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Equilibrium Expressions for Heterogeneous Equilibrium

SECTION 13 - Chemical Equilibrium

These have all been instances of homogeneous equilibria, where only one phase is involved.

Heterogeneous Equilibrium

Many equilibria however, involve more than one phase.  Here's an example of a heterogeneous equilibrium...

ex:    CaCO3(s)  ⇌  CaO(s)  +  CO2(g)

How to Write Heterogeneous Equilibrium Expression

The correct equilibrium expression is K = [CO2]

Why?

Because the concentrations of pure substances such as liquids and solids cannot change!

The take-home message here, is the following:

"Equilibrium expressions only include aqueous (aq) and gaseous (g) reactants and products."

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Heterogeneous Equilibrium Examples

ex:  Write equilibrium expressions for Kc and Kp

  ( K is the same thing as Kc )

  a.   P4(s)  +  5O2(g)  ⇌  P4O10(s)

Heterogeneous Equilibrium Example
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  b.   NH4NO3(s)  ⇌  N2O(g)  +  2H2O(g)

Examples of Heterogeneous Equilibria
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  c.    NH4NO3(aq)  ⇌  N2O(g)  +  2H2O(l)

 Equilibrium Expression for NH4NO3

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Coming up next as we continue with SECTION 13 - Chemical Equilibrium,

We'll talk about the Extent of a Reaction, and the Reaction Quotient, Q