Doc Brown's Chemistry  - Advanced Level Inorganic Chemistry Periodic Table Revision Notes

Part 8. The p-block elements: 8.4 Group 6/16 oxygen and sulfur in particular

(1) Group 6/16 Position in the periodic table - introduction, data, trends and electron configurations

• Generally speaking down a p block group the element becomes more metallic in chemical character.

• Oxygen and sulfur are non-metals, selenium and tellurium are semi-metals, polonium is essentially a metal

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(2) OXYGEN - summary of a few points about its chemistry

• The structure of the element:

  • Non-metal existing as diatomic molecule, O₂, with a double covalent bond.

  • It has two allotropes: 'normal oxygen' O₂ (dioxygen above) and the highly unstable and reactive gas ozone, O₃ (trioxygen).

• Physical properties of the element: 

  • O₂ is a colourless gas; mpt -218^oC; bpt -183^oC;  poor conductor of heat/electricity.

    • O₃ is a pale blue gas.

• Group, electron configuration (and oxidation states): 

  • Gp6; e.c. 2,6  or 1s²2s²2p⁴;  Normally (-2) e.g. H₂O, CO₂ etc. but can have other oxidation states ...

  • e.g. H₂O₂ (-1), F₂O (+2).

• Reaction of element with oxygen: 

  • O₂ molecules won't react with themselves BUT in the upper atmosphere oxygen atoms are formed by high energy radiation/particle collision with oxygen molecules causing homolytic bond fission to produce free oxygen atom (free radicals). These combine with oxygen molecules to form ozone. Ozone can be synthesised by an electric discharge through oxygen.

    • (i) O₂ = hv => 2O^. (ii) O^. + O₂ ==> O₃ 

• Reaction of oxide with water, acids or bases/alkalis: Not applicable.

• Reaction of element with chlorine: 

  • None, but unstable chlorine(I) oxide (chlorine monoxide) can be made indirectly and there are other chlorine oxides. (see chlorine)

• Reaction of chloride with water:

  • Slowly hydrolyses to form weak chloric(I) acid.

    • Cl₂O_(g) + H₂O_(l) ==> 2HClO_(aq)

• Reaction of element with water:

  • Slightly soluble but no reaction.

• Other comments:

  • Formed in plant photosynthesis. Consumed in respiration.

• Links to other pages on site: 

  • Preparation of oxygen gas : Test for gas :  

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(3) SULFUR - summary of a few points about its chemistry

• The structure of the element:

  • Three solid allotropes. Two are crystalline lattices based on S₈ molecules (rhombic and monoclinic sulfur). A 3rd form is an unstable dark brown-black polymeric form called plastic sulfur, formed when boiling sulfur is poured onto cold water, great fun, but of little use!

• Physical properties: 

  • Colourless gas; mpt 117^oC; bpt 445^oC; poor conductor of heat/electricity.

• Group, electron configuration (and oxidation states): 

  • Gp6; e.c. 2,6  or 1s²2s²2p⁶3s²3p⁴; ranges from (-2 to +6) e.g.

  • Na₂S (-2), S₂Cl₂ (+1), SO₂ (+4) and H₂SO₄, SF₆, SO₃ (all +6).

• Reaction of element with oxygen: 

  • Burns in air with a pale blue flame to form sulfur dioxide (sulfur(IV) oxide), with a little sulfur trioxide.

    • S_(s) + O_2(g) ==> SO_2(g) 

  • Sulfur trioxide (sulfur(VI) oxide) has to be made by the industrial Contact Process.

    • 2SO_2(g) + O_2(g) == V₂O₅ catalyst, 450^oC ==> 2SO_3(g) 

• Reaction of the oxides with water: Both dissolve to form acid solutions.

  • Sulfur dioxide forms the weak 'fictitious' sulfurous acid.

    • SO_2(g) + H₂O_(l) ==> H₂SO_3(aq) 

      • the reaction is better represented ionically as ..

      • SO_2(aq) + H₂O_(l) H⁺_(aq) + HSO₃^-_(aq) 

  • Sulfur trioxide reacts very violently and exothermically to form the oily liquid, strong sulfuric acid.

    • SO_3(g) + H₂O_(l) ==> H₂SO_4(l) 

    • In water, the sulfuric acid is almost fully ionised.

      • H₂SO_4(aq) + 2H₂O_(l) ==> 2H₃O⁺_(aq) + SO₄^2-_(aq) 

• Reaction of oxide with acids:

  • None, only acidic in nature.

• Reaction of oxide with bases/alkalis:

  • Sulfur dioxide dissolves in strong bases to form sulfites

  • 2NaOH_(aq) + SO_2(g) ==> Na₂SO_3(aq) + H₂O_(l) formation of sodium sulfite/sulfate(IV)

  • ionic equation: 2OH^-_(aq) + SO_2(g) ==> SO₃^2-_(aq) + H₂O_(l)

  • You would NOT attempt to react sulfur trioxide with water, the reaction is very violent and exothermic.

  • but theoretically: 2NaOH_(aq) + SO_3(g) ==> Na₂SO_4(aq) + H₂O_(l)

• Reaction of element with chlorine: 

  • When chlorine is passed over molten sulfur a variety of chlorides are formed.

  • The main product is disulfur dichloride

  • 2S_(s) + Cl_2(g) ==> S₂Cl_2(l) (also SiCl₂, SiCl₄)

• Reaction of chloride with water:

  • Slowly hydrolyses in water, via a complex reaction, to form an acid solution of several products (not meant to be a balanced equation).

    • S₂Cl_2(g) + H₂O_(l) ==> HCl_(aq), S_(s), SO_2(aq), H₂SO_3(aq),  H₂SO_4(aq), H₂S_(aq)

      • a rather complex redox/hydrolysis reaction! 

• Reaction of element with water:

  • None.

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 (4) Shapes and bond angles of molecules and ions of oxygen and sulfur

oxygen molecule O₂

electrons: two bond pairs and two lone pairs, ANGULAR or BENT shape: e.g. hydrogen sulfide, H₂S, or water H₂O, i.e. H₂X with H-X-H bond angle of approximately 109^o and similarly ions like NH₂^-. Note: the exact H-O-H angle is 104.5^o due to the extra repulsion of two lone pairs. (Q = H, X = O, S etc. in group 6)

electrons: 6 bond pairs, OCTAHEDRAL SHAPE: e.g. sulfur(VI) fluoride (sulfur hexafluoride molecule) SF₆ with Q-X-Q bond angles of 90^o and 180^o (Q = F, X = S)

Sulfur(IV) oxide/sulfur(VI) oxide, SO2 (sulfur dioxide/sulfur dioxide) molecule is a bent shape (angular), O-S-O bond angle ~120o due to two groups of bonding electrons and one non-bonding lone pair of electrons. The sulfate(IV) ion/sulfate(IV) ion, SO32- (sulfite ion/sulfite ion) is a trigonal pyramid shape, O-S-O bond angle ~109o due to three groups of bonding electrons and one lone pair of electrons. Sulfur(VI) oxide/sulfur(VI) oxide, SO3 (sulfur trioxide/sulfur trioxide), is a trigonal planar shape, O-S-O bond angle of 120o due to three bonding groups of electrons and no lone pairs of electrons. Sulfate(VI) ion/sulfate(VI) ion, SO42- (sulfate ion/sulfate ion) is tetrahedral in shape, O-S-O angle of 109.5o. due to four groups of bonding electrons and no lone pairs of electrons. The shapes are deduced below using dot and cross diagrams and VSEPR theory and illustrated valence bond dot and cross diagrams below.

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(5) Why is sulfuric acid a useful material? How is it made? Contact Process

Because sulfuric acid has so many uses the industrial development of a country is sometimes measured by the amount of sulfuric acid that is used each year. Sulfuric acid is made starting from the element sulfur which is found in the Earth's crust.

• Sulfuric acid is used as car battery acid and is used to make fertilisers,  dyes and detergents.

  • e.g. ammonia + sulfuric acid ==> ammonium sulfate (a fertiliser salt)

  • 2NH_3(aq) + H₂SO_4(aq) ==> (NH₄)₂SO_4(aq) => evaporation to get crystals

  • Its acid action make it good for cleaning metal surfaces in industry.

• Sulfuric acid is manufactured from the raw materials sulfur, air and water and involves the production of sulfur trioxide in the Contact Process.

• (1) Sulfur is burned in air to form sulfur dioxide (exothermic).

  • In the reaction the sulfur is oxidised (O gain)  (1a) S_(s) + O_2(g) ==> SO_2(g)

  • Sulfur dioxide can also be indirectly obtained from the process of extracting copper from copper sulfide ores e.g. in a copper smelter: (1b) Cu₂S_(s) + O_2(g) ==> 2Cu_(l) + SO_2(g)

• Note: Sulfur dioxide itself is a useful chemical in its own right:

  • It is used as a bleach in the manufacture of wood pulp for paper manufacture

  • and its toxic nature makes it useful as a food preservative by killing bacteria.

• (2) The Contact Process of sulfur trioxide production must be economically efficient for the manufacture of the important industrial chemical sulfuric acid.

  • In the Contact Process reactor the sulfur dioxide is mixed with air (the required stoichiometric volume/mole SO₂:O₂ ratio is 2:1, in practice 1-2:1 is used) and the mixture passed over a catalyst of vanadium(V) oxide V₂0₅ at a relatively high temperature of about 450°C and at a pressure of between 1-2 atm.

• In the reactor the sulfur dioxide is oxidised in the reversible exothermic reaction ...

  •   (2) 2SO_2(g) + O_2(g) 2SO_3(g)

  • Kp =	pSO32
    	-------------------
    	pSO22 pO2

• The reaction forms sulfur trioxide and the equilibrium is very much to the right hand side ...

  • The reaction forms sulfur trioxide and the equilibrium is very much to the right hand side because despite the reaction being exothermic a relatively high temperature is used which favours the reverse reaction R to L, from the energy change equilibrium rule, i.e. increasing temperature shifts the equilibrium in the endothermic direction. However the value of K_p is high enough to give a 99% yield.

  • The reaction is favoured by high pressure (pressure equilibrium rule, 3 => 2 gas molecules, LHS ==> RHS), but only a small increase in pressure is used to give high yields of sulfur trioxide, because the formation of SO₃ on the right hand side is so energetically favourable (approx. 99% yield, i.e. only about 1% SO₂ unreacted).

  • The use of the V₂O₅ catalyst ensures a fast reaction without having to use too a higher temperature which would begin to favour the left hand side too much (energy change equilibrium rule), but remember a catalyst does not affect the % yield or equilibrium concentration of SO₃, you just get there more economically faster.

  • Multiple reactor beds are used to ensure the maximum % conversion and heat exchange systems are used to control the temperature, and pre-heat incoming reactant gases.

• (3) The sulfur trioxide is dissolved in concentrated sulfuric acid to form fuming sulfuric acid (oleum).

  • SO_3(g) + H₂SO_4(l) ==> H₂S₂O_7(l)

• (4) Water is then carefully added to the oleum to produce concentrated sulfuric acid (98%).

  • H₂S₂O_7(l) + H₂O_(l) ==>  2H₂SO_4(l)

  • If the sulfur trioxide is added directly to water an acid mist forms which is difficult to contain because the reaction to form sulfuric acid solution is very exothermic!

  • If you 'add' equations (3) + (4) you get

    • (5) SO_3(g) + H₂O_(l) ==>  H₂SO_4(l)

• Good anti-pollution measures need to be in place since the sulfur oxides are harmful and would cause local acid rain! To help this situation AND help the economics of the process, any unreacted sulfur dioxide is recycled through the reactor.

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(6) Some chemical reactions of concentrated sulfuric acid

• Concentrated sulfuric acid can be used in the laboratory as a dehydrating agent.

  • Dehydration is the removal of water or the elements of water from a compound and can be described as an elimination reaction. Usually and adjacent H and OH in a molecule are removed to form the water.

  • When added to some organic compounds containing hydrogen and oxygen, e.g. sugar, concentrated sulfuric acid removes the elements of water from the compound leaving a 'spongy' black carbon residue.

  • If alcohols are heated with conc. sulfuric acid, they are dehydrated to alkenes.

    • e.g. ethanol ==> ethene + water

    • CH₃CH₂OH ==> CH₂=CH₂ + H₂O

  • When added to blue copper sulfate crystals concentrated sulfuric acid removes the water of crystallisation leaving white anhydrous copper sulfate. In this case the water already exists BUT not in a mixture and so the following reaction is classified as a chemical change.

    • CuSO₄.5H₂O_(s) CuSO_4(s) + 5H₂O_{(absorbed into the H2SO4 which it reacts with)}

  • Conc. H₂SO₄ catalyses the reaction between an alcohol and carboxylic acid to form an pleasant smelling ester liquid but it isn't considered a dehydration reaction (H comes from one molecule and OH from the other).

    • e.g. the esterification ethanoic acid + ethanol ==> ethyl ethanoate + water

    • CH₃COOH + CH₃CH₂OH ==> CH₃COOCH₂CH₃ + H₂O

  • Concentrated sulfuric acid can be used as a drying agent e.g. in the preparation of gases.

    • The prepared gas is bubbled through a dreschel/dreschler bottle (illustrated on the right), containing the concentrated sulfuric acid. In this case the water vapour is just a component in a gaseous mixture. Most gases can be dried in this way except the alkaline gas ammonia which will exothermically react to form a solid salt. In this case the water vapour is just a component in a gaseous mixture.

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(7) Analysis of some REDOX reactions involving sulfur based molecules or ions

• The oxidation of sulfur dioxide with bromine

  • SO_2(aq) + Br_2(aq) + 2H₂O_(l) ===> SO₄^2-_(aq) + 2Br^-_(aq) + 4H⁺_(aq)

  • (i) the oxidation half reaction is: SO_2(aq) +  2H₂O_(l) ===> SO₄^2-_(aq) + 4H⁺_(aq) + 2e^-

    • The sulfur changes from ox. state +4 to +6 (SO₂ to SO₄^2-).

  • (ii) the reduction half-reaction is: Br_2(aq) + 2e^- ===> 2Br^-_(aq)

    • Two bromine atoms (as molecule) change from ox. state 0 to -1.

  • The hydrogen (+1) and oxygen (-2) do not change oxidation state.

    • (i) + (ii) equals the balanced equation, 2 electrons gained and lost or an ox. state rise and fall of 2 units.

    • Bromine is the oxidising agent (gain/accept e^-s, lowered ox. state),

    • and sulfur dioxide is the reducing agent (loses e^-s, inc. ox. state of S).

  • Sulfur dioxide does ionise to a small extent in water to give the sulfite ion, and adding a strong non-oxidising acid like dilute hydrochloric acid to sodium metabisulfite produces the ion, which means another equation can also adequately describe the redox change in terms of sulfur and bromine.

    • e.g. if the sulfite ion acts as the reducing agent the reaction with chlorine would be written as:

    • SO₃^2-_(aq) + Cl_2(aq) + H₂O_(l) ===> SO₄^2-_(aq) + 2Cl^-_(aq) + 2H⁺_(aq)

• The oxidation of hydrogen sulfide by iron(III) ions

  • If an iron(III) salt (old name, ferric salt) is added to hydrogen sulfide solution a precipitate of sulfur forms and the orange-brown solution turns pale green.

  • H₂S_(aq) + 2Fe³⁺_(aq) ===> 2Fe²⁺_(aq) + 2H⁺_(aq) + S_(s)

  • Oxidation: 1 S at (-2) change to 1 S at (0), H₂S ==> S, a loss of 2 electrons, inc. 2 ox. state units.

  • Reduction: 2 Fe at (+3) change to 2 Fe at (+2), gain in total of 2 electrons, decrease in 2 ox. state units.

  • No change in the oxidation state of the 2H's (+1) involved.

  • The iron(III) ion acts as the oxidising agent (gains/accepts e^-s, lowered ox. state of Fe) and the hydrogen sulfide is the reducing agent (loses/donates e^-s, inc. ox. state of S).

• The decomposition of hydrogen peroxide

  • Hydrogen peroxide decomposition, catalysed by the black solid manganese(IV) oxide, MnO₂.

    • 2H₂O_2(aq) ===> O_2(g) + 2H₂O_(l)

    • Ox. state changes: 4O at (-1) change to 2O at (0) in O₂ and 2O at (-2) in H₂O

    • and H is unchanged at (+1).

  • A case of disproportionation where an element in a species simultaneously changes into a higher and lower oxidation state i.e. here two oxygen atoms increase their oxidation state and two oxygen atoms decrease their oxidation state.

  • It also means that hydrogen peroxide simultaneously acts as a reducing agent and oxidising agent.

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