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Completion Reactions
Some reactions go to completion.
• Here, all of the reactants are used up, as
they all turn into products.
es
Equilibrium 9.1
e.g. HCl(aq) Æ H+(aq) + Cl-(aq)
N
ot
Dynamic Equilibrium
Equilibrium Constants (Keq and Kc)
The Equilibrium Expression, Kc
ts
'L
ec
tu
re
For these reactions, a one way arrow is used.
Dynamic Equilibrium
N2O4(g)
2NO2(g)
of
e.g. N2O4(g)
[NO2]
[N2O4]
Time
C
op
y
For these reactions, a two way arrow is used.
Concentration (mol/L)
St
ud
• Here, all of the reactants do not get used up.
• The system reaches a dynamic state where
reactants are continually turning into products,
and products are continually turning back into
reactants.
en
Most reactions do not go to completion.
2NO2(g)
2NO2(g)
Chemical Equilibrium
• Forward and reverse reactions continue
to take place.
• The rate of the forward reaction equals
the rate of the reverse reaction.
Sa
Rate of Reaction
m
pl
e
N2O4(g)
N 2O 4
• Concentrations of all species remain
constant, yet they will rarely be equal to
one another.
NO2
Time
• Takes place in a closed system
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The Equilibrium Constant (Keq)
Ex1) Equilibrium Expressions
Ex1) Write the equilibrium expression for:
Species involved in the reaction
aA + bB U dD + eE
N 2( g ) + O 2( g ) U 2 NO( g )
N
ot
Units for Concentration
mol/L = M
re
[D]d [E]e
[A]a [B]b
ec
tu
concentration
constant
Kc =
es
stoichiometric coefficients
square brackets represent
concentration in [mol/L]
ts
'L
Solids are left out of the
equilibrium expression
Ex2) Equilibrium Expressions
St
[CO 2 ]
[CO]
of
Kc =
ud
NiO( s ) + CO( g ) U Ni( s ) + CO 2( g )
en
Ex2) Write the equilibrium expression for:
• Concentration is measured in mol/L.
• Both pieces have the same density, so both contain the same
number of particles per unit volume.
C
op
y
Solids are left out of the equilibrium expression.
Volume of
NiO(s) at
Equilibrium
• The concentration of Nickel (II) Oxide has not changed.
Initial Volume
of NiO(s)
m
pl
e
Ex3) Equilibrium Expressions
Liquids are left out of the
equilibrium expression
Ex3) Write the equilibrium expression for:
Sa
4 HCl( aq ) + O2( g ) U 2 Cl2( g ) + 2 H 2 O(l )
[Cl2 ]2
Kc =
[HCl]4 [O 2 ]
Adding or taking
away small
amounts of water
in a reaction that
takes place in an
aqueous solution
does not affect
the overall
concentration of
H2O.
Liquids are left out of the equilibrium expression.
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Equilibrium Expressions
Ex) Equilibrium Constant
Ex) Find Kc for the following system at 400 K, if the
equilibrium concentrations are: [NH3] = 0.031 M,
[N2] = 0.85 M, and [H2] = 0.0031 M.
When writing an equilibrium expression:
• Include gases (g) and aqueous species (aq).
es
N 2( g ) + 3 H 2( g ) U 2 NH 3( g )
Sa
m
pl
e
C
op
y
of
St
ud
en
ts
'L
ec
tu
re
N
ot
• Do not include liquids (l) or solids (s).
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