Iron

Main page

Chemistry • Periodic table • Polymers • Organic compounds • Inorganic compounds • R/S-phrases

Education • Main page • Innovation in R&D

The site • Disclaimer • About the site • Site history

Other names/abbreviations:

Fe

Element no:	26
Element group:	8 (IUPAC) VIII A (American labelling) VIII A (European labelling)
Element type:	Transition metals
CAS-Number:	7439-89-6
EINECS-Number:	231-096-4

Chemical properties

Reaction of iron with acids
Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen forming Fe(II) ions and H₂. In aqueous solution Fe(II) is present as the complex [Fe(H₂O)₆]²⁺.

Fe (s) + H₂SO₄ (aq) Fe²⁺ (aq) + SO₄^2- (aq) + H₂ (g)

If oxygen is present, some of the Fe(II) oxidizes to Fe(III).

3 Fe(s) + 4 H₂SO₄ (aq) Fe²⁺ (aq) + 2 Fe³⁺ (aq) + 4 SO₄^2- (aq) + 4 H₂ (g)

Concentrated nitric acid, HNO₃, reacts on the surface of iron and passivates the surface.


Reaction of iron with ammonia
Fe(II) and Fe(III) does not form complexes with ammonia but are both precipitated as hydroxides:

NH₃ (aq) + H₂O (l) NH₄⁺(aq) + OH^-(aq)
Fe²⁺ (aq) + 2 OH^- (aq) Fe(OH)₂ (s) [white]
Fe³⁺ (aq) + 3 OH^- (aq) Fe(OH)₃ (s) [reddish brown]


Reaction of iron with carbonate
Fe(II) is precipitated by carbonate ions:

Fe²⁺ (aq) + CO₃^2- (aq) FeCO₃ (s) [white]

The carbonate is readily oxidized to Fe(OH)₃ when exposed to O₂(g).

Fe(III) is not precipitated by carbonate ions, but will precipitate as the hydroxide:

CO₃^2- (aq) + H₂O (l) HCO₃^- (aq) + OH^- (aq)
Fe³⁺ (aq) + 3OH^- (aq) Fe(OH)₃ (s) [reddish brown]


Reaction of iron with halogens
Iron reacts with excess of the halogens F₂, Cl₂, and Br₂, to form Fe(III) halides.

2 Fe (s) + 3 F₂ (g) 2 FeF₃ (s) [white]
2 Fe (s) + 3 Cl₂ (g) 2 FeCl₃ (s) [dark brown]
2 Fe (s) + 3 Br₂ (l) 2 FeBr₃ (s) [reddish brown]

For I₂, Fe is only oxidized to Fe(II)

Fe (s) + I₂ (s) FeI₂ (s) (grey)

Fe(III) forms yellow chloro complexes. The complexed are readily hydrolyzed by heat.

Fe(III) is reduced to Fe(II) by I^-:

2 Fe³⁺ (aq) + 2 I^- (aq) 2 Fe²⁺ (aq) + I₂ (aq)


Reaction of iron with hydroxide ions
Fe(II) is precipitated by hydroxide ions:

Fe²⁺ (aq) + 2 OH^- (aq) Fe(OH)₂ (s) [white]

Fe(III) is precipitated by hydroxide ions:

Fe³⁺ (aq) + 3 OH^- (aq) Fe(OH)₃ (s) [reddish brown]


Reaction of iron with phosphate
Fe(II) is not precipitated by phosphate ions in acetic acid.

Fe(III) is precipitated by phosphate ions in acetic acid:

Fe³⁺ (aq) + H₂PO₄^- (aq) FePO₄ (s) [light yellow] + 2 H⁺ (aq)


Reaction of iron with hexacyanoferrate
Fe(II) is precipitated by hexacyanoferrate(II)ions. Under acidic conditions, the precipitate is white, however.

Fe²⁺ (aq) + 2 K⁺ (aq) + [Fe(CN)₆]^4- (aq) K₂Fe[Fe(CN)₆] (s) [light blue]

Fe(III) is precipitated by hexacyanoferrate(II)ions under acidic conditions.

4 Fe³⁺ (aq) + 3 [Fe(CN)₆]^4- (aq) Fe₄[Fe(CN)₆]₃ (s) [Turnbull's blue]

Fe(II) is precipitated by hexacyanoferrate(III)ions under acidic conditions:

21 Fe²⁺ (aq) + 14 [Fe(CN)₆]^3- (aq) 5 Fe₄[Fe(CN)₆]₃ (s) [Turnbull's blue]

Turnbull's blue is destroyed by base:

Fe₄[Fe(CN)₆]₃ (s) + 12 OH^- (aq) 4 Fe(OH)₃ (s) + 3 [Fe(CN)₆]^4- (aq)


Reaction of iron with sulfide
Fe(II) is not precipitated by sulfide under acidic conditions (i.e. H₂S) but pricipitates with Na₂S:

Fe²⁺(aq) + HS^-(aq) + OH^- (aq) FeS(s) [black] + H₂O(l)

Fe(III) is not precipitated by sulfide under acidic conditions (i.e. H₂S) but becomes reduced to Fe(II):

Fe³⁺ (aq) + H₂S (aq) Fe²⁺ (aq) + S (s) + 2 H⁺ (aq)


Reaction of iron with thiocyanate
Fe(II) forms colorless complexes with SCN^-

Fe(III) forms red complexes with SCN^-:

Fe³⁺ (aq) + 3 SCN^- (aq) + 3 H₂O (l) Fe(SCN)₃(H₂O)₃ (aq)


Reaction of iron with water
Iron(III) reacts with water [1]:

Fe³⁺ (aq) + 4 H₂O (l) FeO₄^2- (aq) + 8 H⁺ (aq) + 3e^-, E° = -2.20 V

Quantitative analysis

Method 3500-Fe C Inductively Coupled Plasma Method [2]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.01 mg/L
Method limit of detection in soil = 100 mg/kg

Safety

Symbol: -
R-phrases: -
S-phrases: -

Physical properties

Appearance
	Physical state @ 20°C:	Solid [4]
	Color:	Steel grey to black [5]
	Luster:	Metallic [5]
	Transparancy:	Opaque [5]
	Streak:	Gray [5]
	Odor:	Odourless [4]

Bulk properties
	Molecular weight (g/mol):	55.845(2) [1]
	Melting point (°C):	1535 [1]
	Boiling point (°C):	2750 [1][1]
	Ignition temp. (°C):	> 200 [4]
	Density (g/cm3):	7.874 [1]
	Molar volume (cm3):	7.10 (20 °C) [3]
	Velocity of sound (m/s):	4910
	Elastic properties: Young's modulus (GPa): Rigidity modulus (GPa): Bulk modulus (GPa): Poisson ratio:	211-215.5 [3] 81.6 [3] 170 [3] 0.28-0.33 [3]
	Hardness: Mineral hardness: Brinell hardness (MN/m2): Vicker hardness (MN/m2):	4 [5] 490 608

Optical properties
	Reflectivity (%):	65

Thermodynamic properties
	ΔH°atomization (KJ/mol):	415
	ΔfH°gas (KJ/mol):	398±17 (Monoatomic) [1]
	ΔH°fusion (KJ/mol):	13.8 [1]
	ΔH°vaporization (KJ/mol):	340±13 [1]
	Cp (solid) (J/K·mol):	24.776 (20 °C) [3] 26.412 (100 °C) [3] 28.984 (200 °C) [3] 33.892 (400 °C) [3] 38.800 (600 °C) [3] 43.943 (800 °C) [3]
	Coeff. of linear thermal expansion (106 K-1):	12.1 (20 °C) [3] 12.2 (100 °C) [3] 12.9 (200 °C) [3] 13.8 (400 °C) [3] 14.5 (600 °C) [3] 14.6 (800 °C) [3]

Crystal structure
	Crystal type:	Body centered cubic [3] Isometric; 4/m bar 3 2/m
	Lattice constant (Å):	2.87
	Cleavage:	Perfect [5]
	Fracture	Hackly [5]

Electronic properties
	Electron configuration:	1s2-2s2-2p6-3s2-3p6-3d6-4s2
	Atomic radius (Å):	1.72
	Ionic radius (Å):	Fe(VI): 0.25 (4-coordinate) [1] Fe(IV): 0.585 (6-coordinate) [1] Fe(III): 0.55 (6-coordinate, low-spin) [1] Fe(III): 0.645 (6-coordinate, high-spin) [1] Fe(II): 0.61 (6-coordinate, low-spin) [1] Fe(II): 0.78 (6-coordinate, high-spin) [1]

Conductivity
	e°(V):	Fe(II)/Fe: -0.447 [1] Fe(III)/Fe: -0.037 [1]
	Electrical resisitvity (μΩ·cm):	10.00 (20 °C) [3] 14.70 (100 °C) [3] 22.60 (200 °C) [3] 43.10 (400 °C) [3] 69.80 (600 °C) [3] 105.50 (800 °C) [3]
	Thermal (W/m·K):	73.3 (20 °C) [3] 68.2 (100 °C) [3] 61.5 (200 °C) [3] 48.6 (400 °C) [3] 38.9 (600 °C) [3] 29.7 (800 °C) [3]

References

1: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

2: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

3: Buch,A. Pure Metals Properties. A Scientific-Technical Handbook
1st edition (1999) Edited by Buch,A. pp. 1-306, ASM International and Freund Publishing House Ltd. Ohio. USA

4: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5

5: Korbel,P., Novák,M. The Complete Encyclopedia of Minerals
1st edition (1999) pp1-296, Chartwell Books, Inc. New Jersey, USA

---

© Michael Pilgaard
Created: June 15, 2006
Last update: October 31, 2008