SULPHUR AND ITS COMPOUNDS - Chemistry Notes Form 3

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Sulphur

  • It takes the form of a yellow solid naturally and can be found in this state near volcanoes.
  • Sulphur is also present in a number of metal ores, for example zinc blende (zinc sulphide, ZnS ).
  • Sulphur has chemical symbol S.
  • It has 16 protons and 16 neutrons.
  • An atom of S is represented as 3216S.
  • Sulphur is a non-metal and exists in the earth’s crust either as pure sulphur or as a metal-sulphide.
  • Since S has 16 protons, it also has 16 electrons; the electronic configuration of S is 2, 8, 6.
  • S is placed in Group VI A of the periodic table, just after phosphorus, and below oxygen.
  • The reaction of S is similar to oxygen.
  • Sulphur is found as a free element or in combined state in nature.
  • Free sulphur is found in at a large depth below the earth’s surface.
  • Metal sulphides such as Zn, Fe, Ag, Ca, Pb, Cu are found in abundant quantities. Mineral ores containing S are:
    • Copper pyrites : CuFeS2
    • Galena : PbS
    • Zinc blende : ZnS
    • Iron pyrite : FeS2
  • Sulphur is found as H2S gas in petroleum gas, coal gas.
  • H2S is the familiar pungent smell of onions. It is present in hair, eggs, many proteins and wool.

Extraction of Pure Sulphur : Frasch Process

  • Since sulphur in free state is found at depths of more than 150 to 300 meters below the earth’s surface, the method of extraction of sulphur differs from other metal or non-metal extractions.
    frasch process
  • Sulphur’s relatively low melting point (115oC) is utilized in this process.
  • This is known as the Frasch process.
  • Here compressed super heated water (at 170oC) is pressed into a pipe which reaches up to the sulphur deposits.
  • The sulphur here melts. Introducing hot compressed air through another pipe brings it up.
  • The molten sulphur and water mixture is forced up and is collected in a settling tank.
  • The sulphur is cooled and water is evaporated.
  • The sulphur extracted in this way is more than 99% pure.
  • The sulphur obtained by Frasch process is a yellow and brittle solid or powder.

Physical Properties of Sulphur

  • Since S has 6 electrons in its outermost shell, it needs 2 more electrons to complete its shell.
  • But S combines with 7 other atoms to make a sulphur molecule that has a total of 8 sulphur atoms.
  • Thus each S atom shares 2 electrons with its neighboring atom.
  • The bonds are covalent in nature.
  • A molecule of sulphur is represented as S8. It is a ringed molecule. The structure is shown below.
    structure of sulphur molecule
  • These large molecules each have many electrons , so the Van der Waals forces are quite strong and the melting point is quite high (119oC).
  • There are two ways of packing the sulphur rings, so solid sulphur exists in two crystalline forms, called rhombic and monoclinic.


Allotropes of sulphur
allotropes of sulphur

Physical properties of Sulphur

  • Sulphur is a yellow non-metallic crystalline solid at room temperature.
  • Brittle
  • It is tasteless and odourless.
  • The melting point of S is 115oC.
  • Sulphur is an insulator and is a poor conductor of heat and electricity.
  • S is insoluble in water but is soluble in CS2 and in organic solvents, for example methylbenzene.
  • Sulphur forms covalent bonds and shows allotropic forms.
  • The allotropes have different crystalline shapes such as rhombic and monoclinic. There is another allotrope which has no shape and is called plastic sulphur.
  • Vapours of sulphur are pungent and although not poisonous, they can cause health problems to humans.
  • Non-conductor of electricity whether solid, molten of dissolved.
  • Relatively low melting point and boiling point.

Chemical Properties of Sulphur 

  1. Valence : Since S has 6 electrons in its outer shell. Hence S does not give off its electrons easily. It readily forms covalent bonds to complete its outer shell. It shows variable valence of 2 or 6. S is quite a reactive element and forms oxides, chlorides and sulphides readily.
  2. Action of oxygen: Sulphur reacts with oxygen and burns with a blue flame. It forms sulphur dioxide which is a colourless gas having a pungent smell.
    reaction of sulphur with oxygen
    Sulphur dioxide forms an acidic solution, sulphurous acid, when dissolved in water i.e. it turns damp blue litmus paper red.
    It will also react with oxidising agents to produce sulphate ions e.g. Orange acidified dichromate (VI) ions are turned green and purple acidified manganese (VII) ions are turned colourless.
  3. S reacts with other non-metals also. In all cases sulphur has to be heated or boiled for the reaction to take place.
    reaction of sulphur with non metals
  4. Reaction with metals : Heated S reacts with metals like Fe, Cu, Zn, Sb directly to give metal-sulphide. A mixture of powdered zinc and sulphur, when heated up to a high temperature, will react together to produce an extremely exothermic change. A few reactions are shown below.reaction of sulphur with metals
  5. Reaction with acids : S is oxidized by strong concentrated oxidizing acids such as sulphuric acid and nitric acid.
    reaction of sulphur with acids
    In both the reactions S acts as a reducing agent.

Effect of Heat on Sulphur

  • A sulphur molecule consists of 8 atoms in a ring form.
  • When heated, S melts at 115oC and a pale yellow liquid is formed.
  • The S8 ringed molecules are connected to other molecules in a long chain.
  • On heating, the long chain breaks up.
  • The individual molecules can slip over each other when melted.
  • On further heating, the liquid becomes dark brown and viscous.
  • When the temperature rises beyond 1600C, the intra-molecular bonds break.
  • Sulphur boils at 444oC. At this temperature the large molecule breaks up into pieces of S2 molecule.
  • This molecule is pale yellowish-brown in colour.
  • The vapours of S contains S2 molecules.

Vulcanization of Rubber 

  • Natural rubber is a soft and sticky solid.
  • Rubber is a long chain polymer made out of isoprene (2 – methyl butadiene) monomer.
  • The long molecule forms a coil like structure.
    vulcanization of rubber
  • The unique property of rubber is that it is elastic.
  • When rubber is stretched, the molecular bonds can be extended out.
  • When released, the molecules coil back to their original shape.
  • Natural rubber looses its rubber-like properties at temperatures above 60oC.
  • Also its wear resistance and tensile strengths are low.
  • The process of vulcanization can improve the quality of rubber. Raw rubber is heated with sulphur during vulcanization.
  • This makes the rubber hard, more elastic and strong.
  • During the process of vulcanization, the sulphur atoms attach themselves to extra loose bonds in the rubber molecule and also cross-link the molecules.
  • The cross-linking locks the molecules in place and prevents slipping.
  • Thus making the vulcanized rubber more strong.
  • Vulcanized rubber is non-sticky and has higher elasticity.
  • It does not loose its properties easily and can be used in a temperature range of – 40oC to 100oC.

Uses of Sulphur

  • S is used to make H2SO4 acid, which is used in the manufacture of many compounds such as detergents, plastics, explosives, etc.
  • S is used for making CS2 molecule, gun powder, matches etc.
  • S is used for manufacture of fire works.
  • S is used in the rubber industry for vulcanization of rubber.
  • S is used for making germicides, fungicides.
  • S is used in many medicines.
  • S is used in photographic development (sodium thiosulphate or hypo).
  • S is used for making bleaching agents.
  • S is used in making artificial hair colours or dyes.


Sulphur Dioxide

  • Sulphur dioxide is a colourless gas, about 2.5 times as heavy as air, with a suffocating smell, faint sweetish odour.

Occurrence

  • Sulphur dioxide occurs in volcanic gases and thus traces of sulphur dioxide are present in the atmosphere.
  • Other sources of sulphur dioxide are the combustion of the iron pyrites which are contained in coal. Sulphur dioxide also results from various metallurgical and chemical processes.

Preparation of Sulphur Dioxide

  • Sulphur dioxide is prepared by burning sulphur in oxygen or air.
    S + O2 → SO2
  • Sulphur dioxide is usually made in the laboratory by heating concentrated sulphuric acid with copper turnings.
    Cu(s) + 2H2SO4(aq) → CuSO4(s) + SO2(g) + 2H2O(l)
    laboratory preparation of sulphur dioxide
  • Sulphur dioxide is released by the action of acids on sulphites or acid sulphites (e.g. by dropping concentrated sulphuric acid into a concentrated solution of sodium hydrogen sulphite).
    NaHSO3(aq) + H2SO4(aq)  → NaHSO4(aq) + SO2(g) + H2O(l)

Properties of Sulphur Dioxide

  • Sulphur dioxide is a colourless liquid or pungent gas, which is the product of the combustion of sulphur on air.
  • Its melting point is -72.7 oC, its boiling point is -10oC and its relative density is 1.43.
  • Sulphur Dioxide is an acidic oxide which reacts with water to give sulphurous acid.
    SO2(g) + H2O(l) → H2SO3(aq)
  • Sulphur dioxide is a good reducing and oxidising agent.

Chemical Properties of Sulphur Dioxide

  • Moist sulphur dioxide (or sulphurous acid) is a reducing agent.
  • This fact is used as a test for the detection of sulphur dioxide
    1. There is a colour change from purple (pink in dilute solution) to colourless on the addition of the gas to a solution of potassium manganate (VII) (permanganate)
      2MnO4(aq)- + 5SO2(aq) + 2H2O(l) → 2Mn2+(aq) + 5SO42-(aq) + 4H+(aq)
    2.  There is a colour change from orange to blue on adding the gas to a solution of potassium dichromate (VI).
      Cr2O72-(aq) + 3SO2(aq) +2H+(aq) → 2Cr3+(aq) + 3SO42-(aq) + H2O(l)

Sulphurous Acid and Sulphites

  • Sulphur dioxide dissolves in water forming sulphuric (IV) acid (sulphurous acid).
    SO2 (aq) + H2O(l) → H2SO3 (aq)
  • This is a weak dibasic acid and ionises producing hydrogen ions and sulphite SO32- ions.
    H2SO3(aq) → 2H+(aq) + SO32-(aq)
  • Sulphites give sulphur dioxide on heating with dilute acids.
    Na2SO3(aq0 + 2HCl(aq) → NaCl(aq) + SO2(g) + H2O(l)
  • With barium chloride they give a white precipitate of barium sulphite which is soluble in dilute hydrochloric acid.
    Ba2+(aq) + SO32-(aq)  → BaSO3(s)(White precipitate)
    BaSO3(s) + 2HCl(aq) → BaCl2(s) + SO2(g) + H2O(l)
    NOTE: This reaction is used as a test for sulphite ions in solution.

Summary 

Physical Properties
Colour   Colourless
 Odour  Pungent odour
 Density compared to air (heavier or lighter)  Heavier than air

 

Chemical Properties  
 Solubility in water  Soluble. It reacts with water to form a strong acid
 Burning  Does not support combustion
 Moist pH paper  Acidic reaction
 Red rose petals  Are bleached and lose their color
 Specific test  None

Uses of Sulphur Dioxide

  1. Sulphur dioxide is a reducing agent and is used for bleaching and as a fumigant and food preservative.
  2. Large quantities of sulphur dioxide are used in the contact process for the manufacture of sulphuric acid.
  3. Sulphur dioxide is used in bleaching wool or straw, and as a disinfectant.
  4. Liquid sulphur dioxide has been used in purifying petroleum products
  5. It is used as a bleaching agent.


Sulphuric Acid

The Contact Process

  • This is the Process used in the manufacture of sulphuric acid

A brief summary of the Contact Process

  • The Contact Process:
    • Makes sulphur dioxide;
    • Converts the sulphur dioxide into sulphur trioxide (the reversible reaction at the heart of the process);
    • Converts the sulphur trioxide into concentrated sulphuric acid.

Making the Sulphur Dioxide

  • This can either be made by burning sulphur in an excess of air: 
    S(s) + O2(g) → SO2(g)
  • or by heating sulphide ores like pyrite in an excess of air:
    4FeS2(s) + 11O2(g) → 4Fe2O3(s) + 8SO2(g)
  • In either case, an excess of air is used so that the sulphur dioxide produced is already mixed with oxygen for the next stage.

Converting the Sulphur Dioxide into Sulphur Trioxide

  • This is a reversible reaction, and the formation of the sulphur trioxide is exothermic.
  • A flow scheme for this part of the process looks like this:
    flow scheme of the contact process

Converting the Sulphur Trioxide into Sulphuric Acid

  • This can't be done by simply adding water to the sulphur trioxide - the reaction is so uncontrollable that it creates a fog of sulphuric acid.
  • Instead, the sulphur trioxide is first dissolved in concentrated sulphuric acid:
    H2SO4(l) + SO3(g)  → H2S2O7(l)
  • The product is known as fuming sulphuric acid or oleum.
  • This can then be reacted safely with water to produce concentrated sulphuric acid - twice as much as you originally used to make the fuming sulphuric acid.
    H2S2O7(l) + H2O(l) → 2H2SO4(l)

Summary

conversion of sulphur trioxide

Explaining the conditions

The Proportions of Sulphur Dioxide and Oxygen

  • The mixture of sulphur dioxide and oxygen going into the reactor is in equal proportions by volume.
  • Avogadro's Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
  • That means that the gases are going into the reactor in the ratio of 1 molecule of sulphur dioxide to 1 of oxygen.
  • That is an excess of oxygen relative to the proportions demanded by the equation.
    2SO2(g) + 02(g) ⇌ 2SO3(g) ΔH = - 196 kJmol-1
  • According to Le Chatelier's Principle, Increasing the concentration of oxygen in the mixture causes the position of equilibrium to shift towards the right.
  • Since the oxygen comes from the air, this is a very cheap way of increasing the conversion of sulphur dioxide into sulphur trioxide.
  • Why not use an even higher proportion of oxygen? This is easy to see if you take an extreme case. Suppose you have a million molecules of oxygen to every molecule of sulphur dioxide.
  • The equilibrium is going to be tipped very strongly towards sulphur trioxide - virtually every molecule of sulphur dioxide will be converted into sulphur trioxide. Great! But you aren't going to produce much sulphur trioxide every day.
  • The vast majority of what you are passing over the catalyst is oxygen which has nothing to react with.
  • By increasing the proportion of oxygen you can increase the percentage of the sulphur dioxide converted, but at the same time decrease the total amount of sulphur trioxide made each day.
  • The 1: 1 mixture turns out to give you the best possible overall yield of sulphur trioxide.

The Temperature

Equilibrium considerations

  • You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of sulphur trioxide in the equilibrium mixture.
  • The forward reaction (the production of sulphur trioxide) is exothermic.
    2SO2(g) + 02(g) ⇌ 2SO3(g) ΔH = - 196 kJmol-1
  • According to Le Chatelier's Principle, this will be favoured if you lower the temperature.
  • The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat.
  • In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as low a temperature as possible. However, 400 - 450°C isn't a low temperature!

Rate considerations

  • The lower the temperature you use, the slower the reaction becomes.
  • A manufacturer is trying to produce as much sulphur trioxide as possible per day.
  • It makes no sense to try to achieve an equilibrium mixture which contains a very high proportion of sulphur trioxide if it takes several years for the reaction to reach that equilibrium.
  • You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.

The compromise

  • 400 - 450°C is a compromise temperature producing a fairly high proportion of sulphur trioxide in the equilibrium mixture, but in a very short time.

The pressure

Equilibrium considerations

  • 2SO2(g) + 02(g) ⇌ 2SO3(g) ΔH = - 196 kJmol-1
  • Notice that there are 3 molecules on the left-hand side of the equation, but only 2 on the right.
  • According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules.
  • That will cause the pressure to fall again.
  • In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as high a pressure as possible. High pressures also increase the rate of the reaction.
  • However, the reaction is done at pressures close to atmospheric pressure!

Economic considerations

  • Even at these relatively low pressures, there is a 99.5% conversion of sulphur dioxide into sulphur trioxide
  • The very small improvement that you could achieve by increasing the pressure isn't worth the expense of producing those high pressures.

The catalyst

Equilibrium consideration
  • The catalyst has no effect whatsoever on the position of the equilibrium.
  • Adding a catalyst doesn't produce any greater percentage of sulphur trioxide in the equilibrium mixture.
  • Its only function is to speed up the reaction.
Rate considerations
  • In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time.
  • The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.

Properties of Sulphuric Acid

 Chemical Formula  H2SO4
 Molar Mass  98gmol-1
 Melting Point  10oC
 Boiling Point  340oC
 Density  1.83 gcm-3 
  • Sulphuric acid is a dense, oily liquid once known as oil of vitriol.
  • Pure sulphuric acid is almost twice as dense as water (1.98 g cm-2).
  • As water is added the density drops. Car batteries contain concentrated sulfuric acid. As the battery is discharged, the concentration of the acid falls.
  • By measuring the density of the acid the driver can check whether the battery is flat or not.

Action as an oxidising agent

  • It behaves as an oxidising agent only when hot and concentrated: 
    Cu(s) + 2H2SO4(aq) → CuSO4(s) + H2O(l) + SO2(g)
  • The sulphuric acid is reduced to sulphur dioxide.

Action as a Dehydrating Agent

  • Concentrated sulphuric acid has a great affinity for water. (It is important when diluting the concentrated acid to add the acid to water and NEVER water to acid.)
  • The reaction is highly exothermic.
  • So great is its affinity for water that it can dehydrate compounds containing hydrogen and oxygen:
    Sucrose + sulphuric acid → Carbon + (water + sulphuric acid)
    C12H22O11(aq) + nH2SO4(aq) → 12C(s) + 11H2O(l) + nH2SO4(aq))
    Ethanol + sulphuric acid → ethene + (water + sulphuric acid)
    CH3CH2OH(l) + nH2SO4(aq) → CH2 = CH2(g) + (H2O(l) + nH2SO4(aq)
  • It is used for drying gases, especially SO2 and HCl, but cannot be used to dry a reducing gas such as H2S or an alkaline gas such as NH3.


Other Properties

  • The properties of acids are due to the hydrogen ions in solution.
  • Concentrated sulphuric acid contains molecules, rather than ions.
  • Since it contains very few hydrogen ions it does not react significantly with metals and can safely be stored in steel containers.
  • A piece of magnesium ribbon does not dissolve in concentrated sulphuric acid.
  • Diluted with water, sulphuric acid behaves as a typical acid:
    • it reacts with metals to form sulphates plus hydrogen gas
    • it reacts with metal carbonates to form metal sulphates plus carbon dioxide plus water
    • It neutralises bases to form sulphates plus water.

Industrial Uses
indsutrial uses of sulphurioc acid


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