Possible oxidation states of oxygen in chemical reactions

Oxy­gen is an el­e­ment of the 6ᵗʰ group (un­der the new clas­si­fi­ca­tion the 16ᵗʰ group) of the main sub­group of the pe­ri­od­ic ta­ble. It is a rep­re­sen­ta­tive of the chalco­gens group (they also in­clude sul­fur, se­le­ni­um, tel­luri­um and polo­ni­um). Oxy­gen is a di­atom­ic col­or­less gas with­out smell or taste. It sup­ports breath­ing, com­bus­tion and de­com­po­si­tion. It is en­coun­tered in the form of 3 iso­topes – in na­ture, oxy­gen with the atom­ic num­bers of 16, 17 and 18 is en­coun­tered.

Oxy­gen is a strong ox­i­diz­er (only flu­o­rine dis­plays stronger ox­i­da­tion prop­er­ties be­cause of its greater elec­tri­cal neg­a­tive­ly and its more pro­nounced non-metal­lic prop­er­ties (by its po­si­tion in the pe­ri­od­ic ta­ble)). Oxy­gen is ca­pa­ble of dis­play­ing sev­er­al ox­i­da­tion states in chem­i­cal re­ac­tions: -2, -1, 0, +2.

Oxy­gen in the ox­i­da­tion state of -2

The low­est ox­i­da­tion state of oxy­gen is -2. As this non-met­al is a strong ox­i­diz­er, it fre­quent­ly dis­plays this ox­i­da­tion state in com­pounds. We may pro­vide many ex­am­ples of such com­pounds among salts, acids, ox­ides and bases: KClO₄, H₂­SO₄, N₂O₃, NaOH etc. In wa­ter and in a hy­dro­ni­um ion, the ox­i­da­tion state of oxy­gen is also two.

The va­lence of oxy­gen in these two com­pounds is dif­fer­ent, how­ev­er. In wa­ter oxy­gen shows typ­i­cal va­lence of 2, and in the hy­dro­ni­um ion, from the for­ma­tion of the third, donor-ac­cep­tor bond, va­lence (abil­i­ty to form a cer­tain num­ber of bonds) grows to three. The donor-ac­cep­tor bond forms be­cause the un­shared pair of elec­trons in the oxy­gen atom are lo­cat­ed on the free or­bital of the hy­dro­gen cation Н⁺.

Many re­ac­tions take place with­out a change in ox­i­da­tion states:

H₂­SO₄ + 2NaOH = Na₂­SO₄ + 2H₂O;

CaO + H₂O = Ca(OH)₂;

But usu­al­ly, even in ox­i­da­tion-re­duc­tion re­ac­tions, oxy­gen does not ox­i­dize to high­er ox­i­da­tion states, and pre­serves the val­ue of -2:

10KI + 2KM­nO₄ + 8H₂­SO₄ = 5I₂ + 2Mn­SO₄ + 6K₂­SO₄ + 8H₂O.

Ox­i­da­tion of oxy­gen takes place in the break­down of sub­stances (for ex­am­ple, wa­ter or ox­i­diz­ers), or when wa­ter re­acts with flu­o­rine:

• 2H₂O = O₂ + 2H₂ (car­ried out in the pres­ence of an al­ka­li);

• 2KClO₃ = 3O₂ + 2KCl (with heat­ing);

• 2KM­nO₄ = O₂ + MnO₂ + K₂M­nO₄ (with heat­ing);

• 2KNO₃ = O₂ + 2KNO₂ (with heat­ing);

• 2H₂O₂ = O₂ + 2H₂O (in the pres­ence of man­ganese (IV) ox­ide MnO₂ or with heat­ing);

• 2F₂ + H₂O = 4HF + O₂.

Oxy­gen in the ox­i­da­tion state of -1

In per­ox­ides, the ox­i­da­tion state of oxy­gen is -1. The for­ma­tion of per­ox­ides is char­ac­ter­is­tic for hy­dro­gen (H₂O₂) and cer­tain met­als (Na₂O₂, BaO₂, CaO₂ etc.).

In the case with per­ox­ides and su­per­ox­ides (such as KO₂, where the ox­i­da­tion state of oxy­gen is -0.5), both an in­crease and a de­crease of the ox­i­da­tion state of oxy­gen in re­ac­tions are pos­si­ble:

• 2H₂O₂ = O₂ + 2H₂O (in the pres­ence of man­ganese (IV) ox­ide MnO₂ or when heat­ed);

• 2Na₂O₂ + 2CO₂ = O₂ + 2Na₂­CO₃;

• Ag₂O + Н₂О₂ = 2Ag + H₂O + O₂.

• 4KO₂ + 2H₂­SO₄ = 2H₂O + 3O₂ + 2K₂­SO₄;

• 4KO₂ + 2H₂O = 4KOH + 3O₂;

• 2КM­nO₄ + 5Н₂О₂ + 3H₂­SO₄ = K₂­SO₄ + ₂Mn­SO₄ + 5O₂ + 8H₂O;

Here there are oth­er ex­per­i­ments with potas­si­um per­man­ganate.

• Na₂O₂ + CO = Na₂­CO₃;

• Na₂O₂ + SO₂ = Na₂­SO₄ (in the pres­ence of sul­fu­ric acid or hy­dro­gen per­ox­ide the re­ac­tion takes place more quick­ly);

• 4Na₂O₂ + NH₃ = NaNO₃ + 3NaOH + 2Na₂O.

The ox­i­da­tion state of oxy­gen does not change in the im­pact on per­ox­ides of di­lut­ed acids:

Na₂O₂ + H₂­SO₄ = H₂O₂ + Na₂­SO₄.

As hy­dro­gen per­ox­ide has weak­ly pro­nounced acidic prop­er­ties, it can re­act with al­ka­lis with­out a change in the ox­i­da­tion state of oxy­gen:

Ва(ОН)₂ + Н₂О₂ = ВаО₂ + 2Н₂О.

Oxy­gen in the ox­i­da­tion state of 0

In a free state, oxy­gen has an ox­i­da­tion state of 0, like oth­er sim­ple sub­stances. As oxy­gen is a strong ox­i­diz­er, it re­acts with many met­als and non-met­als, and also com­pounds, dis­play­ing ox­i­diz­ing prop­er­ties (the ox­i­da­tion state of oxy­gen drops to -2, but if per­ox­ide forms, to -1). De­pend­ing on the con­di­tions, the same sub­stances may re­act with oxy­gen dif­fer­ent­ly:

• S + O₂ = SO₂ (with heat­ing);

Melt­ing test tube - re­ac­tion with sul­fur

• 4Li + O₂ = Li₂O (with heat­ing);

• 2Na + O₂ = Na₂O₂ (prod­uct of com­bus­tion of sodi­um in air – sodi­um per­ox­ide);

• Na₂O₂ + O₂ = 2NaO₂ (on re­act­ing with oxy­gen per­ox­ides, it ox­i­dizes them to su­per­ox­ides with an ox­i­da­tion state of oxy­gen of -1/₂);

• 2BaO + O₂ = 2BaO₂ (bar­i­um ox­ide ab­sorbs oxy­gen, form­ing per­ox­ide);

• 4NH₃ + 3O₂ = 2N₂ + 6H₂O (com­bus­tion);

• NH₃ + 5O₂ = 4NO + 6H₂O (cat­alyt­ic ox­i­da­tion with heat­ing in the pres­ence of a plate);

• 2SO₂ + O₂ = 2SO₃ (re­ac­tion takes place with heat­ing and ad­di­tion of cat­a­lyst);

• H₂S + 3O₂ = 2SO₂ + 2H₂O (sul­fur (IV) ox­ide is formed in an abun­dance of oxy­gen);

• 2H₂S + O₂ = 2S + 2H₂O (with lack of oxy­gen);

• 4Fe(OH)₂ + O₂ + 2H₂O = 4Fe(OH)₃;

• 4FeS₂ + 11O₂ = 8SO₂ + 2Fe₂O₃ (takes place with heat­ing);

• СН₄ + 2О₂ = СО₂ + 2Н₂О (com­plete ox­i­da­tion of met­al - com­bus­tion);

• C₂H₅OH + 3O₂ = 2CO₂ + 3H₂O (com­plete ox­i­da­tion of al­co­hol);

• C₂H₅OH + O₂ = CH₃­COOH + H₂O (mild ox­i­da­tion of al­co­hol by oxy­gen to acetic acid).

Oxy­gen in the ox­i­da­tion state of +2

As a sin­gle atom, oxy­gen has a pos­i­tive ox­i­da­tion state of +2 – in a com­pound with flu­o­rine, OF₂. As flu­o­rine is a more elec­tri­cal­ly neg­a­tive el­e­ment, it and not oxy­gen ac­quires the neg­a­tive ox­i­da­tion state of -1 in the com­pound.

Oxy­gen flu­o­ride forms by the re­ac­tion:

2F₂ + 2NaOH = OF₂ + 2NaF + H₂O (in the re­ac­tion, ozone and hy­dro­gen per­ox­ide H₂O₂ can also form).

The com­pound with the pos­i­tive ox­i­da­tion state of oxy­gen +1 ex­ists, O₂F₂ (oxy­gen monoflu­o­ride). Oxy­gen monoflu­o­ride is an un­sta­ble com­pound, and it can be ob­tained in the re­ac­tion of molec­u­lar gas­es – oxy­gen and flu­o­rine.

Oxy­gen and re­ac­tions with it have found wide ap­pli­ca­tion in lab­o­ra­to­ry prac­tice (for ob­tain­ing ox­ides and oth­er sub­stances) and in in­dus­try (for ex­am­ple in smelt­ing cast-iron and steel). It is also used for cut­ting met­als (with acety­lene) and in medicine.