Interaction with simple substances and corrosion
Absolutly pure iron does not react with atmospheric oxygen and water, however, ordinary steel easily undergoes corrosion (rusting). This process is triggered by the simultaneous exposure to moisture and oxygen, resulting in the formation of partially hydrated iron(III) oxide.
Upon heating, the metal demonstrates high reactivity toward non-metals:
- With oxygen: iron powder burns with the formation of bright sparks. The reaction product is mill scale — a mixed oxide.
- With halogens (e.g., chlorine): oxidation proceeds up to the +3 state, forming black iron(III) chloride. Interestingly, its crystal hydrate has a distinct orange color.
- With sulfur: heating a mixture of powders causes a vigorous exothermic reaction, resulting in the synthesis of black iron(II) sulfide.
Reactions with acids and water
The position of iron in the electrochemical series of standard electrode potentials (to the left of hydrogen) determines its ability to displace hydrogen from dilute acids, while converting into cations with an oxidation state of +2.
The interaction with oxidizing acids has certain characteristics:
- Dilute nitric acid: acting as a strong oxidizing agent, converts iron directly to the +3 oxidation state.
- Concentrated acids: sulfuric and nitric acids under normal conditions cause passivation — the formation of a protective surface film that halts the reaction. Strong heating is required for dissolution to begin.
Pure iron does not react with liquid water. However, heating the metal in a stream of water vapor initiates a reversible process whose products are iron(II) oxide and hydrogen gas. Alkali solutions do not affect metallic iron, but it is capable of displacing less active metals (positioned to the right in the reactivity series) from solutions of their salts.
Oxides, hydroxides, and hydrolysis features
Iron forms two key series of compounds corresponding to the +2 and +3 oxidation states.
Iron(II) hydroxide is a white precipitate that can only be obtained in the complete absence of oxygen. If dissolved oxygen is present in the solution, the precipitate instantly acquires a gray-green tint due to partial oxidation to the +3 state.
Iron(III) hydroxide is an extremely sparingly soluble orange-brown substance that precipitates completely even in an acidic medium. It exhibits weak amphoteric properties, reacting exclusively with highly concentrated alkalis, and forms ferrites upon fusion with oxides or carbonates of other metals.
Iron(II) salt solutions are usually light green and undergo negligible hydrolysis. Iron(III) salts hydrolyze much more strongly, imparting a yellow-brown color to the solutions. A characteristic example: adding sodium carbonate to iron(III) sulfate leads to mutual enhancement of hydrolysis, and iron(III) hydroxide precipitates instead of the carbonate, with the release of carbon dioxide.
Redox properties and complexes
In an acidic medium, iron(III) salts act as strong oxidizing agents: they are capable of dissolving metallic copper and oxidizing iodides to free iodine. In turn, iron(II) compounds are easily oxidized to +3 by hydrogen peroxide, nitric acid, or chlorine.
In a strongly alkaline medium, unique compounds can be obtained — ferrates (oxidation state +6). These are red solutions that become extremely powerful oxidizing agents in neutral and acidic media, capable of decomposing water.
Iron is prone to complex formation. The most famous examples are potassium ferrocyanide and potassium ferricyanide (yellow and red prussiates). They serve as analytical reagents for the qualitative detection of iron ions:
- Yellow prussiate with Fe³⁺ salts yields Prussian blue.
- Red prussiate with Fe²⁺ salts yields Turnbull's blue.
Chemically, this is the same dark blue precipitate. An important feature of the element is its ability to form complexes with a zero oxidation state through the pairing of 3d-electrons, for example, volatile iron pentacarbonyl(0).
Briefly: answers to topical questions
Which proteins are responsible for iron transport and intracellular storage in the body?
Specific non-heme proteins are responsible for iron transport and intracellular storage in the body.
- Transferrin is a plasma transport glycoprotein that delivers free oxidized iron to tissues.
- Ferritin is the main intracellular iron storage protein, localized in the liver, spleen, and bone marrow.
- Hemosiderin is a protein in which iron accumulates when in excess. It is formed from ferritin when the amount of iron exceeds the capacity of the ferritin storage pool.
Which human body enzymes contain iron?
Iron is a component of numerous heme and non-heme enzymes involved in oxidation-reduction reactions.
- Cytochromes (including cytochrome c oxidase and the cytochrome P450 system, e.g., 14-α-demethylase) ensure tissue respiration and electron transfer.
- Catalase and peroxidase (including glutathione peroxidase) neutralize reactive oxygen species and hydrogen peroxide.
- Xanthine oxidase is involved in purine metabolism.
- Ribonucleotide reductase is involved in metabolism.
- Dehydrogenases (e.g., NADH dehydrogenase and QH₂ dehydrogenase) contain iron-sulfur clusters and catalyze redox reactions.
What is the biochemical role of iron ions in tissue respiration?
During tissue respiration, iron ions ensure electron transfer along the mitochondrial respiratory chain, which is necessary for ATP synthesis. Iron is part of key components of the electron transport chain:
- Cytochromes (b, c₁, a, a₃) are heme proteins involved in the function of complexes III and IV (cytochrome c oxidase).
- Iron-sulfur proteins (FeS clusters) are non-heme components that are part of complex I (NADH dehydrogenase) and complex III (QH₂ dehydrogenase).
A deficiency of these iron-containing compounds leads to impaired electron transport, inhibition of ATP synthesis, and the development of tissue hypoxia.
To avoid confusion with qualitative reactions: Prussian blue and Turnbull's blue are the same substance (a dark blue precipitate). The only difference is the detection target: yellow prussiate is a reagent for Fe³⁺ ions, while red prussiate is for Fe²⁺ ions.
Class and exam questions
Why do concentrated acids fail to dissolve iron under normal conditions?
Passivation occurs: a dense oxide film rapidly forms on the metal surface, protecting it from further oxidation. The reaction starts only upon heating.
Does iron(III) hydroxide form complexes with ammonia?
No, it does not. Due to the extremely low solubility of iron(III) hydroxide, adding ammonia to iron salts simply results in precipitation, unlike copper and other d-elements.
Are iron cyanide complexes toxic?
No. In "prussiates", cyanide ions are firmly fixed in the inner sphere of the complex by coordination bonds with the iron atom, so they are chemically stable and do not exhibit the toxicity of free cyanides.
What else to study on this topic
- Industrial production of iron: the blast furnace process and its chemistry
- Methods for producing ultra-pure iron (electrolysis, thermal decomposition)
- Electronic structure and hybridization of iron pentacarbonyl(0)
- Mutual enhancement of hydrolysis using iron(III) salts and carbonates as an example
- Application of iron complex compounds in photography
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