DETERMINATION OF THE EQUILIBRIUM
CONSTANT FOR A CHEMICAL REACTION
LAB #6
BY
DAKSIKAN NAVANITHAN
STUDENT ID: 2153689
VITO VIGLIENSONI HERNANDEZ
STUDENT ID: 2087821
COURSE: 202-NYB-05
CHEMISTRY OF SOLUTIONS SECTION 00003
PERFORMED ON MARCH 23rd, 2023
SUBMITTED TO PHILIPPE BRUNET
ON APRIL 3rd, 2023
INTRODUCTION
Objective:
The objective of this experiment is to be able to determine the equilibrium constant for a given
chemical reaction. By using different quantities of the compounds in the reaction and then
detecting the concentration of a compound in it, the equilibrium constant will be calculated.
Theory:
In this experiment, different amounts of the components of a reaction,
3+
2+
+
𝐹𝑒(𝑎𝑞)
+ 𝐻𝑆𝐶𝑁(𝑎𝑞) ⇌ 𝐹𝑒𝑆𝐶𝑁(𝑎𝑞)
+ 𝐻(𝑎𝑞)
,
were combined to see how they would “shift” to maintain equilibrium. The equation for the
equilibrium constant of the reaction is:
𝐾𝑐 =
[𝐹𝑒𝑆𝐶𝑁 2+ ][𝐻 + ]
[𝐹𝑒 3+ ][𝐻𝑆𝐶𝑁]
The concentrations of the four species all have to always equal the 𝐾𝑐 , which remains constant.
For example, if [𝐹𝑒𝑆𝐶𝑁 2+ ] goes up, then one of [𝐹𝑒 3+ ] or [𝐻𝑆𝐶𝑁] also have to go up to keep the
𝐾𝑐 constant, and vice-versa. The 𝐹𝑒𝑆𝐶𝑁 2+ ion has a reddish colour, so as more of it forms in
solution, the solution will get more red. This will be the aspect that will be used to detect and
calculate the amount of 𝐹𝑒𝑆𝐶𝑁 2+ in solution and will further be used to calculate the amount of
each ion in solution. A Spectronic-20 colorimeter will be used to detect how much light gets
through the test tube containing the solution.
Part 1 of the experiment will just focus on how different quantities of 𝐹𝑒 3+ and 𝐻𝑆𝐶𝑁 shift the
reaction to the reactant side or product side, while part 2 of the experiment will look at how raising
the temperature of the solution will shift the equilibrium. If the value of the 𝐾𝑐 reflects that the
reaction prefers the reactant side when heated, it will indicate that the reaction is endothermic,
while if the 𝐾𝑐 shows that the product side is preferred, it will imply that the reaction is exothermic.
This is because the reaction with heat added will favour the side which absorbs heat, meaning that
in an exothermic reaction, the reverse reaction will be favoured and vice-versa.
1
EXPERIMENTAL
Procedure:
The experiment was conducted by following the procedure presented in the lab manual
CHEMISTRY OF SOLUTIONS 202-NYB, LABORATORY EXPERIMENTS, on page 34 and
35.1
Results:
TABLE 1: Preparation of the solutions
3+
Reagent
Solution1
Solution 2
Solution 3
Fe
3.00mL
2.00mL
1.00mL
+
HSCN
H
1.00mL
2.00mL
3.00mL
2.00mL
2.00mL
2.00mL
TABLE 2: Absorbance of the solutions at different temperature
PART I
Room Temperature
PART II
High Temperature
Solution 1
Temp. (o C)
24.2
Absorbance
0.172
[FeSCN2+]
3.44E-05
Temp.(o C)
60.0
Absorbance
0.129
[FeSCN2+]
2.58E-05
Solution 2
24.2
0.241
4.82E-05
60.0
0.188
3.76E-05
Solution 3
24.2
0.193
3.86E-05
60.0
0.156
3.12E-05
Part 1: At 24.2℃
TABLE 3: Determination of equilibrium constant of solution 1
Solution 1
Initial Conc.
Change in Conc.
Equilibrium Conc.
Equilibrium Constant
Fe3+
1.00E-03
-3.44E-05
9.7E-04
FeSCN2+
0
3.44E-05
H+
5.00E-04
3.44E-05
2.99E-04
3.44E-05
6.4E-02
5.34E-04
HSCN
3.33E-04
-3.44E-05
2
TABLE 4: Determination of equilibrium constant of solution 2
Solution 2
Initial Conc.
Change in Conc.
Fe3+
6.67E-04
-4.82E-05
HSCN
6.67E-04
-4.82E-05
FeSCN2+
0
4.82E-05
H+
5.00E-04
4.82E-05
Equilibrium Conc.
Equilibrium Constant
6.18E-04
6.18E-04
4.82E-05
6.91E-02
5.48E-04
TABLE 5: Determination of equilibrium constant of solution 3
Solution 3
Initial Conc.
Change in Conc.
Fe3+
3.33E-04
-3.86E-05
HSCN
1.00E-03
-3.86E-05
FeSCN2+
0
3.86E-05
H+
5.00E-04
3.86E-05
Equilibrium Conc.
Equilibrium Constant
2.95E-04
9.6E-04
3.86E-05
7.3E-02
5.39E-04
Average value of kc :6.87 × 10−2
Part 2: At 60.0℃
TABLE 6: Determination of equilibrium constant of solution 1
Solution 1
Initial Conc.
Change in Conc.
Equilibrium Conc.
Equilibrium Constant
Fe3+
1.00E-03
-2.58E-05
9.7E-04
HSCN
3.33E-04
-2.58E-05
FeSCN2+
0
2.58E-05
H+
5.00E-04
2.58E-05
3.08E-04
2.58E-05
4.5E-02
5.26E-04
TABLE 7: Determination of equilibrium constant of solution 2
Solution 2
Initial Conc.
Change in Conc.
Fe3+
6.67E-04
-3.76E-05
HSCN
6.67E-04
-3.76E-05
FeSCN2+
0
3.76E-05
H+
5.00E-04
3.76E-05
Equilibrium Conc.
Equilibrium Constant
6.29E-04
6.29E-04
3.76E-05
5.11E-02
5.38E-04
3
TABLE 8: Determination of equilibrium constant of solution 3
Solution 3
Initial Conc.
Change in Conc.
Fe3+
3.33E-04
-3.12E-05
HSCN
1.00E-03
-3.12E-05
FeSCN2+
0
3.12E-05
H+
5.00E-04
3.12E-05
Equilibrium Conc.
Equilibrium Constant
3.02E-04
9.7E-04
3.12E-05
5.7E-02
5.31E-04
Average value of kc :5.10 × 10−2
Determination of the sign of enthalpy change of reaction (∆𝐻𝑟𝑥𝑛 ):
Low temperature: 24.2℃
𝐾𝑐 𝑙𝑜𝑤 𝑇 = 6.87 × 10−2
High temperature:60.0℃
𝐾𝑐 ℎ𝑖𝑔ℎ 𝑇 = 5.10 × 10−2
4