In the diagrams below, indicate the outer electron arrangements of the atoms and ions shown. (Use the symbol $\uparrow\downarrow$ to show two electrons paired in one orbital.)
Use the diagrams above to account for why $\text{Fe}^{2+}$(aq) ions are coloured, whereas $\text{Zn}^{2+}$(aq) ions are colourless.
When concentrated $\text{HCl}$ is added to a solution of $\text{Cu}^{2+}$(aq) ions, the solution becomes yellow. State the formula of the species responsible for the yellow colour and name the type of reaction that has taken place.
Ammonia may behave either as a base or as a ligand. Describe the colour changes observed as $\text{NH}_3$(aq) is added gradually, with stirring, to the yellow solution, until $\text{NH}_3$(aq) is in excess. Identify the three ions or compounds responsible for the new colours.
When aqueous solutions of $\text{KI}$ and $\text{K}_2\text{S}_2\text{O}_8$ are mixed, almost no reaction happens at first, but when a few drops of $\text{Fe}^{2+}$(aq) or $\text{Fe}^{3+}$(aq) are added, iodine, $\text{I}_2$(aq), is produced at a steady rate. Write an equation for the overall reaction.
State the exact role of the iron ions in this reaction.
By means of equations or otherwise, explain why the presence of either $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$ can speed up the reaction.