DETERMINATION OF AN

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Experiment 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT
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There are two aspects to chemical reactions which
though, interrelated, are dealt with as two
separate topics. The first of these is the study
of the reaction from it initiation to a
point where the system seems to undergo no
further change, called chemical kinetics. The
deals with the system after all apparent
change have stopped, and is called chemical
equilibrium.
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Reversible reactions
A 3B 2C 2D
When reactants are mixed in stoichiometric
quantities, they are not completely converted
to products. Reactions that do not go to
completion and that can occur in either direction
are called reversible reactions.
forward
aA bB cC dD
backward
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What is chemical equilibrium?
Chemical equilibrium exists when two opposing
reactions occur simultaneously at the same rate
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Equilibrium, an analogy
Amount of water being pumped in
Amount of water being pumped out
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Equilibrium Constant
Kforward
nAA nbB nCC nDD
kbackward
At equilibrium
At equilibrium
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Upper case
Lower case
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What is equilibrium concentration?
Concentration of a reactant or a product left
after the equilibrium has been established.
Mathematically expressed
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A comparison of rate law and equilibrium constant
Rate law
Equilibrium constant
kforward
k
nAA nbB nCC nDD
nAA nbB nCC nDD
kbackward
bidirectional
unidirectional
k is called the rate constant and has units that
depend on order.
Keq is unitless
m and n are the order of the reaction due to A
and B respectively. They have to be determined
experimentally
nAA, nb, nCC and nD are stoichiometric coefficient
s of A,B,C and D respectively. obtained from the
balanced chemical reaction
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Experiment
Determination of equilibrium constant of
the following reaction
Fe(H2O)63 SCN- FeSCN(H2O)52
H2O
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For H2O
H2Oreacted is much smaller compared to
the total amount of H2O in the solution. So we
can assume that H2O essentially remains
constant for this equilibrium.
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How do we follow this equilibrium?
Fe(H2O)63 SCN- FeSCN(H2O)52
H2O
Fe3 SCN- FeSCN2
Yellow
Colorless
Red color
FeSCN2 absorbs maximum amount of light at 447 nm
447 nm corresponds to blue light region in the
visible light spectrum. FeSCN2 absorbs all the
blue light but lets all the red light pass
through. Hence the red color.
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How do we determine the equilibrium Constant (Kc)
of the reaction?
Fe3 SCN- FeSCN2
Yellow
Colorless
Red color
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What does the subscripts initial and equilibrium
refer to?
Initial concentration refers to the concentration
of the species at a time, when the reactants
have been mixed but the reaction has not begun.
equilibrium concentration refers to the
concentration of the species left at a time,
when the reactants have reacted and an
equilibrium has been achieved.
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How do we determine FeSCN2eqb?
Fe3 SCN- FeSCN2
Yellow
Colorless
Red color
Mix specific amounts of Fe3 solution to SCN-
solution. The resulting FeSCN2will make the
solution look red and absorbs 447 nm light.
From the absorbance of the solution we can figure
out the concentration of FeSCN2 using Beers
law.
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But the FeSCN2formed will still be unknown?
Firstly, we need to mix Fe3 and SCN- such that
we know the concentration of FeSCN2 in the
resulting solution
For the known concentrations of FeSCN2, we can
measure the absorbance.
Plot a graph of Absorbance Vs. Concentration.
Obtain the relationship between concentration
and absorbance from the equation of the best-fit
line.
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But the FeSCN2formed will still be unknown?
y
Best-fit line
Equation of the best-fit line
c5,A5
x
cn
n
c4,A4
YmX z
Concentration
m
m slope
c3,A3
c2,A2
x
cm
c1,A1
x
Am
An
Absorbance
intercept
C mA Z
concentration
Absorbance
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But the FeSCN2formed will still be unknown?
Plug in the absorbance of solution whose
concentration of FeSCN2 is unknown in the
equation of best-fit line to obtain the value of
concentration.
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How do I make up a solution where FeSCN2 is
known?
Fe3 SCN- FeSCN2
1 ion
1 ion
1 ion
1 mole
1 mole
1 mole
1 mole
100 moles
1 mole
Limiting reagent
If we make one of the reactants rate limiting,
the FeSCN2 formed will be equal to the
concentration of the limiting reagent.
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How is this done in the experiment?
Fe3 SCN- FeSCN2
0.002 M
0.2 M
25 mL
0 mL
25 mL
2 mL
25 mL
4 mL
25 mL
6 mL
25 mL
8 mL
0
2
4
6
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Limiting reagent
SCN-known FeSCN2known
0.002 M Fe3
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y
Best-fit line
Equation of the best-fit line
c5,A5
x
cn
n
c4,A4
YmX z
FeSCN2, M
m
m slope
c3,A3
c2,A2
x
cm
c1,A1
x
Am
An
Aknown
intercept
C mA Z
concentration
Absorbance
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How to find FeSCN2unknown?
Fe3 SCN- FeSCN2
0.002 M
0.002 M
Solution "0.002 M" Fe(NO3)3 "0.002 M"
KSCN 0.1 M HNO3 1
2.50 mL 0.50 mL 2.00 mL
2 2.50 1.00 1.50
3 2.50 1.50 1.00
4 2.50 2.00 0.50
5 2.50 2.50 0.0
Measure the absorbance of these solutions.
C mA Z
concentration
Absorbance
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How to find FeSCN2eqb?
FeSCN2unknown FeSCN2formed FeSCN2eqb
How to find Fe3eqb and SCN-eqb?
Known
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How to find Fe3reacted and SCN-reacted?
Fe3 SCN- FeSCN2
1 ion
1 ion
1 ion
1 mole
1 mole
1 mole
FeSCN2eqb Fe3reacted SCN-reacted
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How do we determine the equilibrium Constant (Kc)
of the reaction?
Fe3 SCN- FeSCN2
Yellow
Colorless
Red color
FeSCN2unknown FeSCN2formed FeSCN2eqb
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When preparing 0.1 M HNO3

• Wear SAFETY GOGGLES AND GLOVES
• Use graduated cylinder to dispense the acid
• from the bottle
• 3. Please have about 100 mL of water in
• 500 mL volumetric flask, before adding
• acid in to it.
• 4. Add acid to the flask slowly in small
• aliquots.