Halogen displacement reactions

Picture two test tubes, both colourless. Into the first goes a solution of potassium bromide, and into the second, a solution of potassium chloride. A few drops of pale chlorine water go into each. The second tube does nothing at all. The first turns orange, and nothing orange was added to it. Something already in that tube has been thrown out of its compound and left floating loose.

Halogen displacement reactions is video six of nine in The periodic table, and it follows on from Group 7: the halogens. If the trend down Group seven is not solid yet, watch that one first.

A more reactive halogen can take the place of a less reactive one in a solution of its salt. That swap is called displacement. This video is for Foundation and Higher students alike. It rests on one line from the video on the halogens: reactivity falls down the group. Chlorine is more reactive than bromine, and bromine more than iodine. Keep the two words apart. The halogen is the element you add, a two-atom molecule with a colour. The halide is the ion, a charged particle, already in the salt, dissolved and colourless. Think of musical chairs where the stronger player always wins the seat. Chlorine is stronger than bromine, so chlorine takes the seat and bromine is left standing. The colour tells you who was left standing. Chlorine water is pale, bromine solution is orange, and iodine solution is brown. Here is the hook's first tube in full. Chlorine water is added to colourless potassium bromide solution. Pause and give what you would see, and the balanced symbol equation. You would see the solution turn orange, because bromine has been set free. Unbalanced, it reads C l two plus K B r gives K C l plus B r two. The C l two brings two chlorines, so you need two K C l on the right, and then two K B r to match. C l two plus two K B r gives two K C l plus B r two, every one of them aqueous. Aqueous means dissolved in water, and nothing here ever leaves the water. Look at the potassium. It starts dissolved and it finishes dissolved, unchanged. Only the halogen and the halide trade places. In that equation, which is the halide: the C l two, or the bromide in K B r? The bromide. It is the ion sitting in the salt. C l two is the halogen, the element that was added. Now the other way round. Iodine solution is added to potassium bromide solution. Why does nothing happen when iodine meets potassium bromide? Iodine is less reactive than bromine, so it cannot take bromine's place. The only colour in the tube is the brown of the iodine you poured in.

Results like these are usually set out as a grid: halogens down one side, halide solutions along the top, and a result in each box. One exam board meets this through a required practical, in separate chemistry only. On the other two boards it is ordinary Group seven content for everyone. Here are three results. Bromine water and potassium iodide: turns brown. Chlorine water and potassium iodide: turns brown. Iodine solution and potassium bromide: no change. Pause and put the halogens in order, then find the experiment the grid is missing. Iodine goes at the bottom. Both of the others displaced it, and it displaced nothing. But no result sets chlorine against bromine, so the top two are not settled. The missing experiment is one that does, such as chlorine water with potassium bromide, and the orange from the hook settles it: chlorine is the most reactive. One examiner's report on a Foundation paper, about a table of results like this, says: "Very few students could articulate their reasoning in terms of displacement." It goes on: "Several tried to answer in terms of reactivity of the potassium halides rather than the halogens, or were misusing the term 'halide' instead of 'halogen', making it difficult to gain any credit." What works instead is one sentence, said the same way for every box: the more reactive halogen displaces the less reactive one from its salt. The same report adds: "Others thought that darkness of colour was an indication of reactivity, whilst some referred to melting or boiling points, which were not relevant." Put two facts side by side. Iodine is the darkest of the three, and it is also the least reactive. The way to read a colour is as evidence of which halogen was set free, nothing more.

Now an element nobody has a bottle of. Astatine sits below iodine at the bottom of Group seven, and it is so rare and so radioactive that no one has ever seen a visible lump of it. That makes anything you say about it a prediction, not a fact. It is the furthest down the group, so on the trend it comes last of all. Which one reacts: iodine with potassium astatide, or astatine with potassium iodide? Iodine with potassium astatide. Iodine is more reactive than astatine, so it displaces it and sets astatine free. The other way round, astatine cannot displace iodine, so nothing happens. Asking both directions is what makes the prediction a real one: only one of the two can react, and the order of the group says which.

Some displacements to referee now, starting with a salt this video never used. Bromine water is added to sodium iodide solution. What colour appears, and why? Brown. Bromine is more reactive than iodine, so it displaces iodide from the salt and sets iodine free. Now, in that reaction, which is the halogen and which is the halide? Bromine is the halogen, the element added. Iodide is the halide, the ion in the salt. Next, iodine is the darkest halogen. Does that make it the most reactive? No. It is the least reactive of the three; the colour only shows which halogen was set free. One more: what is the balanced equation for bromine with potassium iodide? B r two plus two K I gives two K B r plus I two, all of them aqueous. And the orange tube from the start was chlorine taking bromine's place. The orange was bromine, left floating loose.

In The periodic table, Halogen displacement reactions comes sixth of the nine videos. Once you've got this down, a thumbs up means you never have to sit through it again. If it has not clicked yet, keep it saved for another look, because it often lands on a second viewing.

Next in the chapter: Group 0: the noble gases.

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