Intro to Dynamic Equilibrium in Chemical Systems
Dynamic equilibrium: A balance between the forward and reverse
processes occurring at the same rate.
There are 3 kinds:
1.
SOLUBILITY EQUILIBRIUM
-In a closed system
-between solute and solvent in a saturated solution
Example: NaCl Crystal in solution.
-Once NaCl ions are dissolved, collisions between water, collisions
between water and remaining crystal continue:
a) if a dissolved ion collides with crystal a new ionic bond can form (=
more crystal)
b) if a water molecule collides with the crystal, ions can break off of
the crystal and dissolve
-Early on, the # molecules dissolving > # crystallizing
-When equilibrium is near, the rates approach one another.
-When equilibrium is reached, rate of dissolution = rate of crystallization.
And there are no observable changes in [ions] or quantity of solid present.
2.
PHASE EQUILIBRIUM
-occurs in a closed system
Example:
-When liquid is placed in a closed container, initially, only evaporation
occurs.
-as the # of molecules in gas phase increases, more gas molecules collide
with the surface of the liquid and lose enough energy to join that phase.
-When rate of condensation = rate of evaporation, equilibrium is reached.
-Note: in these cases, evaporation increases as temperature increases
-Note: in an open system/container, equilibrium can’t be established
because molecules that leave the system can’t get back
3. CHEMICAL REACTION EQUILIBRIUM
-quantitative reactions are chemical reactions that go to completion: ALL
reactants are converted to products
Example 1: In an open system, CO2 escapes, preventing the reverse
reaction from occurring.
If it was in a closed system, you would still find products after the
reaction should have been completed.
Example 2:
Whether you start with N2O4 or NO2 , the same equilibrium
concentrations are achieved. This is called a reversible reaction.