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The Equilibrium Expression

(Mass Action Expression)

 

-->FLASH The Meaning of the Equilibrium Constant<--

 

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

 

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