Materials: Refer to the Heath Lab Manual p.209.
Procedure: Refer to the Heath Lab Manual p.209-211.
Data and Observations: Refer to attached sheet labeled Data and Observations.
1. Increasing the molarity of the NaOH would decrease the number of drops required in order to observe the desired color change since there would be more OH- particles available to collide with the Hydrogen ions causing the reaction to shift faster.
2. The addition of the NaOH is the application of a stress to the equilibrium system. The NaOH dissociates into hydroxide ions and sodium ions. Hydroxide ions react with the iron (III) ions to produce iron (III) hydroxide. Therefore the concentration of the reactant iron ions is decreased causing the reaction to shift towards the reactant side to replenish the low iron ion levels. Thus the system forms a new equilibrium at a level where the concentration of the iron ions is slightly lower than before the stress.
3. If you were to refrigerate the hydrated cobalt (II) ion complex then the result would either be a quite pink color since the reaction would be proceeding to the left for quite a while due to the need to replace a lot of heat being lost to the refrigerator.
4. From our data when the temperature was increased the reaction shifted to the right and when the temperature was decreased the reaction shifted to the left. This means that the forward reaction was the endothermic reaction in this lab. Le Chateliers principle states that in an endothermic reaction increasing the temperature increases the forward rate and this is what is observed in our results.
5. Adding sodium chloride would cause an increase in the concentration of the chloride ions upon the dissociation of the NaCl into chloride ions and sodium ions. This causes a stress on the equilibrium system causing a shift towards the products to occur in order to use up the excess chloride ions. Thus both the forward and reverse rates increase. However the forward rate would increase more to allow the excess chloride ions to get used up. Eventually the system would establish a new equilibrium with the chloride ion concentration slightly higher then it was at the original equilibrium.
6. Cr2O72- + 2OH- 2CrO4 + H2O
7. For the second step, when the NaOH is added it dissociates into hydroxide ions and sodium ions and then the hydroxide ions combine with the hydrogen ions to make water. Thus the hydrogen ion concentration decreases and therefore the reaction shifts to the left. This causes the potassium dichromate solution to turn to a yellow color. The potassium chromate solution simply becomes more yellow since it is yellow in the first place. In step 3, when the HCl is added it dissociates into hydrogen ions and chloride ions. Thus the hydrogen ion concentration is increased and this causes the reaction to shift right. The potassium dichromate returns to its original color. The