What if the halide ions were in solution rather than in a solid?
We have concentrated on the energetics of the process starting from solid halide ions because that’s what you use if you try to oxidise them using concentrated sulphuric acid. What about oxidising them in solution using some different oxidising agent? The trend is exactly the same. Fluoride ions are difficult to oxidise and it gets easier as you go down the Group towards iodide ions. Looked at another way, fluoride ions aren’t good reducing agents, but iodide ions are. The explanation this time has to start from the hydrated ions in solution rather than solid ions. In a sense, this has already been done on another page. Fluorine is a very powerful oxidising agent because it very readily forms its negative ion in solution. That means that it will be energetically difficult to reverse the process. By contrast, for the energetic reasons you will find discussed, iodine is relatively reluctant to form its negative ion in solution. That means that it will be relatively easy to persuade it to
