Kc Help!

[Scienuk]

Hello,

1. Why is Kc only effected by temperature and not pressure/volume and concentration? shouldn't Kc decrease for example say if volume is increased in the Haber process and equilibrium shifts to the left so Kc is supposed to decrease as concentration of reactants increases whereas concentration of products decreases? 

[kw0573]

1) Concentration does not change the equilibrium position, but only how fast you get to equilibrium (or the rate of reaction). Equilibrium constant depends on the ratio of forward and reverse rate constants, not on the actual initial concentration. Equilibrium constant is only expressed in concentrations because those are equilibrium concentrations.

2) By Le Chatelier's Principle, increase in pressure tends to shift equilibrium towards the side of the reaction with less gaseous moles of chemical species per mole of reaction (such as in the Haber process). Pressure only affects equilibrium position when a gas is involved. Most solids, liquids, and aqueous solutions do not vary much in volume with pressure.

3) By the same logic, volume will not affect reactions involving only solids, liquids, and aqueous solutions, when pressure and temperature are constant. However, the effect of changes in volume on gases cannot be assessed on it's own; it has to be translated to changes in pressure and temperature. By the ideal gas law
PV = nRT --> V = nRT/P

If volume increases because temperature increased at the same pressure, then you look at the enthalpy values; if volume increases because pressure decreased at the same temperature, then you look at the molecularity values. The specifics of gas expansion and compression is studied in thermodynamics, and in the simplest forms, through PV diagrams and carnot engines. Because such topics are not included in any part of the 2016 IB chemistry syllabus, you will not be assessed on effects of changing volume on gaseous reactions, when it is not known that either pressure or temperature is constant. 

Edited June 10, 2016 by kw0573