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8.3.1 Group VII properties and reactions

The halogens

Definition

Halogen

An element in Group VII of the Periodic Table, such as chlorine, bromine or iodine, whose atoms have seven electrons in the outer shell.

Definition

Diatomic molecule

A molecule made of two atoms joined by a covalent bond.

  1. The elements in Group VII are called the halogens, and the three you need to know in detail are chlorine, bromine and iodine.
  2. Fluorine sits above chlorine at the top of the group, and astatine sits below iodine at the bottom.
  3. The halogens are non-metals, so they are on the right-hand side of the Periodic Table.
  4. Every halogen atom has seven electrons in its outer shell, so chlorine is 2,8,72,8,72,8,7.
  5. Each halogen exists as diatomic molecules, in which two atoms are joined by a single covalent bond: Cl2\text{Cl}_2Cl2​, Br2\text{Br}_2Br2​ and I2\text{I}_2I2​.
  6. In the molecule, each atom shares one of its electrons with the other atom, so both atoms reach a full outer shell of eight electrons.
  7. When a halogen reacts with a metal, each halogen atom gains one electron to form an ion with a charge of −1-1−1, such as the chloride ion Cl−\text{Cl}^{-}Cl−.

Appearance at room temperature

  1. At room temperature and pressure (r.t.p.), the three halogens are in different states and have different colours.
  2. Chlorine: a pale yellow-green gas.
  3. Bromine: a red-brown liquid, which gives off a red-brown vapour because it evaporates easily.
  4. Iodine: a grey-black solid, which turns into a purple vapour when it is warmed.
  5. The change from gas to liquid to solid shows that melting point and boiling point increase down the group.
  6. The colour also gets darker down the group, from pale yellow-green to red-brown to grey-black.
Common Mistake
  • Write red-brown for bromine and grey-black for iodine, because "brown" for bromine or "purple" for solid iodine does not match the description the marks are given for.
  • Do not give the colour of a halogen to its halide ion, because chloride, bromide and iodide ions in solution are colourless.

Trends down Group VII

  1. Density increases down Group VII.
    1. Chlorine is a gas with a very low density, bromine is a liquid with a density of about 3.1 g/cm33.1\ \text{g/cm}^33.1 g/cm3 and iodine is a solid with a density of about 4.9 g/cm34.9\ \text{g/cm}^34.9 g/cm3.
  2. Reactivity decreases down Group VII.
    1. Chlorine is the most reactive of the three and iodine is the least reactive.
    2. A halogen atom reacts by gaining one electron to complete its outer shell.
    3. Further down the group, the outer shell is further from the nucleus, so the attraction for an incoming electron is weaker.
    4. The electron is therefore gained less easily, so the halogen is less reactive.
  3. The reactivity trend in Group VII is the opposite of the trend in Group I, because halogens react by gaining electrons while alkali metals react by losing them.

Displacement reactions

Definition

Displacement reaction

A reaction in which a more reactive element takes the place of a less reactive element in a compound.

Definition

Halide ion

A negative ion with a charge of −1-1−1, formed when a halogen atom gains one electron.

  1. A halide ion is the negative ion formed from a halogen, so chlorine forms chloride ions, bromine forms bromide ions and iodine forms iodide ions.
  2. A more reactive halogen displaces a less reactive halogen from a solution of its halide ions.
  3. The more reactive halogen takes the electron from the halide ion, so it becomes a halide ion itself and the less reactive halogen is released as molecules.
  4. When chlorine water is added to colourless potassium bromide solution, the solution turns orange because bromine is released.
    1. The word equation is chlorine +++ potassium bromide →\rightarrow→ potassium chloride +++ bromine.
    2. The symbol equation is Cl2+2KBr→2KCl+Br2\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2Cl2​+2KBr→2KCl+Br2​.
    3. The ionic equation is Cl2+2Br−→2Cl−+Br2\text{Cl}_2 + 2\text{Br}^{-} \rightarrow 2\text{Cl}^{-} + \text{Br}_2Cl2​+2Br−→2Cl−+Br2​.
  5. The potassium ions take no part in the reaction, which is why they do not appear in the ionic equation.
  6. When chlorine water is added to potassium iodide solution, the solution turns brown because iodine is released: Cl2+2KI→2KCl+I2\text{Cl}_2 + 2\text{KI} \rightarrow 2\text{KCl} + \text{I}_2Cl2​+2KI→2KCl+I2​.
  7. When bromine water is added to potassium iodide solution, the solution also turns brown: Br2+2KI→2KBr+I2\text{Br}_2 + 2\text{KI} \rightarrow 2\text{KBr} + \text{I}_2Br2​+2KI→2KBr+I2​.
  8. A less reactive halogen cannot displace a more reactive one, so bromine does not react with chloride ions and iodine does not react with chloride or bromide ions.
  9. In these combinations there is no colour change, apart from the colour of the halogen solution that was added.
  10. In every displacement, the halogen gains electrons and the halide ion loses electrons, so the reaction is a redox reaction in which the halogen is the oxidising agent.
Practical

Displacement reactions of the halogens

  • Aim: to find out which halogens displace which halide ions from solution, and to use the results to place chlorine, bromine and iodine in order of reactivity.
  • Apparatus: a white spotting tile or six test-tubes in a rack, dropping pipettes, chlorine water, bromine water, iodine solution, and 0.1 mol/dm30.1\ \text{mol/dm}^30.1 mol/dm3 solutions of potassium chloride, potassium bromide and potassium iodide, plus eye protection and access to a fume cupboard.
  • Method:
    • Place the spotting tile on a white background so that small colour changes are easy to see.
    • Add five drops of potassium chloride solution to each of two wells, and label the wells.
    • Repeat this with potassium bromide solution in two more wells and potassium iodide solution in the last two wells.
    • Add five drops of chlorine water to one potassium bromide well and one potassium iodide well.
    • Add five drops of bromine water to one potassium chloride well and one potassium iodide well.
    • Add five drops of iodine solution to one potassium chloride well and one potassium bromide well.
    • Put a few drops of each halogen solution on its own in three spare wells as controls, so you can tell a new colour from the colour of the solution you added.
    • Record the colour in each well against the controls.
  • Results:
    • Chlorine water with potassium bromide turns orange, and with potassium iodide turns brown.
    • Bromine water with potassium iodide turns brown, while bromine water with potassium chloride keeps the pale orange colour of the bromine water.
    • Iodine solution with potassium chloride or potassium bromide keeps the pale brown colour of the iodine solution.
  • Conclusion: chlorine displaces both bromine and iodine, bromine displaces only iodine, and iodine displaces neither, so the order of reactivity is chlorine, then bromine, then iodine.
  • Watch out: dilute bromine and dilute iodine can look similar, so a few drops of cyclohexane can be shaken with the mixture, where bromine dissolves to give an orange layer and iodine gives a purple layer.
  • Safety: chlorine water and bromine water give off toxic, irritating vapours, so keep the bottles in a fume cupboard and wear eye protection throughout.
Example

Writing a displacement equation

  • Bromine water is added to sodium iodide solution.
  • Bromine is above iodine in Group VII, so bromine is more reactive and displaces iodine.
  • The word equation is bromine +++ sodium iodide →\rightarrow→ sodium bromide +++ iodine.

Br2+2NaI→2NaBr+I2\text{Br}_2 + 2\text{NaI} \rightarrow 2\text{NaBr} + \text{I}_2Br2​+2NaI→2NaBr+I2​

  • Removing the sodium ions, which do not change, gives the ionic equation.

Br2+2I−→2Br−+I2\text{Br}_2 + 2\text{I}^{-} \rightarrow 2\text{Br}^{-} + \text{I}_2Br2​+2I−→2Br−+I2​

  • The solution changes from colourless to brown as iodine forms.
Exam technique

Explaining a displacement result

  • When you explain why a reaction happens, compare the two halogens directly, such as "chlorine is more reactive than bromine".
  • When you explain why no reaction happens, state that the added halogen is less reactive than the halogen in the halide.
  • Describe the colour change of the solution, such as "the solution turns orange", rather than describing the colour of a pure halogen at r.t.p.

Predicting other halogens

  1. To predict the properties of an unfamiliar halogen, decide whether it is above or below a known halogen in the group.
  2. Fluorine, above chlorine, is predicted to be a gas that is less dense and more reactive than chlorine.
    1. Fluorine is predicted to displace chlorine, bromine and iodine from their halide ions.
  3. Astatine, below iodine, is predicted to be a dark solid that is denser and less reactive than iodine.
    1. Astatine is predicted not to displace any of the other halogens from their halide ions.
  4. Every halogen is predicted to exist as diatomic molecules, such as F2\text{F}_2F2​ and At2\text{At}_2At2​, and to form halide ions with a charge of −1-1−1.
  5. When data for melting points or boiling points are given, continue the increase down the group to make a sensible estimate.
Self review
  • Why are the halogens described as diatomic non-metals?
  • State the appearance of chlorine, bromine and iodine at r.t.p.
  • How do density and reactivity change down Group VII?
  • Explain why chlorine displaces bromine from potassium bromide solution, and give the colour change.
  • Predict whether astatine would displace iodine from potassium iodide solution, and give a reason.
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8.3.1 Group VII properties and reactions Revision Guide

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Revision notes for CIE IGCSE Chemistry 8.3.1 Group VII properties and reactions: explanations and worked examples.

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