How to Investigate Chemical Equilibrium in Lab 12A
Chemical equilibrium is a state where the forward and reverse reactions occur at the same rate, resulting in no net change in the concentrations of the reactants and products. In this article, we will explain how to perform Lab 12A, which involves investigating chemical equilibrium in five different systems using Le Chatelier's principle.
Le Chatelier's principle states that if a stress is applied to a system at equilibrium, the system will shift to relieve the stress and restore equilibrium. The stress can be a change in concentration, temperature, pressure, or volume. Depending on the direction of the shift, the equilibrium constant (K) may change or remain the same.
Lab 12A consists of three parts: Part 1 involves equilibrium involving bromcresol green, a pH indicator that changes color depending on the acidity or basicity of the solution. Part 2 involves equilibrium involving thiocyanatoiron (III) ion, a complex ion that forms a blood-red solution with thiocyanate ion. Part 3 involves equilibrium involving cobalt (II) complexes, which have different colors depending on the ligands attached to the metal ion.
The color of the solution depends on the ratio of HBcg to Bcg. If the solution is acidic (high concentration of H), HBcg predominates and the solution is yellow. If the solution is basic (low concentration of H), Bcg predominates and the solution is blue. If the solution is neutral (equal concentrations of H and OH), HBcg and Bcg are present in equal amounts and the solution is green.
To perform this part of the experiment, you will need:
2 Erlenmeyer flasks (250 mL)
0.01 M HCl
0.01 M NaOH
Bromcresol green solution
The procedure is as follows:
Add 50 mL of water and 10 drops of bromcresol green into each flask and record the color of the solution.
In the first flask, add one drop of 0.01 M HCl and swirl the flask. Record any color change. Continue adding drops of HCl until a definite color change occurs. HCl dissociates into H and Cl, increasing the concentration of H. This shifts the equilibrium to the left, favoring HBcg and making the solution more yellow.
In the second flask, add one drop of 0.01 M NaOH and swirl the flask. Record any color change. Continue adding drops of NaOH until a definite color change occurs. NaOH dissociates into Na and OH, increasing the concentration of OH. The OH reacts with H, forming water and decreasing the concentration of H. This shifts the equilibrium to the right, favoring Bcg and making the solution more blue.
To reverse the color changes, add drops of NaOH to the first flask until it turns green again. Then add drops of HCl to the second flask until it turns green again.
Part 2: Equilibrium Involving Thiocyanatoiron (III) Ion
The equilibrium reaction for thiocyanatoiron (III) ion is: