Formulas and reactions of bivalent and trivalent iron with alkalis

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).

KClO₃ and gum­my bear

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.