Halogens are a group of elements that are known for their reactivity, and they can be used in a variety of reactions. For example, bromine can be used to reduce mercury emissions from coal power plants. This is done by using activated carbon to trap mercury particles released when coal is burnt, and bromine helps the activated carbon oxidise mercury and trap more of the harmful element.

But there are many other reactions of halogens. For instance, halogens can undergo redox and displacement reactions with halides, hydrogen, metals, sodium hydroxide and organic molecules [^3]. As you move down the group, the reactivity of the halogens decreases.

What are halogens?

The halogens are a family of nonmetals found in group 17 in the periodic table. They include fluorine, chlorine, bromine, iodine, and astatine.

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If you've read about halogens before, you might know that some scientists include tennessine as a halogen, but we won't talk about it in this article since it's an artificial element that hasn't been studied much. Now, let's dive into the reactions of halogens! They can react in different ways, such as displacing other halogens, reacting with hydrogen, metals, sodium hydroxide, and even organic molecules like alkanes and benzene. In the following sections, we'll explore these reactions in more detail.

Reactions of halogens as oxidising agents

Halogens can act as oxidising agents. An oxidising agent oxidises other species and is reduced in the process. You may already know the acronym OIL RIG. It helps you remember the movement of electrons in redox reactions:

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Similarly, there is another acronym, RAD OAT. It helps you remember the actions of oxidising and reducing agents, once again in terms of electrons:

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This means that an oxidising agent takes electrons from another species and gains them itself. It is reduced. In general, the oxidising power of the halogens decreases as you go down the group. In fact, fluorine is one of the most potent oxidising agents out there! To recap redox reactions, check out Redox.

Displacement reactions of halogens

Let's take a closer look at one type of reaction that involves halogens - a displacement reaction. This occurs when a more reactive halogen replaces a less reactive halide in a compound. In a displacement reaction, the more reactive halogen atoms oxidise the less reactive halide ions, causing the halide ions to lose electrons and form halogen atoms. The halogen atoms then gain electrons to form halide ions, which are reduced. For example, if you add chlorine to a sodium bromide solution, chlorine would displace bromine since it's more reactive. Remember that halogens become less reactive as you move down the periodic table, so chlorine is more reactive than bromine in this case.

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So chlorine atoms displace bromide ions from an aqueous solution. The bromide ions are oxidised and lose electrons, whilst the chlorine atoms are reduced and gain electrons. The bromide ions form bromine, and the chlorine forms chloride ions. Here's the equation: You can also formulate this as two half equations:

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It's important to note that we can observe these displacement reactions because they cause a noticeable color change in the solution. For example, when adding chlorine to a sodium bromide solution, the bromide ions turn the solution orange-brown.

However, if you were to add bromine to a solution containing chloride ions, nothing would happen because bromine is less reactive than chlorine and isn't a potent enough oxidizing agent to oxidize the chloride ions. The table below shows the displacement reactions between different combinations of chlorine, bromine, and iodine and their aqueous ions, along with any observable color changes. Fluorine is not included in this table because it is too strong of an oxidizing agent and can even oxidize water into oxygen, making the reaction more complicated. Astatine is also not included because it is extremely rare and highly reactive.

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Reactions of halogens with hydrogen

Another example of a redox reaction involving halogens is when they react with hydrogen to form hydrogen halides, such as HX. In this reaction, the halogens act as oxidizing agents, and their reactivity decreases as you move down the group.

For example, when fluorine reacts with hydrogen, it does so explosively, forming hydrogen fluoride gas. In this reaction, hydrogen is oxidized and loses electrons, while fluorine is reduced and gains electrons. However, when iodine reacts with hydrogen, the reaction is only partial, and an equilibrium is formed. This means that the reaction can go in both directions, and the products can react to form the reactants again. It's important to note that the reversibility of this reaction is due to the weaker oxidizing power of iodine compared to fluorine.

Reactions of halogens with metals

Sure! The reactions between halogens and metals also form salts and are redox reactions. As we move down the group, the reactivity decreases, and the reactions become less vigorous. The word halogen comes from the Greek words "hal" or "halo," meaning salt, and "gen," meaning to produce.

One example is the reaction between chlorine and sodium. When chlorine gas is bubbled through a solution of sodium chloride, a chemical reaction occurs, forming sodium hypochlorite and sodium chloride:

2NaCl + Cl2 → 2NaClO

In this reaction, chlorine acts as an oxidizing agent and gains electrons, while sodium is reduced and loses electrons. This reaction is commonly used in the disinfection of water.

Another example is the reaction between bromine and iron. When bromine is added to iron filings, a redox reaction occurs, forming iron(III) bromide:

2Fe + 3Br2 → 2FeBr3

In this reaction, bromine acts as an oxidizing agent, gaining electrons and becoming reduced, while iron loses electrons and is oxidized.

These reactions demonstrate the halogens' ability to form salts with metals and their oxidizing power, which decreases as we move down the group.

Reaction with sodium

The halogens react vigorously with hot sodium metal to produce a sodium halide. They oxidise the sodium into Na(I) ions with a charge of +1. Sodium fluoride, chloride, bromide and iodide are all white solids.

For example, the reaction between chlorine and sodium: Sodium fluoride is commonly added to toothpaste and even drinking water to improve dental health. Fluorine helps build fluorapatite, a naturally occurring part of tooth enamel. On the other hand, sodium chloride is the common salt found in oceans and table salt shakers around the globe.

Reaction with iron

Halogens can oxidise iron into iron(III) ions. The overall reaction produces an iron(III) halide. However, this reaction only happens with fluorine, chlorine and bromine – iodine isn't a potent enough oxidising agent for the reaction to occur. For example, the reaction between chlorine and iron produces iron(III) chloride, which is used in sewage treatment and is a common catalyst for the reaction between ethene and chlorine, forming 1,2-dichloroethane.1,2-dichloroethane is the predecessor of 1,2-dichloroethene, the monomer used to make PVC.

Another redox reaction involving halogens is their reaction with sodium hydroxide. This reaction is an example of a disproportionation reaction, where the oxidation states of some of the atoms of a particular element increase and the oxidation states of others decrease. In the reaction between chlorine and cold sodium hydroxide, sodium chloride, sodium chlorate(I) and water are produced. In sodium chloride, chlorine has an oxidation state of -1, while in sodium chlorate, chlorine has an oxidation state of +1.

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Reacting chlorine with hot sodium hydroxide produces a slightly different product: sodium chlorate(V). In this compound, chlorine has an oxidation state of +5:

In Chlorine Reactions, you'll explore this reaction again. Bromine and iodine react similarly. However, you can produce sodium bromate(V) at a much lower temperature than needed to make sodium chlorate(V) since bromine is a better reducing agent than chlorine. Likewise, producing sodium iodate(V) is even easier. In general, whilst halogens become better oxidising agents as you go up the group, they become better reducing agents as you go down the group.

Reactions of halogens with organic molecules

Finally, halogens can react with organic molecules. You'll see these again as part of organic chemistry, but we'll look at them now too.

Reaction with alkanes

Yes, that's correct! Alkanes are hydrocarbons that contain only single C-C and C-H bonds and are therefore known as saturated hydrocarbons.

When an alkane is mixed with a halogen and exposed to UV light, a chemical reaction called free radical substitution occurs. In this reaction, the halogen atoms replace some of the hydrogen atoms in the alkane, resulting in the formation of a halogenoalkane. If the halogen used is chlorine, the reaction is called chlorination.

Free radical substitution can produce a variety of products depending on the specific conditions of the reaction. For example, the reaction between ethane and chlorine can produce chloroethane:

C2H6 + Cl2 → C2H5Cl + HCl

In this reaction, one of the hydrogen atoms in ethane is replaced by a chlorine atom, resulting in the formation of chloroethane and hydrogen chloride.

Overall, free radical substitution reactions are important for the synthesis of halogenoalkanes, which have various industrial and commercial applications.

Reaction with benzene

Benzene is a cyclic hydrocarbon with a molecular formula of C6H6. It is unique because it is neither an alkane nor an alkene, but an aromatic compound. Halogens can react with benzene in a substitution reaction known as electrophilic substitution. This reaction requires a catalyst such as an aluminium halide or iron. For example, the reaction between benzene and chlorine in the presence of aluminium chloride produces chlorobenzene and hydrochloric acid.

The reactivity of halogens decreases as you move down the group when reacting with both alkanes and benzene. Fluorination of benzene is explosive, while iodination does not tend to occur. Similarly, iodine does not react with alkanes. Chlorine and bromine are the preferred reactants. These reactions do not involve the transfer of electrons but instead form covalent molecules. In summary, halogen reactions are not too difficult to understand once you grasp the concepts of reactivity and oxidizing ability. As you move down the group, the oxidizing power decreases, and the reducing power increases.

Reactions of Halogens – Key takeaways The halogens are a family of nonmetals found in group 17 in the periodic table. Halogens can act as oxidising agents. Oxidising agents oxidise another species by taking electrons from them. The oxidising ability of halogens decreases as you move down the group. A more reactive halogen will displace a less reactive halide from an aqueous solution, known as a displacement reaction. Halogens oxidise hydrogen to form hydrogen halides. Reactivity decreases as you move down the group. Halogens oxidise metals to form salts. Reactivity decreases as you move down the group. Halogens react with sodium hydroxide in a disproportionation reaction. The products vary depending on the temperature used. Halogens react with alkanes and benzene in substitution reactions. Reactivity decreases as you move down the group.

Reactions of Halogens

In what group are the halogens?

The halogens are in group 17 on the periodic table. However, this group is often known as group 7.

How does the carbon-halogen bond affect the rate of reaction? 

Carbon-halogen bonds get more reactive as you move down the group. As the bond becomes weaker, the halogen becomes larger. For more information, check out 'Halogenoalkanes'.

How do the halogens react with metals?

Halogens oxidise metals to form salts. For example, iron reacts with chlorine to give iron chloride.

Which groups react easily with the halogens? 

Halogens react readily with all sorts of metals, including groups 1, 2, 3 and transition metals. They also react with hydrogen. When reacting with metals, halogens form salts with a giant ionic structure, and when reacting with hydrogen, they form hydrogen halides.

What happens when halogens react with alkali metals?

Halogens react violently with alkali metals to form a salt. For example, sodium reacts with chlorine to produce sodium chloride, also known as table salt.

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