SPM Chemistry Form 5 Notes – Terminology and Concepts: Carbon Compounds (Part 9 – Final)

by BerryBerryTeacher

in Berry Reference (Notes)

Finally, Berry Berry Easy will be ending the long running Carbon Compound series for SPM Form 5 Chemistry with Part 9. This final part won’t be a recapitulation of what have been learnt but rather focuses on polymers. One of the most important natural polymers is natural rubber. And since rubber is historically an important export for Malaysia, it would be beneficial to know about rubber in general. So we present you with the final part of the long series.

SPM Form 5 – Terminology and Concepts: Carbon Compounds (Part 9 – Final)

Polymers

1. Polymer – many small units (monomers) joining together to formed large molecule.

2. Polymer can be classified into two groups:

  • synthetic polymers / man-made polymers (polythene; PVC – polyvinyl chloride; artificial silk; and polypropene)
  • natural polymers (natural rubber; starch; cellulose; and proteins)

3. Natural polymer: Carbohydrates (polysaccharides) (starch, glycogen and cellulose)

  • General formula: Cx(H2O)y with the ratio of H:O = 2:1
  • Carbohydrates have cyclic structure.
  • Monomer: glucose (C6H12H6)
  • Reaction to form polymer: condensation reaction (- H2O)

4. Natural polymer: Protein (polypeptide)

  • Protein consists of carbon, hydrogen, oxygen and nitrogen (some have sulphur, phosphorus and other elements)
  • Monomer: amino acids
  • Amino acids have two functional group which are carboxyl group (-COOH) and amino group (-NH2)
  • Reaction to form polymer: condensation reaction (- H2O)

5. Natural polymer: Natural rubber

  • Extracted from the latex of rubber tree (Hevea brasiliensis) which the tree originates from Brazil.
  • A molecule of rubber contains 5000 isoprene units.
  • Monomer: isoprene, C5H8 or 2-methylbuta-1,3-diene.
  • Reaction to form polymer: additional polymerisation (one of the double bond in isoprene becomes single bond)

6. Structure of rubber molecule

  • Latex is colloid (35% rubber particles and 65% water).
  • Rubber particle contains rubber molecules which are wrapped by a layer of negatively-charged protein membrane. Same charge of rubber molecules repels each other. This prevent rubber from coagulate.

7. Coagulation process of latex

The process for the coagulation of latex is summarised as:

  1. Acid (H+) can neutralise the negatively-charged protein membrane. Example of acid: formic acid, methanoic acid, suphuric acid and hydrochloric acid.
  2. The rubber molecules will collide after the protein membrane is broken.
  3. Rubber molecules (polymers) are set free
  4. Rubber molecules combine with one another (coagulation).

8. Natural coagulation process of latex

For the natural coagulation of latex:

  1. Latex is exposed to air without adding acid (duration – overnight).
  2. Coagulation process occurs in slower pace due to the bacteria (microorganism) action which produce acid)

9. Prevent coagulation process of latex

The following are latex coagulation prevention method:

  1. Alkaline / Basic solution is added to the latex. Example: ammonia (NH3).
  2. Positively-charged hydrogen ion / H+ produced by bacteria can be neutralised by negatively-charged hydroxide ion / OH- from ammonia solution.

10. Properties of natural rubber

  • elastic
  • cannot withstand heat (become sticky and soft – above 50°C; decompose – above 200°C; hard and brittle – cooled)
  • easily oxidised (present of C=C)
  • insoluble in water (due to the long hydrocarbon chains)
  • soluble in organic solvent (propanone, benzene, petrol etc.)

11. Vulcanisation of rubber

Vulcanisation – process of hardening rubber and increases rubber elasticity by heating it with sulphur or sulphur compounds.

Methods:

  • heating natural rubber with sulphur at 140°C using zinc oxide as catalyst or
  • dipping natural rubber in a solution of disulphur dichloride (S2Cl2) in methylbenzene.

12. Properties of vulcanisation of rubber

  • The sulphur atoms are added to double bonds in the natural rubber molecules to form disulphide linkages (-C-S-S-C-) / sulphur cross-links between the long polymer chains. Therefore, vulcanised rubber is more elastics and stronger.
  • This increases the molecular size and the intermolecular forces of attraction between rubber molecules. Therefore, vulcanised rubber is more resistant to heat (does not become soft and sticky when hot).
  • This also reduces the number of carbon-carbon double bonds in rubber molecules. Therefore, vulcanised rubber is more resistant to oxygen, ozone, sunlight and other chemicals.

13. Comparison between the properties of vulcanised rubber and unvulcanised rubber

Properties Vulcanised rubber Unvulcanised rubber
Double bonds Decreases (formation of sulphur cross-links) More number of double bonds
Melting point High (presence of sulphur) Low
Elasticity More elastic (sulphur cross-links prevents the polymer chain or rubber from slipping past. Less elastics
Strength and hardness Strong and hard (depends on degree of vulcanisation) Weak and soft (polymer chain of rubber will break when rubber is over stretched.
Resistant to heat Resistant to heat Poor resistant to heat
Oxidation Resistant to oxidation (reduction of number of double bonds per rubber molecule) Easily oxidised by oxygen, UV light (presence of many double bonds per rubber molecules)

14. R & D of rubber

  • RRIM – Rubber Research Institute of Malaysia
  • MRB – Malaysian Rubber Board
  • Rubber Technology Centre
  • Various local higher institutions of learning

So finally you are done with the chapter of Carbon Compound if you have mastered all nine parts which was published on Berry Berry Easy.

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