Sunday 9 December 2012

Sulfur

Properties of Sulfur

  • Yellow solid at room temperature
  • Second only to carbon, sulfur is found in large number of allotropes (various forms). For examples: disulfur (S2)trisulfur (S3), tetrasulfur (S4), cyclo-pentagon (cyclo-S5), and so-on. 
Here are the common sulfur allotrope:

Rhombic sulfur
Monoclinic sulfur



Powder

  • Sulfur reacts with metal to form metal sulfide (sulphide)
             mg(s)          +     S(s)    --->      MgS(s)
             magnesium  +    sulfur   --->     magnesium sulfide


             Zn(s)           +     S(s)    --->      ZnS(s)
             zinc             +     sulfur  --->     zinc sulfide
  • Sulfur reacts with oxygen to produce sulfur dioxide 
             S(s)           +     O(g)     --->    SO(g)

             sulfur         +     oxygen    --->   sulfur dioxide



  • Sulfur is a non-metallic element, does not conduct electricity and insoluble in water
  • Sulfur atomic number = 16
Sulfur atom



Sources of Sulfur

Sulfur may be obtained from the following sources:

  1. Copper pyrite (CuFeS2)
  2. Zinc blend (ZnS) is heated in air (to produce zinc oxide and sulfur dioxide)
  3. Fossil fuel/ natural gas and oil/petroleum
  4. Sulfur ore (sulfide/sulphide) from volcanic activity
  5. Elemental sulfur from sulfur beds in Poland, Russia and US (Louisiana)  
Recovery sulfur from refinery of petroleum/natural gas  using Claus process
Mining sulfur ore from active volcano area
Sulfur minor carrying sulfur ore


Frasch process, named after Hermann Frasch. 

Highly heated  water and compressed air is forced into the sulfur beds. 

Sulfur becomes molten and is forced out together with water. 

Molten sulfur can be easily separated from water. 

99.5% purity may be obtained.


Uses of Sulfur

Sulfur is very important in the chemical industry as the following:
  1. to produce sulfuric acid
  • millions of tonnes of sulfuric acid are produced worldwide every year. 
  • sulfuric acid is used to make chemicals, paints & pigments, detergents, soaps, synthetic fibres, dyes, metal salts, fertilisers, and so-on.
  1. to vulcanise rubber, a process which makes the rubber harder and increases its elasticity.
  2. in manufacture of matches, fireworks and fungicides
  3. as a sterilising agent in medicines
  4. to produce bleaching agent (sulfur dioxide gas) for paper manufacture 
  5. to produce food preservative (sulfur dioxide gas) by killing bacteria ( also called fumigation; to exterminate pests or disinfect)
For knowledge only:
The standard method to control postharvest decay of grapes is to fumigate with sulphur dioxide gas the fruit immediately after harvest and during storage.  However, the concentration of sulphur dioxide necessary to inhibit fungal infections may induce injuries in both rachis and berries, in addition sulphur dioxide application has been restricted in many Countries for the human health risks due to the sulphite residues. Several studies have shown the efficacy of heat treatments on harvested fruits to control biological agents, delay ripening and preserve quality of fresh-cut table grapes.
(source:http://www.freshplaza.com/article/97631/How-to-preserve-the-quality-of-ready-to-eat-grapes#SlideFrame_1)

Saturday 15 September 2012

Salts - preparing insoluble salts by precipitation

Tips: Before preparing a salt, find out if the salt is soluble or insoluble. A soluble salt is made from an acid. An insoluble salt is prepared by precipitation.


Table 1 Soluble Salts and Insoluble Salts

Soluble Salts
Insoluble Salts
All (group I) sodium, potassium, and ammonium


All nitrates 


Most chlorides 

Silver chloride and lead chloride
Most sulfates (sulphates)

Calcium sulfate, barium sulfate and lead sulfate
Most bromide and iodides

Silver bromide, silver iodide, lead bromide and lead iodide
Sodium, potassium(group I), and ammonium carbonates
All other carbonates

Preparing Insoluble Salts by Precipitation

Example: 
preparing insoluble barium sulfate salt.


  • Barium sulfate salt is made of Ba2+ ions and SO42- ions.
  • So, we have to prepare two soluble salts solutions of which one contains Ba2+, its positive metal ions and the other contains the SO42-, its negative non-metal ions. 
  • These two ions will react and form a precipitation as barium sulfate salt. 
          Ba2+ (aq) + SO42-  (aq) ---> BaSO(s)

The precipitate of barium sulfate salt(residue) is gathered using a filter funnel, rinsed with distilled water to remove any impurities and dried. The filtrate contains the spectator ions which remained unchanged in the reaction.


Another way to separate the precipitate from the solution is using a centrifuge. This technique is used when the solid particles are very small and dispersed in the solution. The fast spinning of the tube in a centrifuge (see picture below) causing the solid gathers at the bottom of the tube due to higher density and gravity force. The liquid can be then decanted leaving the solid in the tube.

A centrifuge

More examples of preparing insoluble salts:

Table 2. Preparing Insoluble Salts 
Preparing insoluble salts
‘+’ ions
‘- ' ions
Solution used
Solution used
Salt precipitation
Spectator ions (no changes in reaction)
Barium sulfate BaSO4
Ba2+
SO42-
Barium nitrate with Ba2+

Sodium sulfate with SO42-
Barium  sulfate BaSO4

NaNO3
sodium nitrate
soluble and remained unchanged as Na+ ions and NO3ions  
                                     Reaction: Ba(NO3)2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaNO3 (aq)

Barium sulfate BaSO4
Ba2+
SO42-
Barium chloride  with Ba2+

Sodium sulfate with SO42-
Barium  sulfate BaSO4

NaCl 
sodium chloride
soluble and remained unchanged as Na+ ions and Clions  
                                        Reaction: BaCl2 (aq) + Na2SO4(aq) ---> BaSO4 (s) + 2 NaCl2 (aq)

Barium sulfate BaSO4
Ba2+
SO42-
Barium chloride  with Ba2+

Magnesium sulfate with SO42-
Barium  sulfate BaSO4

MgCl2 
magnesium chloride
soluble and remained unchanged as Na+ ions and Clions 
                                        Reaction: BaCl2 (aq) + MgSO4(aq) ---> BaSO4 (s) + 2 MgCl2 (aq)

Silver chloride
AgCl
Ag+
Cl-
Silver nitrate with 
Ag+
Sodium chloride with Cl-
Silver chloride AgCl
NaNO3 
sodium nitrate
soluble and remained unchanged as Na+ ions and NO3ions  
                                        Reaction: AgNO3 (aq) + NaCl(aq) ---> AgCl (s) + NaNO3 (aq)

Lead iodide
PbI2
Pb2+
I-
Lead nitrate with 
Pb2+
Sodium iodide with
 I-
Lead iodide
PbI2

NaNO3 sodium nitrate
soluble and remained unchanged as Na+ ions and NO3ions  
                                        Reaction: Pb(NO3)2 (aq) + NaI (aq) ---> PbI2 (s) + NaNO3 (aq)





Pictures of insoluble salts:
White precipitate
silver chloride, AgCl 
Creamy precipitate
silver bromide, AgBr
Yellow precipitate
silver Iodide, AgI
White precipitate
barium sulfate, BaSO4
Yellow precipitate
lead iodide, PbI2






















Saturday 8 September 2012

Salts - normal salts, acid salts

Salts are very important to us.

Table 1 Example of salts
Salt
Use
Ammonium chloride
Torch batteries
Ammonium nitrate
Fertilisers
Calcium carbonate
Extraction of iron, making cement, making glass
Calcium chloride
Extraction of sodium, drying agent
Calcium sulphate
Making plaster boards and casts for injured limbs
Iron(II) sulphate
Iron tablets
Magnesium sulphate
Medicines
Potassium nitrate
Fertiliser, gunpowder
Silver bromide
Photography
Sodium carbonate
Making glass, washing powders, water softener
Sodium chloride
Making hydrochloric acid, food flavouring, hospital saline, making sodium carbonate
Sodium stearate
Soaps
Tin(II) flouride
Additive to toothpaste


battery 
Ammonium chloride in a battery

Ammonium nitrate fertilisers
Calcium carbonate
Iron sulphate for making Iron supplement,
also named as ferrous tablet
Magnesium sulphate


File:Bromid stříbrný.PNG
Silver bromide




Sodium carbonate

Sodium chloride


Sodium stearate

Normal Salts

A Normal salt is formed when all the hydrogen ions(H+) of an acid, have been replaced by metal ions or by the ammonium ions(NH4+), All the salts listed in table 1 are normal salts. 

Normal salts may be soluble or insoluble in cold water.
  • all nitrates salts are soluble
  • all common sodium, potassium and ammonium salts are soluble
  • all chlorides salts all soluble except lead, silver and mercury 
  • all sulfates salts are soluble except lead, barium and calcium
Table 2 Normal Salts
Acid
Type of salt
Example
Carbonic acid H2CO3
Carbonates
Sodium carbonate Na2CO3
Sulfuric acid H2SO4
sulphates
Sodium sulfate Na2SO4
Ethanoic acid CH3COOH
Ethanoates
Sodium ethanoate CH3COONa
Hydrochloric acid HCl
Chlorides
Potassium chloride KCl
Nitric acid HNO3
Nitrates
Potassium nitrate KNO3


Acid Salts

An acid salt is formed where not all of the hydrogen ions of an acid have been replaced by metal ions or the ammonium ions. For examples:
Acids such as sulfuric acid and carbonic acid which have two hydrogen ions per molecule, when only one of the hydrogen ions is replaced with a metal ion, the salts formed still contain hydrogen ions.


Table 3 Acid Salts
Acid
Type of acid salt
Example
Carbonic acid H2CO3
Hydrogencarbonate
Sodium hydrogencarbonate NaHCO3
Sulfuric acid HsSO4
Hydrogensulfate
Potassium hydrogensulfate KHSO4