EQUILIBRIUM
SYSTEMS
SYSTEMS
Recall that there are THREE types of systems:
● CLOSED: a system which may change
energy but not matter with its surroundings
● OPEN: a system which may exchange both
energy and matter with its surroundings
● ISOLATED: ideal system in which neither
matter nor energy can move in or out
CHEMICAL EQUILIBRIUM
● A state of reaction in which
all reactants and products
have reached constant
concentrations in a closed
system
● Any chemical reaction in a
closed system will
eventually reach
equilibrium
DYNAMIC EQUILIBRIUM
A balance between
forward and reverse
processes that are
occurring
simultaneously at equal
rates
H2O (g) + CO (g) 🡪 H2 (g) + CO2 (g)
•[reactants] decreases as [products]
increases
•Dashed line is when reactants and
products remain stable
•This is when chemical equilibrium
has been reached
•No further changes will occur to
concentrations (unless chemical
system is changed)
EQUILIBRIUM POSITION
● The relative
concentration of
reactants and
products in a system
in dynamic
equilibrium (dashed
line on graph)
REVERSE REACTIONS
Chemical reactions that proceed in BOTH the
forward and reverse directions
EXAMPLE:
H2O (g) + CO (g) ⇌ H2 (g) + CO2 (g)
FORWARD AND REVERSE
REACTIONS
● For a closed chemical equilibrium system in
constant environmental conditions, the same
equilibrium concentrations are reached
regardless of the direction by which the
equilibrium was reached
EXAMPLE:
N2O4 (g) ⇌ 2 NO2 (g)
Exp. 1
Exp. 2
Initial [] (mol/L)
Final [] (mol/L)
N2O4 (g)
NO2(g)
N2O4(g)
NO2(g)
0.750
0
0
1.50
0.721
0.721
0.0580
0.0580
STOICHIOMETRY AND
CHEMICAL EQUILIBRIA
Changes in the concentration of products
and reactants can be predicted as a
system approaches equilibrium based on
the coefficients of a balanced chemical
equation
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)
● This reaction produces ammonia which are
used in fertilizer production
● Reactions with N2, H2, NH3 occur very slowly
at 25 ⁰C due to the strength of the bonds
● Catalysts are used to increase the reaction rate
● For every 1 mol of N2, 3 mol of H2 CONSUMED
2 mol NH3 are FORMED (vice versa for reverse
rxn)
EQUILIBRIUM
CALCULATIONS
1. Use the ICE table to perform equilibrium
calculations
2. Add INITIAL CONCENTRATIONS to the I row.
3. Use the variable x to indicate the CHANGE IN
CONCENTRATION in the C row
4. Place the EQUILIBRIUM CONCENTRATION in the
E row
5. Use algebra to solve for the variable x , and the
concentrations.
BALANCE CHEMICAL EQUATION
I
C
E
PRACTICE
● When ammonia is heated, it decomposes into nitrogen
gas and hydrogen gas:
2 NH3(g) ⇌ N2(g) + 3 H2(g)
● When 4.0 mol of ammonia is introduced into a 2.0 L
container, the system reaches equilibrium after 6.0 s. At
equilibrium, there are 2.0 mol of ammonia in the
container. Calculate the equilibrium concentration of
nitrogen and hydrogen gas.
PRACTICE
Consider the following reaction:
H2 (g) + F2(g) ⇌ 2HF(g)
● If the rxn begins with 2.0 mol/L of H2 and F2 and no HF,
calculate the concentrations of H2 and HF at equilibrium
if the equilibrium concentration of F2 is measured to be
0.48 mol/L.