Chemical Equilibrium in Reversible Reactions
DO NOW
•
Pick up notes.
•
Turn in Concentration and Reaction Rate lab.
•
23 days to get your late work in by the May 18
deadline and thus your grade up….
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CHEMICAL
EQUILIBRIUM
UNIT 14
REVERSIBLE REACTIONS
•
Most
reactions do not go all the way to
completion
(meaning all the reactants are used
up). Reactions can proceed in
both
directions.
•
This is usually indicated by: N
2
O
4 (g)
2 NO
2 (g)
•
A chemical reaction in which the products
regenerate
the original reactants is called a
REVERSIBLE REACTION
.
double
arrow
REVERSIBLE REACTIONS
•
In theory,
all
reactions are
reversible
.
•
Some
are reversible
on their own
and others are
reversible only under
restricted conditions
.
•
Do
not assume
that all reactions
proceed
until one
of the reactants is entirely
consumed
. GOOD
TEST QUESTION!
DEMO
Heat is absorbed when N
2
O
4
changes into
2 NO
2
molecules
N
2
O
4
(g)
2 NO
2
(g)
colorless
red-brown
EQUILIBRIUM TUBES
CHEMICAL EQUILIBRIUM
•
In equilibrium, it is important to remember that
reaction
rate
is
proportional
to
concentration
.
rate =
Δ
[Reactants or Products]
Δ
Time
•
Reaction
rate
is the
speed
at which a reaction occurs.
When the reaction goes
faster
, the reaction rate
increases
, but the actual time
decreases
.
CHEMIMCAL EQUILIBRIUM
•
How many times have we talked about
equilibrium?
•
What are some examples?
CHEMICAL EQUILIBRIUM
•
Chemical equilibrium is the
state
in which
the
concentrations
of reactants and
products remain
constant
with time.
•
The
rate
at which they are
formed
in each
reaction
equals
the
rate
at which they are
consumed
in the opposite direction.
•
The
concentration remains constant.
CHEMICAL EQUILIBRIUM
•
Chemical equilibrium is a
dynamic
, or changing,
process. But the
net change
in concentration is
zero
.
•
REMEMBER DYNAMIC EQUILIBRIUM?
Here it is again!
CHEMICAL EQUILIBRIUM
Rate
forward
= Rate
reverse
CHEMICAL EQUILIBRIUM
H
2
+ I
2
2HI
•
When a reaction is at equilibrium, the following
is true:
Rate
forward
= Rate
reverse
CHEMICAL EQUILIBRIUM
H
2(g)
+ I
2(g)
2HI
(g)
Rate
forward
= Rate
reverse
Rate
forward
= K
f
[H
2
][I
2
]
Rate
reverse
= K
r
[HI]
2
CHEMICAL EQUILIBRIUM
By moving the rates to the same side and calling
them K
eq
, we get:
K
eq
=
[HI]
2
product
Equilibrium Constant
[H
2
][I
2
]
reactant
CHEMICAL EQUILIBRIUM
TO SUM IT ALL UP
•
The
mass action law
states that if the system is
at
equilibrium
at a given temperature, then the
following ratio is a constant.
aA + bB
cC + dD becomes
Keq =
[C]
c
[D]
d
[A]
a
[B]
b
CHEMICAL EQUILIBRIUM
Keq =
[C]
c
[D]
d
[A]
a
[B]
b
1.
ALWAYS
products
divided by
reactants
;
2.
The
coefficient
becomes the
exponent
;
3.
Brackets
mean
concentration
.
CHEMICAL EQUILIBRIUM
•
The
Law of Chemical Equilibrium
states that at a given
temperature, a chemical system may reach a state in
which a particular ratio of reactant and product
concentrations has a constant value, K
eq
.
•
K
eq
is the
numerical
value of the
ratio
of product
concentration to reactant concentration with each
concentration raised to the power corresponding to its
coefficient.
•
It is
constant
only at a
specific temperature
. If
temperature
changes
, K
eq
changes
.
CHEMICAL EQUILIBRIUM
Write the equilibrium expressions for the following reactions. All
are gases.
Start out by writing “K
eq
= “
1.
3H
2(g)
+ N
2(g)
2NH
3(g)
2.
SO
2(g)
+ NO
2(g)
NO
(g)
+ SO
3(g)
3.
FeCl
3(aq)
+ 3KSCN
(aq)
3KCl
(aq)
+ Fe(SCN)
3(s)
CHEMICAL EQUILIBRIUM
RULES:
Use
GASES
and
AQUEOUS SOLUTIONS
in
the mass action expressions. Leave everything
else out.
Do not include
pure solids
or
pure liquids
in
your mass action expression. The concentration
of pure solids and liquids is
constant
and can be
determined by their densities so we leave them
out of the expression.
If it is an aqueous solution, it is not pure, and
must included
DETERMINING A NUMERICAL
VALUE FOR K
eq
•
Experiments must be performed to determine the
concentrations of reactants and products at equilibrium.
•
To determine the value of K
eq
, we will use the
equilibrium expression we just learned.
STEPS:
•
Balance
the chemical equation.
•
Write
the equilibrium (mass action) expression.
•
Plug
in the equilibrium concentrations to
solve
the
problem.
DETERMINING A
NUMERICAL VALUE FOR K
eq
Table 1
Initial
Initial
EQ
EQ
K
eq
Trial
[R]
M
[P]
M
[R]
M
[P]
M
1 0.0200
0.0
0.0172
0.0014 4.73
2 0.0300
0.0
0.0243
0.0028 4.74
3
0.0
0.0200
0.0310
0.00452 ?
Calculate K
eq
for the third trial. Use the equation: 2 R
P
K
eq
=
[P]
=
(0.00452)
=
[R]
2
(0.0310)
2
K
eq
has no units
NUMERICAL VALUE FOR K
eq
EXAMPLE:
What is the equilibrium constant for the following reaction
if the equilibrium or final concentrations are [I
2
] = 0.302
M
,
[H
2
] = 0.428
M
, and [HI] = 0.655
M
?
H
2
+ I
2
↔ 2HI
K
eq
=
TO REMEMBER
SOME THINGS TO REMEMBER:
1. If K
eq
is
small
(less than one), then the
reactants
are
favored
and equilibrium is established
before the product is formed.
2. If K
eq
is
large
(greater than one), then
products
are
favored
and equilibrium is established after
lots of product is formed.
3. If K
eq
=
1
, then the concentrations of the
reactants and products are
equal
.
TO REMEMBER
4. Each reaction has a
unique
K
eq
for every
temperature
.
If the temperature
changes
, the K
eq
changes
.
5. K
eq
DOES NOT
indicate the
time
it takes to reach
equilibrium. It only provides information about the
mixture of reactants and products at equilibrium.
TO DO
•
Do handout for tomorrow.
Chemical equilibrium occurs when the concentrations of reactants and products remain constant over time in a reversible reaction. Reaction rate is proportional to concentration, and equilibrium is reached when the rate of formation equals the rate of consumption in both directions. Reversible reactions play a key role in maintaining equilibrium, where heat absorption and color changes may indicate shift. Understanding equilibrium dynamics is crucial in chemistry labs and tests.
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Presentation Transcript
Pick up notes. Turn in Concentration and Reaction Rate lab. 23 days to get your late work in by the May 18 deadline and thus your grade up . DO NOW
UNIT 14 CHEMICAL EQUILIBRIUM
Most reactions do not go all the way to completion (meaning all the reactants are used up). Reactions can proceed in both directions. This is usually indicated by: N2O4 (g) 2 NO2 (g) double arrow A chemical reaction in which the products regenerate the original reactants is called a REVERSIBLE REACTION. REVERSIBLE REACTIONS
In theory, all reactions are reversible. Some are reversible on their own and others are reversible only under restricted conditions. Do not assume that all reactions proceed until one of the reactants is entirely consumed. GOOD TEST QUESTION! REVERSIBLE REACTIONS
Heat is absorbed when N2O4 changes into 2 NO2 molecules N2O4 (g) 2 NO2 (g) colorless red-brown DEMO
In equilibrium, it is important to remember that reaction rate is proportional to concentration. rate = [Reactants or Products] Time Reaction rate is the speed at which a reaction occurs. When the reaction goes faster, the reaction rate increases, but the actual time decreases. CHEMICAL EQUILIBRIUM
How many times have we talked about equilibrium? What are some examples? CHEMIMCAL EQUILIBRIUM
Chemical equilibrium is the state in which the concentrations of reactants and products remain constant with time. The rate at which they are formed in each reaction equals the rate at which they are consumed in the opposite direction. The concentration remains constant. CHEMICAL EQUILIBRIUM
Chemical equilibrium is a dynamic, or changing, process. But the net change in concentration is zero. REMEMBER DYNAMIC EQUILIBRIUM? Here it is again! CHEMICAL EQUILIBRIUM
Rateforward = Ratereverse CHEMICAL EQUILIBRIUM
H2 + I2 2HI When a reaction is at equilibrium, the following is true: Rateforward = Ratereverse CHEMICAL EQUILIBRIUM
H2(g) + I2(g) 2HI(g) Rateforward = Ratereverse Rateforward = Kf[H2][I2] Ratereverse = Kr[HI]2 CHEMICAL EQUILIBRIUM
By moving the rates to the same side and calling them Keq, we get: Equilibrium Constant [H2][I2] reactant Keq = [HI]2 product CHEMICAL EQUILIBRIUM
TO SUM IT ALL UP The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant. aA + bB cC + dD becomes Keq = [C]c[D]d [A]a[B]b CHEMICAL EQUILIBRIUM
Keq = [C]c[D]d [A]a[B]b 1. ALWAYS products divided by reactants; 2. The coefficient becomes the exponent; 3. Brackets mean concentration. CHEMICAL EQUILIBRIUM
The Law of Chemical Equilibrium states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value, Keq. Keq is the numerical value of the ratio of product concentration to reactant concentration with each concentration raised to the power corresponding to its coefficient. It is constant only at a specific temperature. If temperature changes, Keqchanges. CHEMICAL EQUILIBRIUM
Write the equilibrium expressions for the following reactions. All are gases. Start out by writing Keq= 1. 3H2(g) + N2(g) 2NH3(g) 2. SO2(g) + NO2(g) NO(g) + SO3(g) 3. FeCl3(aq) + 3KSCN(aq) 3KCl(aq) + Fe(SCN)3(s) CHEMICAL EQUILIBRIUM
RULES: Use GASES and AQUEOUS SOLUTIONS in the mass action expressions. Leave everything else out. Do not include pure solids or pure liquids in your mass action expression. The concentration of pure solids and liquids is constant and can be determined by their densities so we leave them out of the expression. If it is an aqueous solution, it is not pure, and must included CHEMICAL EQUILIBRIUM CHEMICAL EQUILIBRIUM
Experiments must be performed to determine the concentrations of reactants and products at equilibrium. To determine the value of Keq, we will use the equilibrium expression we just learned. STEPS: Balance the chemical equation. Write the equilibrium (mass action) expression. Plug in the equilibrium concentrations to solve the problem. DETERMINING A NUMERICAL DETERMINING A NUMERICAL VALUE FOR VALUE FOR K Keq eq
Table 1 Trial 1 0.0200 2 0.0300 3 Initial [R]M Initial EQ [P]M [R]M [P]M 0.0 0.0172 0.0014 4.73 0.0 0.0243 0.0028 4.74 0.0200 0.0310 0.00452 ? EQ Keq 0.0 Calculate Keq for the third trial. Use the equation: 2 R P Keq = [P] = (0.00452) = [R] 2 (0.0310)2 Keq has no units DETERMINING A DETERMINING A NUMERICAL VALUE FOR NUMERICAL VALUE FOR K Keq eq
EXAMPLE: What is the equilibrium constant for the following reaction if the equilibrium or final concentrations are [I2] = 0.302M, [H2] = 0.428M, and [HI] = 0.655M ? H2 + I2 2HI Keq = NUMERICAL VALUE FOR NUMERICAL VALUE FOR K Keq eq
SOME THINGS TO REMEMBER: 1. If Keq is small (less than one), then the reactants are favored and equilibrium is established before the product is formed. 2. If Keq is large (greater than one), then products are favored and equilibrium is established after lots of product is formed. 3. If Keq = 1, then the concentrations of the reactants and products are equal. TO REMEMBER
4. Each reaction has a unique Keq for every temperature. If the temperature changes, the Keqchanges. 5. KeqDOES NOT indicate the time it takes to reach equilibrium. It only provides information about the mixture of reactants and products at equilibrium. TO REMEMBER
Do handout for tomorrow. TO DO
