 9

How does a catalyst affect dynamic equilibrium?

Answer 9

DOES NOT AFFECT POSITION OF EQUILIBRIUM, only increasing rate of achieving equilibrium
Increases the rate of both forward and reverse reactions in an equilibrium by the same amount resulting in an unchanged position of equilibrium

Question 10

How does equilibrium affect industrial processes?

Answer 10

Want high % yield of product, but may have to be compromised between chemical equilibrium and reaction rate e.g. haber process

Question 11

What is equilibrium constant, Kc?

Answer 11

Gives a quantitative measure of the proportion (ratio) of product to reactant in an equilibrium mixture

Question 12

How do you form equilibrium constant?

Answer 12

• equilibrium concentrations raised to the power of balancing number
• products go on top of expression
• reactants go on the bottom of expression
  e.g. Kc = [Product]/[Reactant]

Question 13

What does the value of Kc signify?

Answer 13

Extent of chemical equilibrium
Kc = 1 (lies in the middle)
Kc < 1(lies towards the left, low yield)
Kc > 1 (lies towards the right, high yield

Question 14

The larger the value of Kc…

Answer 14

The further the position of equilibrium to the right and the higher concentration of products compared to reactants