Gases Equilibrium add-in

Question 1

Define:

K_c, the molar concentration equilibrium constant

Answer 1

K_c is the value of K_eq when all reactant and product concentrations are given in concentration units, such as mol/l.

When reactant and product concentrations are given in these units, K_c will be the calculated value of equilibrium.

Question 2

Define:

K_p, the partial pressure equilibrium constant

Answer 2

K_p is the value of K_eq when all reactant and product concentrations are given in pressure units, such as atm.

When reactant and product concentrations are given in these units, K_p will be the calculated value of equilibrium.

Question 3

How can the value of K_p be calculated, given the value of K_c?

Answer 3

The equation relating K_p to K_c is:

K_p = K_c (RT)^Δn

where:

R = the ideal gas constant in l-atm/K-mol
T = the absolute temperature in K
Δn = the change in moles of gas (moles product gas - moles reactant gas)

Question 4

At STP, if the number of moles of gas decreases significantly in a reaction, what will be the relationship between K_p and K_c?

Answer 4

K_c will be greater than K_p.

The equation relating K_p to K_c is:

K_p = K_c (RT)^Δn

Since at STP, R*T is greater than 1, and since Δn is negative if number of moles of gas decreases, this gives :

K_p = K_c (1 / [number greater than 1]), hence K_p is a fraction of the size of K_c.

Question 5

At STP, if the number of moles of gas remains the same in a reaction, what will be the relationship between K_p and K_c?

Answer 5

K_c will be equal to K_p.

The equation relating Kp to Kc is:

K_p = K_c (RT)^Δn

If the number of moles of gas is unchanged, Δn must equal zero, this gives:

K_p = K_c ([RT]⁰)=K_c * 1,
hence K_p = K_c.