Kc

Question 1

Explain the equilibrium law.

Answer 1

The equilibrium states that for the equilibrium

aA +bB cC + dD

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

Kc is the equilibrium constant in terms of concentrations.

The equilibrium concentration of each product and reactant is raised to the power of its balancing number in the overall equation.

Question 2

Dinitrogen tetroxide, N2O4 is a colourless gas.Nitrogen dioxide, NO2, is a dark brown gas.

N2O4 and NO2 are linked by a reversible reaction.

If N2O4 is placed in a sealed container explain what happens to the concentrations of the reactants/products till they reach dynamic equilibrium.

Answer 2

If N2O4 is placed in a seal container:
-N2O4 molecules start to decompose into NO2 molecules- because there will be very few NO2 molecules present, the reverse reaction will take place slowly.

As the reaction proceeds:

• There are fewer N2O4 molecules available to decompose and the N2O4 concentration decreases- the rate of the forward reaction increases.
• There are more NO2 molecules to recombine and the NO2 concentration increases - the rate of the reverse reaction increases.

Eventually, a dynamic equilibrium is established:

• the forward reaction takes place at exactly the same rate as the reverse reaction.
• The concentrations of the reactants and products are constant.

Question 3

What are the units for Kc?

Answer 3

The units for Kc expressions can be worked out afresh after each Kc expression. Each concentration term in the Kc expression is replaced by its units.

Question 4

What does the magnitude of Kc indicate?

Answer 4

The magnitude of Kc indicates the extent of a chemical reaction.

Question 5

What happens in an equilibrium where Kc > 1?

Answer 5

• The reaction is product favoured.

- The products on the right-hand side predominate at equilibrium.

Question 6

What happens in an equilibrium where Kc <1?

Answer 6

• The reaction is reactant-favoured.

- The reactants on the left-hand side predominate at equilibrium.

Question 7

What does the value of Kc change with?

Answer 7

Kc only changes with temperature.

But the way that Kc changes is linked to Change in H.

Question 8

Is Kc affected by changes in pressure and concentration?

Answer 8

No

Question 9

Explain why Kc is unaffected by changes in concentration.

Answer 9

If a certain product is added to an equilibrium the concentrations change and the equilibrium position shifts to restore to the Kc value.

Question 10

Explain why Kc is unaffected by changes in pressure.

Answer 10

If the pressure in an equilibrium is doubled the concentrations of the products and reactants will double.

This will change the equilibrium constant and the equilibrium constant will shift to restore the concentrations back to normal.

Question 11

How does the presence of a catalyst affect Kc?

Answer 11

Catalysts affect the rate of a chemical reaction, but not the position of equilibrium. We reach the equilibrium more quickly with a catalyst.

Question 12

What does a large value for Kc mean?

Answer 12

A large value for k means a fast rate of reaction.