Page 62 - A Working Method Approach For Introductory Physical Chemistry Calculations

46 Chapter 4
increased, there is a shift in the position of equilibrium in the direction
that tends to reduce the pressure as predicted by Le Chiitelier. From
the equation of state of an ideal gas, pY = AnRT, i.e. p = (An/V)RT.
Therefore p oc An For a reduction in pressure to occur, n must
decrease. Therefore, the total number of molecules must decrease.
This is done by shifting the position of equilibrium from left to right,
i.e. four gaseous molecules to two gaseous molecules.
It must be emphasised here that although the position of equili-
brium shifts to the right, the value of the equilibrium constant does
not change. Conversely, if the pressure is decreased, the equilibrium
shifts to the left. In the case of changes in temperature, the value of Kc
does change.

Changes in Concentration

BiC13(aq) is a cloudy solution due to a hydrolysis reaction (reaction
with water): BiCl,,,,) + H200) F-= BiOCI,,) + 2HCl(aq,. If some
concentrated hydrochloric acid is added, the position of equilibrium
shifts in the direction that will absorb the acid, i.e. from right to left.
Therefore the hydrolysis reaction is considerably decreased resulting
in the formation of a clear solution.
However, the solution does not absorb all the acid.

Effect of a Catalyst on Equilibrium

A catalyst is a reagent which accelerates or retards (anti-catalyst) the
rate of a chemical reaction, but is not itself consumed in the reaction,
and it has no effect on the equilibrium concentration or the value of
the equilibrium constant.
An iron catalyst is used in the Haber process, used to manufacture
ammonia according to the equation: N2(g) + 3H,,,) * 2NH3(g), Kp =
P(NH~)~/(P(N~)'P(H~)~}. the role of the catalyst is to make the
Here
reaction attain equilibrium more rapidly at the relatively low tempera-
ture employed (400-600 "C).

SUMMARY

The most important feature of this chapter is the working method for
solving simple equilibrium type problems. Two important equations
should be memorised: Kp = Kc(RT)A"# and AG = AGO + RTln Kp,
where at equilibrium AG = 0.