Question

1.       Consider the reaction for the production of HI from H₂ and I₂.

               H₂ (g)+ I₂(g) D 2HI(g)     

a)       What is the initial concentration of HI?

b)      What is the equilibrium concentration of HI?

c)       How is the change in concentration of HI calculated? Explain:

d)      How are the changes in concentration for H₂ and I₂ calculated? Explain

e)       How are the equilibrium concentrations of H₂ and I₂ calculated? Explain

Answer 1

The reaction

H2 (g)+ I2(g) 2HI(g)

This is a equilibium reaction

a) For the forward reaction the rate can be calculated as

r = k [H₂]^a [I₂]^b

This gives the initial rate and from the initial rate we can calculate the initial concentration of HI

b) For this reaction

The equilibrium constant expression for the reaction is

Kc = [HI]²/ [H₂][I₂]

If you have the value of Kc you can find the concentration of HI.

c) To calculate the change in concentrations of either he product HI or the reactants H₂ or I₂

we will need the initial concentrations of the starting material along with the equilibrium rate constant. Subsequently knowing this we can setup an ICE table which is the INITIAl, Change and equilibrium concentration table. Using this and substituting for the required unknown we can calculate the equilibrium concentrations of all the species involved. From this the change in concentration can be calculated.

Given the equation:

H₂ + I₂ ⇌ 2 HI

when [H₂]_o = X1 M and [I₂]_o = X2 M and K_c = Y

The ICE table

	[H2]	[I2]	[HI]
InitialX	X1	X2	0
Change	- x	- x	+ 2x
Equilibrium	X1 - x	X2 - x	2x

Now put values into the equilibrium expression.

K_c = [HI]² / ([H₂] [I₂])

Plugging values into the expression gives:

Y = (2x)² / ((X1 - x) (X2 - x))

Now all calculations can be made when X1, X2 and Y are known.