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Dynamic
Equilibrium in
Chemical
Systems
Chapter 7.1
Types of Equilibrium
Equilibrium
A dynamic system in which chemical changes are
taking place in such a way that there is no overall
change in the composition of the system
A balance between forward and reverse processes
occurring at the same rate
The Constancy of the Macroscopic
The Inconsistency of the Microscopic
Evidence for Equilibrium
1. Solubility
• In a closed system, the rate of dissolution balances with
the rate of crystallization
2. Phase
• In a closed system, the rate of evaporation balances with
the rate of condensation
3. Chemical Reaction
• In a closed system, the rate of formation of product
balances with the rate of formation of the reactants
Chemical Equilibrium
Equilibrium means balance or steadiness
In chemistry, an equilibrium reaction is when there are two
opposite reactions that are balanced.
Equilibrium Reactions:
This is a reaction that has two directions, a forward and
reverse reaction.
An equilibrium reaction is reversible
We tend to think that all reactions go in only one direction. What
is called the forward reaction.
Forward reaction
CaCO3 (aq) + HCl (aq)  CaCl2 (aq) + CO2 (g) + H2O (l)
In this reaction the reactants react to form products.
It goes to completion, or until one of the reactants runs out
But reactions are not like this!
H2O(g) + CO (g)
H2 (g) + CO2 (g)
Reverse reaction
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Dynamic Equilibrium
Changes in reaction rates of the forward and reverse reactions
for:
H2O + CO
H2 +CO2
If kept in a closed system, then both reactions will occur. If
this reaction is left then it will reach a point of equilibrium
where the rate of the forward reaction equals the rate of the
reverse reaction. So the concentrations of the reactants doesn’t
change.
The equilibrium is dynamic, because even though the
concentrations stay the same and nothing appears to be
happening, both the forward and reverse reactions are continuing
to occur.
[ ] of A decreases while
[ ] of B increases till
equilibrium is reached.
Rate of forward reaction decreases while reverse increases till the
concentrations reach a level at which the rate of the forward and
reverse reactions is the same. The system has reached equilibrium.
Calculating Concentration at
Equilibrium
ICE tables
Equilibrium is reached
when rate of forward
reaction is the same as
the reverse reaction.
Initial
Example
Change
Equilibrium
I.C.E. Charts
H2(g) + F2(g)
Consider the following equation at SATP
H2(g) + F2(g)
Concentrations
(mol/L)
2 HF (g)
2 HF (g)
If the reaction begins with 1.00 mol/L concentrations of
H2(g) and F2(g) and no HF(g), calculate the
concentrations of H2(g) and HF(g) at equilibrium if the
equilibrium concentration of F2(g) is measured to be
0.24 mol/L.
Initial concentration (mol/L)
Change in concentration
(mol/L)
Equilibrium concentration
(mol/L)
1.0
-x
1.0
-x
1.0-x 1.0-x
0.0
+2x
2x
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Since we know that the equilibrium value for F2 (g) is
0.24 mol/L we can solve for x in the F2 (g) column
THE END
1.0 mol/L – x = 0.24 mol/L
-x = 0.24 – 1.0
X = 0.76 mol/L
Last step, use the value of x to solve for all other
Equilibrium concentrations, in ICE table…
[H2(g)] = 1.0 mol/L – x = 1.0 – 0.76 = 0.24 mol/L
[HF(g)]= 2x = 2(0.76 mol/L) = 1.52 mol/L
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