Formulas and reactions of bivalent and trivalent iron with alkalis
Properties of bivalent and trivalent iron
By its physical properties, iron is a malleable metal of a silvery white color, with magnetic properties and which is a good conductor of electric current. The reactive ability of the metal is very high. If you place metal in a damp medium or heat to high temperatures, the metal quickly becomes subjected to corrosion – it spontaneously disintegrates under the impact of the environment. The impact of aggressive substances may also be harmful for metallic iron – for example, mineral acids.
Reactions of metallic iron with alkalis
By its chemical properties, iron is amphoteric – i.e. depending on the conditions, it may play the role of acid or base in a reaction. These properties are especially characteristic for iron with an oxidation state of +3 (compounds are also encountered in which the oxidation state of iron is +2 or +6). Chemical reactions of bivalent iron with alkalis have certain features.
Metallic iron does not react with diluted solutions of alkalis. A reaction is only possible in an alkaline flux of a strong oxidizer (potassium chlorate):
Fe + KClO₃ + 2KOH = K₂FeO₄ + KCl + H₂O (potassium ferrate forms).
Reaction of oxides and hydroxides of bivalent iron with alkalis
Oxides and hydroxides of bivalent iron are compounds with predominant basic properties (but amphoteric qualities are manifested in them to a lesser degree), so they do not react with solutions of alkalis in normal conditions.
With acidic oxides or solutions of acids, FeO, like Fe(OH)₂, reacts according to a typical displacement reaction:
FeO + 2HCl = FeCl₂ + H₂O;
Fe(OH)₂ + 2HCl = FeCl₂ + 2H₂O.
Iron (II) oxide reacts with sodium hydroxide only in alloying – the dual oxide forms with the composition 2Na₂O·FeO (summarily - Na₄FeO₃):
FeO + 4NaOH = Na₄FeO₃ + 2H₂O (temperature of alloying – 400-500 ᵒC or 752-932 ᵒF).
Iron hydroxide can display acidic properties only when it reacts with a concentrated alkali in an inert atmosphere (for example nitric). The reactive mixture must boil:
Fe(OH)₂ + 2NaOH = Na₂[Fe(OH)₄] (sodium tetra hydroxoferrate (II) Na₂[Fe(OH)₄] forms a sediment at a temperature of around 120 ᵒC or 248ᵒF).
Reaction of salts of bivalent iron with alkalis
Salts of bivalent iron are also capable of reacting with alkalis – to obtain Fe(OH)₂, the reaction should be conducted at room temperature, but without access of air – otherwise oxygen will rapidly oxidize Fe(OH)₂ to Fe(OH)₃). Iron II sulfate reacts with an alkali according to the equation:
FeSO₄ + 2NaOH = Na₂SO₄ + Fe(OH)₂ (a greenish sediment of bivalent iron hydroxide forms, which subsequently turns brown in the air).
Other salts with the cation Fe²⁺ react with alkalis in a similar way, for example iron (II) chloride:
FeCl₂ + 2NaOH = 2NaCl + Fe(OH)₂.
Reaction of oxides and hydroxides of trivalent iron with alkalis
Oxides and hydroxides of trivalent iron have pronounced amphoteric properties. With acids, these compounds react typically, acting as a base in the reaction:
- Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O;
- 2Fe(OH)₃ + 3H₂SO₄ = Fe₂(SO₄)₃ + 6H₂O.
With bases, the reaction may take place differently, as the final products depend on the form of the reacting alkali – if it is in a flux or a solution.
In a flux, reactions take place as follows:
With iron 3 oxide 3 – Fe₂O₃ + 2NaOH = 2NaFeO₂ + H₂O (product of alloying – sodium ferrite NaFeO₂);
With iron 3 hydroxide – Fe(OH)₃ + NaOH = NaFeO₂ + 2H₂O.
By the way, experiments with getting Fe(OH)₃ are included in MEL Chemistry subscription.
Iron (III) oxide in an aqueous medium with NaOH does not react. In a solution with heating the product of reaction of NaOH with iron (III) hydroxide:
Fe(OH)₃ + NaOH = Na[Fe(OH)₄] (sodium tetra hydroxoferrate (III)).
In abundance of an alkali, the reaction product may be different:
Fe(OH)₃ + 3NaOH = Na₃[Fe(OH)₆] (sodium hexa hydroxoferrate (III)).
Reaction of salts of trivalent iron with alkalis
In a solution, iron salts with an oxidation state of +3 are heavily hydrolyzed. With full hydrolysis iron (III) hydroxide forms. As iron is trivalent, it can form three series of compounds on hydrolysis – the main salts of two types – with the cations [Fe(OH)]²⁺, [Fe(OH)₂]⁺ and, with the full process, a base with the cation Fe³⁺.
Summary equation of full hydrolysis:
- Fe₂(SO₄)₃ + 6H₂O = 2Fe(OH)₃ + 3H₂SO₄ (hydrolysis takes place on the cation, the medium is acidic, as the strong acid H₂SO₄ forms).
In full form:
- 2Fe³⁺ + 3SO₄²⁻ + 6H₂O = 2Fe(OH)₃ + 6H⁺ + 3SO₄²⁻.
In abbreviated ionic form:
- 2Fe³⁺ + 6H₂O = 2Fe(OH)₃ + 6H⁺.
In carrying out any reactions with Fe³⁺ salts in the solution, the influence of hydrolysis should be taken into account – secondary insoluble products may appear, or the pH of the medium may change (as long as this is not hydrolysis of iron sulfide or other salts with weak cations and anions).
Many salts of trivalent iron (for example, iron (III) sulfate) break down under the impact of alkali. The final product depends on the concentration of the solution of MeOH:
Fe₂(SO₄)₃ + 2NaOH = 2Fe(OH)SO₄ + Na₂SO₄ (iron (III) hydroxosulfate forms);
Fe₂(SO₄)₃ + 6NaOH = 2FeO(OH) + 3Na₂SO₄ + 2H₂O (iron metahydroxide settles).
Reactions between salts of trivalent iron and alkalis do not always take place stoichiometrically – i.e. correspond to a fixed quantitative composition. Sometimes the main component of the mineral jarosite appears in the sediment - KFe₃(OH)₆(SO₄)₂ (with the use of KOH as a reagent).
If you mix iron (II) sulfate and iron (III) sulfate, you can obtain their mixed oxide:
Fe₂(SO₄)₃ + FeSO₄ + 8NaOH = Fe₃O₄ + 4Na₂SO₄ + 4H₂O (an oxide with the formula FeO·Fe₂O₃ forms a sediment – “iron cinder”, in nature – magnetite).
Often, ferrites and ferrates obtained in the reaction of iron 2 and 3 with alkalis are used as filtering and decontaminating components. Iron oxides are used in the manufacture of electrodes, food colorings, ceramics, and also in smelting cast-iron.