1. Experiment #5
DETERMINATION OF AN
EQUILIBRIUM CONSTANT

2. 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.

3. 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.
aA + bB cC+ dD
forward
backward

4. What is chemical equilibrium?
Chemical equilibrium exists when two opposing
reactions occur simultaneously at the same rate
Kinetics
Equilibrium
Apparent
change in
concentration
of reactants
and products
No apparent
2Hl(g) H2(g)+I2(g)

5. Equilibrium, an analogy
Amount of water being pumped in =
Amount of water being pumped out

6. Equilibrium Constant nAA + nbB nCC+ nDD At equilibrium At equilibrium
Kforward
nAA + nbB nCC+ nDD
kbackward
At equilibrium
At equilibrium

7. Upper case
Lower case

8. What is equilibrium concentration?
Concentration of a reactant or a product left after
the equilibrium has been established.
Mathematically expressed:

9. A comparison of rate law and
equilibrium constant
Rate law
Equilibrium constant
kforward k
nAA + nbB nCC+ nDD
nAA + nbB nCC+ nDD
kbackward
unidirectional
bidirectional
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
coefficients of A,B,C and D respectively.
obtained from the balanced chemical reaction

10. Experiment Determination of equilibrium constant of the
following reaction
Fe(H2O) SCN FeSCN(H2O) H2O

11. For H2O
[H2O]reacted 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.

13. How do we follow this equilibrium?
Fe(H2O) SCN FeSCN(H2O) H2O
Fe 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.

14. How do we determine the equilibrium Constant (Kc) of the reaction?
Fe SCN FeSCN2+
Yellow
Colorless
Red color

15. 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.

16. How do we determine [FeSCN2+]eqb?
Fe SCN FeSCN2+
Yellow
Colorless
Red color
Mix specific amounts of Fe3+ solution to SCN- solution.
The resulting FeSCN2+will 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 Beer’s law.

17. But the [FeSCN2+]formed 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.

18. But the [FeSCN2+]formed will
still be unknown? y
Best-fit line
Equation of the best-fit line: cn x
c5,A5 n
c4,A4
Y=mX + z
Concentration m
c3,A3
m = slope =
c2,A2 cm x
c1,A1 x Am An
intercept
Absorbance
C= mA + Z
concentration
Absorbance

19. But the [FeSCN2+]formed 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.

20. How do I make up a solution where
[FeSCN2+] is known?
Fe SCN FeSCN2+
1 ion
1 ion
1 ion
1 mole
1 mole
1 mole
100 moles
1 mole
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.

21. How is this done in the experiment?
Fe SCN FeSCN2+
0.2 M
0.002 M
25 mL
0 mL
25 mL
2 mL
25 mL
4 mL
25 mL
6 mL
25 mL
8 mL 2 4 6 8
Limiting reagent
[SCN-]known = [FeSCN2+]known
0.002 M Fe3+

22. C= mA + Z Equation of the best-fit line: [FeSCN2+], M Aknown intercepty
Best-fit line
Equation of the best-fit line: cn x
c5,A5 n
c4,A4
Y=mX + z
[FeSCN2+], M m
c3,A3
m = slope =
c2,A2 cm x
c1,A1 x Am An
intercept
Aknown
C= mA + Z
concentration
Absorbance

23. How to find [FeSCN2+]unknown?
Fe SCN FeSCN2+
0.002 M
0.002 M
Solution "0.002 M" Fe(NO3) "0.002 M" KSCN M HNO3
mL mL mL
Measure the absorbance of these solutions.
C= mA + Z
concentration
Absorbance

24. How to find [FeSCN2+]eqb?
[FeSCN2+]unknown = [FeSCN2+]formed= [FeSCN2+]eqb
How to find [Fe3+]eqb and [SCN-]eqb?
Known

25. How to find [Fe3+]reacted and
[SCN-]reacted?
Fe SCN FeSCN2+
1 ion
1 ion
1 ion
1 mole
1 mole
1 mole
[FeSCN2+]eqb = [Fe3+]reacted= [SCN-]reacted

26. How do we determine the equilibrium Constant (Kc) of the reaction?
Fe SCN FeSCN2+
Yellow
Colorless
Red color
[FeSCN2+]unknown = [FeSCN2+]formed= [FeSCN2+]eqb

27. 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.