Question

Calculating Equilibrium Concentrations

The concentrations of reactants and products for a chemical reaction can be calculated if the equilibrium constant for the reaction and the starting concentrations of reactants and/or products are known.

Part A

Carbonyl fluoride, COF2, is an important intermediate used in the production of fluorine-containing compounds. For instance, it is used to make the refrigerant carbon tetrafluoride, CF4 via the reaction

2COF2(g)⇌CO2(g)+CF4(g),    Kc=7.20

If only COF2 is present initially at a concentration of 2.00 M, what concentration of COF2 remains at equilibrium?

Express your answer with the appropriate units.

Part B

Consider the reaction

CO(g)+NH3(g)⇌HCONH2(g),    Kc=0.900

If a reaction vessel initially contains only CO and NH3 at concentrations of 1.00 M and 2.00 M, respectively, what will the concentration of HCONH2 be at equilibrium?

Express your answer with the appropriate units.

Answer 1

Part A

2COF2(g)⇌CO2(g)+CF4(g),    Kc=7.20

Now we will make ICE table as follows:

                        2COF2(g)⇌        CO2(g)+                  CF4(g)

Intial Concentration ----------- 2.00 M         0                          0

Change in Conc. ----------- -- - 2x -              x                          x

Equilibrium Conc. ----------- 2 .00- 2x         x -                        x


Kc = [CO2][CF4] / [COF2]^2

Kc = (x)(x) / (2.00 - 2x)^2

Kc = x^2 / (2 - 2x)^2 = 7.20

7.20 =x2 / 4 +4 x2

x = [CO2] = [CF2] = 0.842

[COF2] = 2 - 2x = 2 - (2)(0.842)

[COF2] = 0.316

(b)

CO(g)   +NH3(g)⇌HCONH2(g),

Initial Conc. 1 .00       2 .00         0

Change in Conc. -X       -X         +X

Equilibrium Conc. 1 - X 2 - X       X


Given that Kc= 0.900

Kc = [HCONH2] / [CO][NH3]

Kc = x / (1 - x)(2 - x) = 0.900

0.900 = X/ 2-2X-1X +X2

1.8 - 2.7X+0.900 X^2 = X

0.900 X^2 -3.7 X+1.8= 0

x = [HCONH2] = 0.56