Experiment No. 9
INTEGRATED CONCEPTS OF EQUILIBRIUM RESULTS AND DISCUSSION
A system in equilibrium can be affected by the addition of another reagent leading to a change in chemical equation with a new equilibrium constant. An overall reaction is the sum of two or more reaction steps with different equilibrium constants. The overall equilibrium constant, Koverall, is the product of the equilibrium constants of the individual reaction step. If a reaction step is reversed, the equilibrium constant is set into its reciprocal. If a reaction step is multiplied by a common coefficient n, the new equilibrium constant is raised to the nth.
The first part of the experiment dealt with the equilibrium reaction Cu(OH)2(s) ⇌ Cu2+(aq) + 2OH-(aq). 0.10 M Cu(NO3)2 and 0.10 M NaOH were reacted together in seven test tubes to form the solid Cu(OH)2, a blue precipitate.
Distilled water was added to the first test tube. This served as the control. When 6.0 M H2C2O4 was added to the second test tube, the precipitate turned cloudy blue. The new equilibrium can be attributed to the formation of the solid CuC2H4 which is cloudy blue in color, and the ionization of H2C2O4. The addition of Zn dust in the third test tube resulted into a gray-brown precipitate. The mechanisms in this reaction are the dissociation of Cu(OH)2(s), redox of Cu2+ and Zn, and precipitation of Zn(OH)2, leading to an overall reaction of Cu(OH)2(s) + Zn(s) ⇌ Zn(OH)2(s) + Cu(s).
When 6.0 M HNO3 was added to the fourth test tube, the precipitate disappeared. The H+ ions from the complete dissociation of HNO3 neutralize the OH- ions. This results to the shifting of the system to the right. The addition of 6.0 M NH3 in the fifth test tube caused the formation of the deep blue [Cu(NH3)4]2+ complex. The new equilibrium was established from the dissociation reaction of Cu(OH)2 and NH3, and the formation of complex [Cu(NH3)4]2+.
1.0 M Na3PO4 was added to the sixth test tube, and formed a light blue...
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