Group: VII: Halogens, Halides and Chlorine

1. 1
   Electronegativity

   .

   The electronegativity of halogens decrease down the group. Fluorine is very reactive and is the most electronegative element.

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2. 2
   Boiling Point

   .

   A further physical property is the halogen's boiling point, which increase down Group 7. This is because the atoms get bigger and so the Van der Waals forces get larger as well.

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3. 3
   Oxidization

   .

   And now a chemical property. The oxidizing power of the halogens decreases from Fluorine to Iodine. This property can be demonstrated by displacement reactions.This means if halogen ions are dissolved in water. If a halogen higher up the group is added; this higher halogen will replace it. For example, when chlorine is added to Br-(aq), the following reaction takes place.

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2Br-(aq) + Cl₂ ® 2Cl-(aq) + Br₂

A halide is simply the ion of a halogen. Their ability to reduce increases down the group from F to I. This ability is shown in the reactions of NaX (where X is a halogen) with sulphuric acid, or as chemists like to call it: H₂SO₄. The table below shows the results of the reactions of NaX with sulphuric acid.

What this demonstrates is that more products are formed with the sodium halides further down the group, therefore reducing the ability of these halides is greater at the bottom of the group.

How to Test for Halides

Now - how to test for the presence of the different halides. A classic test is to add silver nitrate, and it produces a precipitate of different colors depending on the halide. However, as you can see below, the difference in color is not very distinct. Therefore, we add another substance: ammonia. First, a few drops of dilute ammonia are added to the precipitate to see how much it dissolves. Chlorine will dissolve quite well. Bromine will dissolve sparingly and Iodine not at all. Then we add concentrated ammonia and the precipitate in both the chloride and bromide solutions will dissolve and the iodide precipitate turns white. The diagram below outlines this testing procedure.

The ionic equation for this reaction is as follows (using Br as the halide).

Cl-(aq) + Ag+(aq) ® AgCl(s)

Halogens will react with metals to form salts. For example, if chlorine gas is passed over a heated iron wire, you will see a brown solid upon cooling.

iron + chlorine ® iron (III) chloride 2Fe(s) + 3Cl₂ (g) ® 2FeCl₂ (s)

The most reactive halogen is fluorine, and they become less reactive as you go down. Due to this, a more reactive halogen will displace a less reactive one. For example, when chlorine gas is bubbled through potassium bromide solution...

2KBr(aq) + Cl₂ (g) ® 2KCl (aq) + Br₂ (aq)

Chlorine and Chlorides

Chlorine has a very interesting reaction with water. It's a reversible reaction and also shows something called disproportionation [dis-pro-por-shon-ay-shon].

Disproportionation defines a reaction where one species (in this case chlorine) is simultaneously oxidized and reduced. If you have a look at chlorine in HCl, it has an [chemistry/physical/redox.html|oxidation number] of -1 and in hydrogen chlorate. This reaction has been used in water treatment to destroy bacteria, however this practice has been replaced by granular calcium hypochlorite Ca(ClO)₂ which is less dangerous.

Another reaction we are going to look at is chlorine with cold, dilute sodium hydroxide (NaOH). This causes the following reaction to take place.

Cl₂ + 2OH- ® Cl- + ClO- + H₂O

This reaction is very important because it is used commercially to produce bleach!

I read that fluorine is the most reactive but I thought it was the least?

Small atom so electrons closer to nucleus- doesn't that mean it's less reactive??

• The reason Fluorine is the (most) reactive is that it's at the top of the Halogen group or 2nd to the right on the periodic table. The higher an element is, the more reactive it is.
• Outer shells that orbit the nucleus lack electrons which makes them enthusiastically grab electrons. Fluorine is a massively reactive element, is nonmetal and halogen, it's atomic weight is 18.9984032, it's atomic number is 9 and contains 10 neurons in the nucleus.

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