4 NaOH Orange-Yellow Cr2O72- + 2OH- ↔ C 2CrO42- + H2O
Reactions for the chromate-dichromate system:
[1] 2CrO42- + 2H+ ↔ Cr2O72- + H2O;
(Yellow) (Orange)
[2] 2CrO42- + 2H+ → Cr2O72- + H2O;
(Yellow) (Orange)
[3] Cr2O72- + 2OH- → 2CrO42- + H2O
(Orange) (Yellow)
Equation 1 is the equilibrium reaction. From this equation, we can derive the Keq.
Keq = [Cr2O72-] / [H+]2 [CrO42-]2
The Le Châtelier’s principle predicts that the forward reaction, which is essentially equation 2, is favored in acidic medium and there will be predominantly Cr2O72- thus giving the solution an orange color when H2SO4 was added. In basic medium, the concentration of H+ was so small, the Q will also be so large that Q will be much larger than the Keq, so the reverse reaction is favored which is the formation of CrO42- thus giving the solution a yellow color when NaOH was added. The data in Table II obtained in the experiment are consistent with the Le Châtelier’s principle.
Table III. The iron (III) chloride-thiocyanate system
Test Tube # Color Shift in Equilibrium
1 Pale Orange -----
2 Red Orange To the right
3 Red Orange To the right
4 Light Yellow Orange To the left
Reaction for the iron (III) chloride-thiocyanate system: Fe3+ + SCN- ↔ [Fe(SCN)]2+
(Pale orange) (Blood red)
According to the Le Châtelier’s principle, the addition of FeCl3 (Fe3+) will favor the forward reaction, leading to the formation of [Fe(SCN)]2+, a blood red complex thus giving the solution a more reddish color (orange + red = red orange). The addition of KSCN (SCN-) will favor the forward reaction, leading to the formation of [Fe(SCN)]2+, a blood red complex thus giving the solution a more reddish color (orange + red = red orange). The addition of saturated NaCl (excess Cl-) will lead to a conversion of Fe3+ to [FeCl4]-, a colorless ion that is more stable than [Fe(SCN)]2+, leading to a deficiency of Fe3+ thus favoring the reverse reaction, which in this case, the formation of Fe3 and SCN-. Because some [FeCl4]- and Fe3+ were formed, they contributed to the color of the solution (yellow + colorless + orange = light yellow orange). The data in Table III obtained in the experiment are consistent with the Le Châtelier’s principle.
Table IV. The cobalt (II) ions system
Color
Before Addition of HCl Pale Pink
After Addition of HCl Light Blue
At Room Temperature Pale Pink
At Boiling-H2O Temperature Light Blue
Reaction for the cobalt (II) ions system:
Co2+ + 4Cl- ↔ [CoCl4]2-
(Pink) (Blue)
In Part A, according to the Le Châtelier’s principle, the addition of concentrated HCl (excess Cl-) will favor the forward reaction which in this case, the formation of [CoCl4]2-, a blue complex, thus giving the solution a blue color. When the solution was heated, its color changed from pale pink to light blue (shifted to the right, theoretically) which indicated the presence of more [CoCl4]2- ions, therefore, according to the Le Châtelier’s principle, the reaction is endothermic because heating the solution favors the forward reaction which in this case the formation of [CoCl4]2-.
Answers to Questions
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According to the Le Châtelier’s principle, the addition of excess NH3 to Cu2+ will lead to a shift in the equilibrium to the right. Its Keq will be a large number and its ΔG will be slightly negative. The addition of HCl (Cl-) will lead to will lead to a shift in the equilibrium to the left thus giving the solution a light blue color. Its Keq will be a small number and its ΔG will be slightly positive. In Part C, according to the Le Châtelier’s principle, the addition of FeCl3 (Fe3+) will favor the forward reaction, leading to the formation of a blood red complex thus giving the solution a more reddish color. Its Keq will be a large number and its ΔG will be slightly negative. The addition of KSCN (SCN-) will favor the forward reaction, leading to the formation of a blood red complex thus giving the solution a more reddish color. Its Keq will be a large number and its ΔG will be slightly negative. The addition of saturated NaCl (excess Cl-) will lead to a shift in the equilibrium to the left thus giving the solution a yellow orange color. Its Keq will be a small number and its ΔG will be slightly positive.
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2CrO42- + 2H+ ↔ Cr2O72- + H2O
In acidic medium (more H+), the reaction will shift to the direction, according to the Le Châtelier’s principle, such that it will favor the production of more Cr2O72- than CrO42- thus resulting in an orange solution, while in basic medium (less H+), the reaction will shift to the direction, according to the Le Châtelier’s principle, such that it will favor the production of more CrO42- than Cr2O72- thus resulting in a yellow solution.
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2HCrO4- ↔ 2H+ + 2CrO42- (3.2 x 10-7)2
Cr2O72- + H2O ↔ 2HCrO4- (37)-1
2H+ + 2OH- ↔ 2H2O (1.0 x 10-14)2
Cr2O72- + 2OH- ↔ 2CrO42- + H2O
Keq = (3.2 x 10-7)2 (37)-1 (1.0 x 10-14)2
= 2.76756 x 10-43
= 2.8 x 10-43
2H+ + 2CrO42- ↔ 2HCrO4- (3.2 x 10-7)-2
2HCrO4- ↔ Cr2O72- + H2O (37)
2CrO42- + 2H+ ↔ Cr2O72- + H2O
Keq = (3.2 x 10-7)-2 (37)
= 3.61328125 x 1014
= 3.6 x 1014
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a. The Solution will become more yellow after adding Ag+.
b. The Solution will become colorless after adding NaCl.
c. The Solution will become colorless after adding HCl.
d. Adding H3PO4 will form a precipitate, FePO4, consuming Fe3+ in the process. Thus, the solution will become more yellow in color.
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a. Q > Keq
b. Q < Keq
c. Q < Keq
d. Q > Keq
REFERENCES
[1] Petrucci, P.H., Herring, F.G., Madura J.D. and Bissonnette C.: General Chemistry, Principles and Modern Application, 10th Ed. Prentice Hall, New Jersey, 2010
[2] Brown, T.L., Le May Jr., H.E., Bursten, B.E.: Chemistry the Central Science, 11th Ed. Prentice Hall, New Jersey, 2010.
[3] Mortimer, C.E.: Chemistry, 6th Ed. Wadsworth Publishing Company, 1986
