Describe what you observe when the SCN- is added to the solution in the petri dish.

What chemical species must have been formed to account for your observation?

What chemical species did the SCN-, that was added to the solution in the petri dish, react with to produce the change you observed?

 

The petri dish on the left has had some Fe3+(aq) added to it, now we will add some SCN-(aq) to the solution in the petri dish on the right.

When we add SCN-(aq) to the solution in the petri dish on the right we observe the solution gets darker. This is VERY interesting. Adding some SCN-(aq) causes the solution to get darker, it appears that adding the SCN-(aq) causes more FeSCN2+(aq) to be formed.

We can understand this to suggest that in the solution in the petri dish on the right there must have been some Fe3+(aq). So when I added SCN-(aq) there was a reaction to produce more of the product, FeSCN2+(aq).

So the original beaker containing the solution of FeSCN2+(aq) must also have some Fe3+(aq) present.