Halogen displacement reactions
A more The tendency of a substance to undergo a chemical reaction. An element placed in group 7 of the periodic table, which starts with fluorine and ends with astatine. The name 'halogen' means 'salt-producing' because halogens produce a range of salts when they react with metals. can Take the place of another substance in a chemical reaction. For example, a metal can displace a less reactive metal from its oxide, removing oxide ions from the less reactive metal and becoming an oxide itself. a less reactive halogen from Mixture formed by a solute and a solvent. of its The substance formed when the hydrogen ion in an acid is replaced by a metal ion.. For example, chlorine is more reactive than iodine. A solution of chlorine can displace iodine from potassium iodide solution:
chlorine + potassium iodide 鈫 potassium chloride + iodine
Cl2(aq) + 2KI(aq) 鈫 2KCl(aq) + I2(aq)
The reaction mixture turns darker and iodine solution forms.
The slideshow shows what happens when solutions of chlorine, bromine and iodine are added to various potassium A halide ion is an ion formed when a halogen atom (an atom from group 7) gains one electron. Halide ions have a single negative charge. Ionic compounds containing halide ions may be called halides. salts.
Image caption, Adding chlorine, bromine and iodine to halogen salts
Chlorine water is added to three solutions
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Determining a reactivity series
A A list of elements in order of their reactivity, usually from most reactive to least reactive. can be produced by attempting some A reaction that occurs when a more reactive element replaces a less reactive element in a compound.. Different combinations of halogen solution and salt solution are tested. The table shows the results of these tests. A halogen cannot displace itself from a solution of one of its salts, so these three tests were not done.
In the table below, the following formula names are used:
- KCl - potassium chloride solution
- KBr - potassium bromide solution
- KI - Potassium iodide solution
| KCl solution | KBr solution | Kl solution | Reactions | |
| Chlorine | Not done | Solution darkens | Solution darkens | 2 |
| Bromine | No visible reaction | Not done | Solution darkens | 1 |
| Iodine | No visible reaction | No visible reaction | Not done | 0 |
| Chlorine | |
|---|---|
| KCl solution | Not done |
| KBr solution | Solution darkens |
| Kl solution | Solution darkens |
| Reactions | 2 |
| Bromine | |
|---|---|
| KCl solution | No visible reaction |
| KBr solution | Not done |
| Kl solution | Solution darkens |
| Reactions | 1 |
| Iodine | |
|---|---|
| KCl solution | No visible reaction |
| KBr solution | No visible reaction |
| Kl solution | Not done |
| Reactions | 0 |
Example
Use the results in the table to deduce an order of reactivity, starting with the most reactive halogen.
The order of reactivity is chlorine > bromine > iodine. This is because chlorine could displace bromine and iodine, bromine could only displace iodine, but iodine could not displace chlorine or bromine.
Question
Astatine is placed below iodine in The vertical column of non-metal elements next to group 0 in the periodic table, also called the halogens.. Predict whether astatine will displace iodine from potassium iodide solution.
Astatine is less reactive than iodine, so it will not displace iodine from potassium iodide solution.
Explaining the trend
When an element in group 7 takes part in a reaction, its The smallest part of an element that can exist. outer shells gain an Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. and form negatively charged Electrically charged particle, formed when an atom or molecule gains or loses electrons., called An atom or group of atoms that have gained electrons and become negatively charged. Most anions are formed from non-metals.. The less easily these anions form, the less reactive the halogen.
Going down group 7:
- the atoms become larger
- the outer shell becomes further from the The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei.
- the force of attraction between the nucleus and the outer shell decreases
- an outer electron is gained less easily
- the halogen becomes less reactive
Displacement reactions as redox reactions - Higher
A A chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. for the reaction between chlorine and potassium bromide solution can be written in terms of the ions involved:
Cl2(aq) + 2K+(aq) + 2Br-(aq) 鈫 2K+(aq) + 2Cl-(aq) + Br2(aq)
Notice that potassium ions, K+, appear on both sides of the equation. They do not take part in the reaction and are called An ion that is exactly the same on both sides of an ionic equation.. The equation can be rewritten without them:
Cl2(aq) + 2Br-(aq) 鈫 2Cl-(aq) + Br2(aq)
This equation is an example of a balanced A chemical equation that shows how positively charged ions join with negatively charged ions to make a compound.. It can be split into two An equation, involving ions and electrons, that describes the process happening at an electrode.:
Cl2(aq) + 2e- 鈫 2Cl-(aq), (reduction)
2Br-(aq) 鈫 Br2(aq) + 2e-, (oxidation)
Notice that:
- chlorine atoms (in chlorine A collection of two or more atoms held together by chemical bonds.) gain electrons - they are A substance is reduced if it loses oxygen or gains electrons.
- bromide ions lose electrons and form molecules - they are Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons.
The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. and The gain of oxygen, or loss of electrons, by a substance during a chemical reaction. happen at the same time, so the reactions are called When reduction and oxidation take place at the same time..
Displacement reactions are just one example of redox reactions. The decomposition (breakdown) of a compound using an electric current. reactions are also redox reactions.