chemical equilibria

Question 1

What is the difference between an irreversible and reversible reaction?

Answer 1

• an irreversible reaction stops when all the reactants have been used up
• a reversible reaction it never stops, it reaches equilibrium when the rate of the forwards and backwards reactions are equal

Question 2

What is dynamic equilibrium?

Answer 2

The position of equilibrium is constantly moving

Question 3

What is the main condition for equilibrium?

Answer 3

The system must be closed

Question 4

What is equilibrium mixture?

Answer 4

A mixture in which the proportions of all the components remain constant (not the same)

Question 5

What is the proportion of reactants and products in an equilibrium mixture?

Answer 5

any proportion (doesn’t have to be 50:50

Question 6

What is the main problem associated with reversible reactions in terms of yield?

Answer 6

You can never obtain a 100% yeild

Question 7

What happens in a reversible reaction at equilibrium?

Answer 7

1. The forwards and backwards reactions occur at equal rates

2. The concentrations of reactants and products remains constant

Question 8

What is Le Chatelier’s principle?

Answer 8

When a change is applied to a system equilibrium, the system responds to oppose the applied change and restore equilibrium

Question 9

What factors affect Le Chatelier’s principle?

Answer 9

1. Concentration
2. Pressure
3. Temperature

Question 10

What effect does the change in the concentration have on an equilibrium reaction?

Answer 10

If the concentration of a reactant is increased:
- the equilibrium shift right/froward (from reactants to products) to oppose the change
- this change reduces the concentration of reactants and increases the concentration of products
If the concentration of a product is increased:
- the equilibrium shift left (from products to reactants) to oppose the change
- this change reduces the concentration of products and increases the concentration of reactants

Question 11

How do you change the pressure of equilibrium mixture?

Answer 11

1. Reducing the container size/volume

2. Increasing the number of molecules

Question 12

Why does changing the pressure affect equilibrium?

Answer 12

particles will collide more frequently with each other and with the container walls

Question 13

What effect this change in the pressure have on an equilibrium reaction with a larger number of mole of reactants than products?

Answer 13

If the pressure is increased:
- the equilibrium will shift right/ forward (from reactants to products) to oppose the change
- this change will reduce the pressure of the reactants and increase the pressure of the products
If the pressure is decreased:
- the equilibrium will shift left/backward (from products to reactants) to oppose the change
- this change will reduce the pressure of the products and increase the pressure of the reactants

Question 14

What is the difference between exothermic and endothermic reactions?

Answer 14

• exothermic reactions release heat

- endothermic reactions take in heat

Question 15

Will ∆H be positive or negative for an exothermic reaction?

Answer 15

negative

Question 16

Will ∆H be positive or negative for an endothermic reaction?

Answer 16

positive

Question 17

If the forward reaction is exothermic what will be the backwards reaction?

Answer 17

Endothermic

Question 18

If the forward reaction equilibrium reaction is exothermic, what effect is changing the temperature have?

Answer 18

If the temperature is increased:
- the equilibrium will shift left/backwards (from products to reactants) in the endothermic direction to oppose the change
- this increases the temperature of the reactants and decreases a temperature of the products
If the temperature is decreased:
- the equilibrium will shift right/forwards (from reactants to products) in the exothermic direction to oppose the change
- this increases the temperature of the products and decreases a temperature of the reactants

Question 19

What effect does the catalyst have on an equilibrium reaction?

Answer 19

• no effect

- catalyst to increase the rate of the forwards and backwards reaction equally

Question 20

Why, in industry, would a compromise between pressure and temperature be needed?

Answer 20

• High temperatures and pressures are expensive to maintain

- the reaction can occur at a lower enough temperature and pressure that it is still producing products effectively

Question 21

How do you work out equilibrium constant?

Answer 21

Kc = concentration of products / concentration of reactants

Question 22

For the equation:

aA + bB cC +dD what would Kc be?

Answer 22

Kc = [C]c X [D]d / [A]a X [B]b /

Question 23

What are the square brackets signify?

Answer 23

Concentration

Question 24

How does the equilibrium constant and Le Chatelier’s principle link?

Answer 24

The equilibrium will shift to keep Kc constant

Question 25

Are Kc’s units the same every time?

Answer 25

no, they very depending on the ration between reactants and products

Question 26

What happens to the units of Kc if the number of reactants and products is the same?

Answer 26

there are no units

Question 27

What would be the units for Kc if:

Kc = [A] / [B]2 X [C]

Answer 27

mol - 2 dm 6

Question 28

What factors affect Kc?

What factors don’t affect Kc?

Answer 28

affecting:
- temperature
not affecting:
- pressure 
- concentration 
- addition of a catalyst