Page 57 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Equilibrium I 41
concentrations. Hence, it is essential that the equation is
correct, with the appropriate stoichiometry factors, uA, uB, uc,
uD, etc.
4. Having checked that the equation is completely balanced in step
3, write down an expression for the equilibrium constant for the
reaction of the form,
K, = products/reactants.
i.e. for the reaction vAA + uBB S ucC + vDD,
K, = [ C] uc [ D]””/ [A] vA [ B]””.
5. If gases are involved, write down an expression for Kp:

and relate Kp to &, by determining the change in the coefficents
of gaseous reagants!
i.e. Au = u (gaseous products) - u (gaseous reactants)
K~ = K,(RT)*”
Remember the order: PC- ‘political correctness’!
6. Convert any concentrations to mol dmW3 (M), etc. Create a
table of the initial and final concentrations at equilibrium,
letting x = the change in concentration. This is related to the
stoichiometry factors, uA, vB, vc, etc.
For example, in the reaction:
N2(g) + 3*2,, A
- 2NH3(,,
Initial conc./M 0.12 0.25 0
Change --x - 3x + 2x
Final conc./M 0.12 - x 0.25 - 3~ 0 + 2x

7. Substitute the final equilibrium concentrations into the expres-
sion for K, (step 4), and solve for x.
8. If the change (i.e. x) is assumed to be considerably less than the
initial concentration, this can be neglected. If x is then found to
be <5% of the initial concentration, the assumption made is
valid. If not, a quadratic equation of the form ax2 + bx + c = 0

-b f I/-
may need to be solved, with solution, x =
2a
9. Determine the final equilibrium concentrations.
10. State the units of K,. Remember also that Kp is dimensionless.