Interaction with simple substances
Chlorine most frequently acts as the typical halogen. Under the influence of ultraviolet rays, its diatomic molecules split into free radical atoms. This transformation serves as the starting point for chain chemical processes, for example, when reacting with hydrogen or organic compounds (ethane, methane).
Upon slight heating, chlorine actively reacts with other non-metals. The process can proceed in stages:
- With phosphorus, a colorless liquid is formed first—phosphorus trichloride—which upon further chlorination turns into white crystals of phosphorus pentachloride.
- With sulfur, the reaction yields a yellow liquid (disulfur dichloride), and the addition of an extra chlorine molecule leads to the synthesis of dark red sulfur dichloride.
As for oxygen, halogens do not interact with it directly. Oxygen-containing compounds are obtained exclusively via indirect pathways. However, halogens react directly with most metals to form salts. The only exceptions are the most resistant metals—iridium and rhodium.
Reactions with water, acids, and oxides
The behavior of halogens in water is quite specific. A reversible disproportionation process takes place in an aqueous medium. About 30% of chlorine molecules in a saturated solution turn into a mixture of two acids: hydrochloric and hypochlorous.
Chlorine is incapable of displacing oxygen from oxides. Interaction with oxygen-containing compounds is possible under only one condition: if the element bonded to oxygen is capable of increasing its valence and oxidation state. A striking example is the reaction with carbon monoxide (II), where carbon transitions to a +4 state.
Chlorine reacts with acids exclusively when the acid anion can be oxidized. For example, chlorine can oxidize nitrous acid to nitric acid (in the presence of water) or displace a less active halogen—bromine—from hydrobromic acid.
Interaction with alkalis and salts
In alkaline solutions, complete disproportionation of the halogen is observed—it simultaneously acts as both an oxidizing agent and a reducing agent. The final products depend on the temperature regime:
- In a cold environment (about 20 °C): chloride and hypochlorite are formed.
- Upon heating (up to 90 °C): chloride and chlorate are formed.
The reaction of chlorine with calcium hydroxide leads to the formation of so-called bleaching powder—a whitish mixture of calcium salts. This substance easily breaks down even under the action of weak acids (including carbon dioxide from humid air), releasing chlorine gas, which causes its specific odor.
Halogens react with salts if there is an opportunity to increase the oxidation state of the cation or anion. Thus, chlorine converts iron (II) salts into iron (III) salts, and oxidizes sulfites to sulfates.
Comparative activity: fluorine, bromine, and iodine
The oxidizing ability of halogens decreases as you move down the group.
- Fluorine possesses extremely strong oxidizing characteristics. It decomposes almost any substance into elements. Interacting with water, fluorine forms hydrogen fluoride and atomic oxygen. The reaction with alkalis produces sodium fluoride, hydrogen fluoride, and fluorine oxide.
- Bromine is weaker than chlorine, but is still capable of oxidizing iron ions from the +2 to the +3 state (in this process, the color of the bromine itself disappears).
- Iodine is the least active oxidizing agent in this group. Unlike bromine, it is unable to oxidize iron (II) ions, so adding iodine simply colors the solution. A qualitative reaction for iodine is its interaction with starch: an intense blue-colored product is formed, which easily decomposes upon heating.
Briefly: answers to topical questions
Which halogens are capable of oxidizing iron (II) salts to iron (III) salts?
Chlorine and bromine are capable of oxidizing iron (II) salts to iron (III) salts. Chlorine reacts with iron (II) salts, oxidizing the cation and forming iron (III) chloride. Bromine is also a stronger oxidizing agent than the iron (III) ion, so the reaction with it proceeds successfully. Iodine has a lower oxidizing capacity compared to chlorine and bromine, which is why it is unable to oxidize iron (II) salts.
What specific compound is formed during the reaction of chlorine with carbon monoxide (II)?
During the reaction of chlorine with carbon monoxide (II), a compound with the formula $C^{+4}OCl_2$ is formed. In the course of this interaction, the oxidation state of carbon increases from +2 to +4. Chlorine does not directly displace oxygen from compounds, so a prerequisite for this reaction to occur is the possibility of increasing the valence of the element bonded to oxygen.
Which halogens are part of thyroid hormones?
Iodine is part of thyroid hormones. This halogen participates in the formation of iodinated tyrosine via electrophilic substitution of hydrogen in the aromatic ring. Iodinated tyrosine acts as a precursor and constituent part of such thyroid hormones as thyroxine and triiodothyronine. A lack of iodine in the diet leads to a decrease in the synthesis of these hormones and the development of endemic iodine-deficiency goiter.
Which halogen-containing compounds are used in medicine as antiseptics?
Halogen-containing preparations based on iodine and chlorine are used as antiseptics in medicine.
- Iodine preparations—alcoholic solution (5%), Lugol's solution, iodoform, iodinol, and iodopyrine. They contain active elemental iodine, which causes protein denaturation and possesses oxidizing properties.
- Chlorine preparations—chloramines, for example Chloramine B (Chloraminum), and chlorites.
To easily remember the products of chlorine disproportionation in alkalis, focus on temperature: "In the cold, there is little oxygen (hypochlorite is formed); in the warmth, there is a lot of oxygen (chlorate is formed)."
Class and exam questions
Why does bleaching powder always have a pungent odor?
Chlorinated lime contains salts that react even with weak acids. Carbon dioxide present in humid atmospheric air reacts with the lime, leading to the continuous release of free chlorine.
Can chlorine displace oxygen from oxides?
No, chlorine does not have the ability to displace oxygen. The reaction with the oxide will proceed only if the element within the oxide is capable of further increasing its valence.
How to distinguish bromine salts from iodine salts using reagents?
Iodine can be identified using starch—a characteristic blue coloration is formed. Bromine does not produce such compounds with starch.
Why doesn't iodine react with iron (II) salts?
The redox potential of iodine is lower than the potential of the iron transition from the +2 to the +3 state. Iodine is too weak an oxidizing agent for this reaction, unlike the stronger bromine.
What else to study on this topic
- The mechanism of chain reactions of halogen decay under the influence of ultraviolet light
- Stepwise halogenation of non-metals (phosphorus and sulfur)
- The effect of temperature on disproportionation reaction products
- Chemical composition and properties of bleaching powder
- Comparative analysis of redox potentials of halogens
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Other topics in the section "Chapter 9. Group VIIA Elements — Halogens"
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