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Reactions of Halogens

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Reactions of Halogens

Did you know that we can use bromine to reduce mercury emissions from coal power plants? Scientists have already discovered that they can use something called activated carbon to trap mercury particles released when coal is burnt. However, bromine has recently come into the spotlight, because it can help the activated carbon oxidise mercury and trap more of the harmful element. This is just one example of the many reactions of halogens.

  • This article looks at many of the different reactions of halogens.
  • You'll explore the trends in various halogen redox and displacement reactions, including the reactions of halogens with halides, hydrogen, metals, sodium hydroxide and organic molecules.
  • You'll learn how reactivity varies as you move down the group.

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.

Reactions of halogens periodic table halogens studysmarterThe halogens. StudySmarter Originals

If you've read the article Halogens, you may recall that some scientists include the artificial element tennessine among the halogens. As before, we won't focus on it in this article. It has only existed for a fraction of a second, and no one quite knows any of its properties!

Halogens can react in many ways. They can:

  • Displace other halogens.

  • React with hydrogen.

  • React with metals.

  • React with sodium hydroxide.

  • React with alkanes, benzene and other organic molecules.

We'll explore these reactions throughout the rest of this article.

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:

Reactions of halogens OIL RIG StudySmarterThe OIL RIG acronym. StudySmarter Orginals

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

Reactions of halogens RAD OAT StudySmarterThe RAD OAT acronym. StudySmarter Originals

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

An example of an oxidation reaction involving halogens is a displacement reaction.

A displacement reaction occurs when one element in a compound replaces another.

In principle, these reactions are pretty simple. A more reactive halogen displaces a less reactive halide from an aqueous solution:

  • The more reactive halogen atoms oxidise the less reactive halide ions.
  • The halide ions lose electrons and form halogen atoms. They are oxidised.
  • The halogen atoms gain electrons to form halide ions. They are reduced.

Suppose you have a sodium bromide solution to which you add chlorine. The halogens get less reactive as you go down the group in the periodic table, meaning that chlorine is more reactive than bromine.

Reactions of Halogens The reactivity of halogens StudySmarterReactivity of halogens. StudySmarter Originals

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:

We know this reaction occurs because we can see a colour change. The bromide ions turn the solution orange-brown.

However, if you add bromine to a solution containing chloride ions, nothing will happen – bromine is less reactive than chlorine. It isn't a potent enough oxidising agent to oxidise the chloride ions.

The following table shows the displacement reactions between different combinations of chlorine, bromine, iodine and their aqueous ions alongside any observable colour changes. Notice that fluorine is not inclluded. The reason is that fluorine is too strong of an oxidising agent. In fact, it manages to oxidise water into oxygen, complicating the reaction further. Likewise, we haven't included astatine because it is both extremely rare and highly reactive, and you'll probably never come across it.

Reactions of halogens displacement StudySmarterHalogen displacement reactions. StudySmarter Originals

Reactions of halogens with hydrogen

Halogens react with hydrogen in another example of a redox reaction. They form a hydrogen halide, HX. Like in the displacement reactions that we looked at above, the halogens act as oxidising agents, and so their reactivity falls as you move down the group.

For example, fluorine and hydrogen react explosively to give hydrogen fluoride gas:

Hydrogen is oxidised and loses electrons, whilst fluorine is reduced and gains electrons.

However, iodine and hydrogen only partially react. The mixture forms an equilibrium:

Notice that the reaction between hydrogen and iodine is reversible.

Reactions of halogens with metals

Let's now look at a few examples of reactions between halogens and metals. All of these reactions form salts. Again, the reactions are redox reactions and reactivity decreases as you move down the group.

The word halogen means 'salt former', from the Greek hal/halo- , meaning 'salt' and -gen, meaning 'to produce'.

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:

Iron(III) chloride is used in sewage treatment. It is also 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. You'll find out more about PVC in the article Properties of Polymers.

Reactions of halogens with sodium hydroxide

Another redox reaction involving halogens is their reaction with sodium hydroxide. However, this reaction is slightly different: it is an example of a disproportionation reaction.

A disproportionation reaction is a reaction where the oxidation states of some of the atoms of a particular element increase and the oxidation states of others decrease. In other words, one element is both oxidised and reduced.

Let's look at the reaction between chlorine and cold sodium hydroxide. The two react to produce sodium chloride, sodium chlorate(I) and water:

Look at the oxidation states of the compounds produced:

  • In sodium chloride, chlorine has an oxidation state of -1. The chlorine has been reduced.
  • In sodium chlorate, chlorine has an oxidation state of +1. The chlorine has been oxidised.

Reactions of halogens disproportionation StudySmarterOxidations states of chlorine is the disproportionation reaction with sodium hydroxide. StudySmarter Originals

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

Alkanes are saturated hydrocarbons containing just C-C and C-H single bonds.

Don't worry if you don't understand those terms! If you want to find out more, check out Alkanes.

If you mix an alkane with a halogen and shine UV light on the mixture, the two molecules will react. Halogen atoms replace some of the alkane's hydrogen atoms to produce a halogenoalkane. This reaction is known as free radical substitution. If the halogen used is chlorine, it's also known as chlorination.

Free radical substitution can produce a variety of products. But to give an example, the reaction between ethane and chlorine could produce chloroethane:

Reaction with benzene

Benzene is a cyclic hydrocarbon with a molecular formula C6H6. It is unique because it is neither an alkane nor an alkene but something between.

You can find out more about benzene in Aromatic Chemistry.

Halogens react with benzene in another substitution reaction known as electrophilic substitution. This reaction needs a catalyst – either an aluminium halide, or iron.

For example, reacting benzene and chlorine in the presence of aluminium chloride produces chlorobenzene and hydrochloric acid:

Regarding reactions with both alkanes and benzene, the reactivity of the halogens decreases as you move down the group. Fluorination of benzene is explosive, whilst iodination doesn't tend to happen. Likewise, you'd be hard-pressed to react iodine with any alkanes. Chlorine and bromine are the preferred reactants of choice. These reactions also aren't redox reactions – no electrons transfer. Instead, they form covalent molecules.

Summary of reactions of halogens

Yes, we know – we've thrown a lot of new information at you! But once you get your head around reactivity and oxidising ability, most halogen reactions aren't too tricky:

  • As you move down the group, oxidising power decreases.
  • As you move down the group, 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.

Frequently Asked Questions about Reactions of Halogens

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

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'.

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

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.

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.

Final Reactions of Halogens Quiz

Question

True or false: Fluorine is more reactive than chlorine.


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Answer

True

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Question

Halogens can act as oxidising agents. Define oxidising agent.


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Answer

A species that oxidises another substance and is reduced itself in the process.

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Question

True or false: Chlorine is a better oxidising agent than fluorine.


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Answer

False

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Question

True or false: Oxidising agents gain electrons.

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Answer

True

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Question

What is a displacement reaction?


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Answer

It is a reaction where one element in a compound replaces another.

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Question

Which of the following is correct?

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Answer

A more reactive halogen will displace a less reactive halide from an aqueous solution.

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Question

What do we observe when we mix chlorine and sodium bromide?


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Answer

The solution turns orange-brown.

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Question

What do we observe when we mix bromine and sodium iodide?


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Answer

The solution turns red/a grey precipitate forms.

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Question

What do we observe when we mix iodine and sodium chloride?


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Answer

No observable reaction.

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Question

The reaction between a metal and a halogen forms ______.

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Answer

A salt.

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Question

How many moles of chlorine are required to react with 2 moles of calcium?


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Answer

1

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Question

When a halogen reacts with a metal, the metal is _____ and the halogen is ______.

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Answer

Oxidised, reduced.

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Question

What is a disproportionation reaction?


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Answer

A reaction where oxidation states of some atoms in an element increase and oxidation states of others decrease.

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Question

  1. State the conditions for the reaction between an alkane and a halogen.
  2. Name this type of reaction.
  3. State the type of product formed.


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Answer

  1. UV light.
  2. Free radical substitution.
  3. Halogenoalkane.

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Question

  1. State the conditions for the reaction between benzene and chlorine.
  2. Name this type of reaction.
  3. State the product formed.

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Answer

  1. Aluminium chloride or iron catalyst.
  2. Electrophilic substitution.
  3. Chlorobenzene.

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Question

What group do the halogens belong in?

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Answer

Group 17

Show question

Question

List the first four members of the halogen family.

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Answer

  • Fluorine
  • Chlorine
  • Bromine
  • Iodine

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