Question

Suppose HI at 0.5 atm, H2 at 0.01 atm and I2 at 0.005 atm are mixed in a 5 L flask. Which direction will the reaction shift to reach equilibrium (right, left, neither)? Calculate the equilibrium concentrations of all species. H2(g) + I2(g)  2 HI(g) Kp = 1.00 x 102 at 25 oC

Answer 1

The reaction will shift to the left to reach equilibrium. The equilibrium concentrations are:

[HI] = 0.483 atm

[H2] = 0.0235 atm

[I2] = 0.0185 atm

Determining the direction of the reaction

Calculate the reaction quotient (Qp):

Qp = (
`[HI]^2`) / (
`[H_2][I_2]`)

Qp = (
`0.5^2`) / (0.01 * 0.005)

Qp = 500

Compare Qp with Kp:

Since Qp (500) is much greater than Kp (1.00 x
`10^2`), the reaction will shift to the left to reach equilibrium.

Calculating equilibrium concentrations

Set up an equilibrium table:

Species Initial Concentration Change Equilibrium Concentration

H_2(g) 0.01 atm +x 0.01 + x

I_2(g) 0.005 atm +x 0.005 + x

HI(g) 0.5 atm -2x 0.5 - 2x

Substitute the equilibrium concentrations into the Kp expression:

Kp = (
`[HI]^2`) / (
`[H_2][I_2]`)

1.00 x
`10^2` = (
`(0.5 - 2x)^2`) / ((0.01 + x)(0.005 + x))

Solve the equation for x:

This is a quadratic equation and can be solved using the quadratic formula.

After solving, we get x = 0.0135 atm.

Calculate the equilibrium concentrations:

[HI] = 0.5 - 2x = 0.483 atm

[H2] = 0.01 + x = 0.0235 atm

[I2] = 0.005 + x = 0.0185 atm