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 The Equilibrium Expression (Mass Action Expression)

At equilibrium the following mathematical relationship exists. Substances that are excluded from this equation are SOLIDS AND LIQUIDS. They CANNOT change concentrations (they are pure), so they can not be included in this equilibrium expression.

Example of equilibrium constants

 Reaction (all substances in the gas phase) Equilibrium Expression 3 O2 <===> 2 O3 N2 + 3 H2 <===> 2 NH3 H2 + I2 <===> 2 HI PCl5 <===> PCl3 + Cl2 SO2 + (1/2) O2 <===> SO3

What does K imply?

Large K > 1 products are "favored"

K = 1 neither reactants nor products are favored

Small K < 1 reactants are "favored"

What do  equilibrium constants with solids look like?

 The equilibrium produced on heating carbon with steam   Everything is exactly the same as before in the equilibrium constant expression, except that you leave out the solid carbon. The equilibrium produced between copper and silver ions   Both the copper on the left-hand side and the silver on the right are solids. Both are left out of the equilibrium constant expression.

The equilibrium produced on heating calcium carbonate

This equilibrium is only established if the calcium carbonate is heated in a closed system, preventing the carbon dioxide from escaping.

The only thing in this equilibrium which isn't a solid is the carbon dioxide. That is all that is left in the equilibrium constant expression.

Why you don't include the solids or liquids?

ANIMATION-Notice the change when more solid is added to the system. Nothing Changes. That is why it needs to be excluded.

More Practice Problems-

 ANSWERS 2SO2(g) + O2(g) <-----> 2SO3(g) K = [SO3]2 /[SO2]2 [O2] SO2(g) + 1/2O2(g) <-----> SO3(g) K= [SO3] / [SO2] [O2]1/2 4NH3(g) + 5O2(g) <-----> 4NO(g) + 6H2O(g) K= [NO]4 [H2O]6 / [NH3]4 [O2]5 Pb(NO3)2(s) <-----> PbO(g) + 2NO2(g) + 1/2O2(g) K = [PbO] [NO2]2 [O2]1/2