Natural rubber is obtained from latex, a milky white substance extracted from the rubber tree Hevea brasiliensis.
What is the monomer of natural rubber?
The monomer of natural rubber is isoprene (2-methylbuta-1,3-diene).
What is the molecular formula of isoprene?
The molecular formula of isoprene is C₅H₈.
What type of polymerisation occurs in the formation of natural rubber?
Natural rubber is formed through addition polymerisation of isoprene.
What is the structural name of the polymer formed from isoprene?
The polymer formed is called cis-1,4-polyisoprene.
What property of natural rubber allows it to stretch and return to its original shape?
Its elasticity, which is due to weak intermolecular forces between polymer chains.
Why does natural rubber soften at high temperatures?
At high temperatures, the polymer chains gain mobility, weakening the material and making it sticky.
What happens to natural rubber when exposed to oxygen or ozone?
It undergoes oxidation, becoming brittle and losing its elasticity.
What is latex?
Latex is a colloidal dispersion of rubber particles in water, stabilized by negatively charged protein membranes.
Why do rubber particles in latex remain dispersed in water?
The rubber particles carry a negative charge, causing them to repel each other and stay dispersed.
What causes latex to coagulate?
Coagulation occurs when acids neutralize the negative charges on rubber particles, causing them to clump together.
What acid is commonly used to coagulate latex?
Ethanoic acid (acetic acid), CH₃COOH.
How can latex coagulation be prevented?
By adding alkaline stabilisers such as ammonia (NH₃) or hydroxides, which maintain the negative charge on the rubber particles.
What is vulcanisation?
Vulcanisation is the process of heating natural rubber with sulfur to create cross-links between polymer chains, improving its properties.
Who discovered the process of vulcanisation?
Charles Goodyear.
What type of bonds are formed during vulcanisation?
Covalent sulfur cross-links (C-S-S-C) between polymer chains.
Why does vulcanised rubber have greater heat resistance than natural rubber?
The sulfur cross-links restrict polymer chain movement, preventing softening at high temperatures.
Give two common applications of vulcanised rubber.
Tyres and gloves.
Why is natural rubber used in making hoses and seals?
Its flexibility, elasticity, and durability make it ideal for these applications.
What happens to natural rubber when exposed to UV light for a prolonged period?
Natural rubber degrades when exposed to UV light for long periods, leading to brittleness and loss of elasticity due to photodegradation.
Explanation Questions
Why is natural rubber soft and weak in its raw form?
Natural rubber lacks strong cross-links between polymer chains, making it easily deformable and weak under stress.
Why does oxidation cause rubber to become brittle?
Oxidation breaks down the polymer chains, reducing flexibility and making the rubber hard and brittle.
Why does adding an acid to latex cause it to coagulate?
Acidic H⁺ ions neutralize the negative charges on rubber particles, allowing them to clump together and form a solid mass.
Why does ammonia prevent latex coagulation?
Ammonia increases the negative charge on rubber particles, keeping them dispersed and preventing clumping.
Why does vulcanisation increase the strength of rubber?
Sulfur cross-links create strong covalent bonds between polymer chains, preventing them from slipping past each other.
Why does vulcanisation improve the elasticity of rubber?
The cross-links allow the polymer chains to return to their original shape after being stretched, improving elasticity.
Why does vulcanised rubber resist heat better than natural rubber?
The cross-links prevent excessive polymer movement at high temperatures, reducing softening and stickiness.
Why is vulcanised rubber more durable than natural rubber?
The sulfur cross-links make it more resistant to tearing, oxidation, and degradation, increasing its lifespan.
Why is natural rubber an ideal material for tyres?
Vulcanised rubber provides durability, elasticity, and heat resistance, making tyres strong and long-lasting.
Why does vulcanisation reduce the oxidation of rubber?
Vulcanisation strengthens rubber by forming sulfur cross-links between polymer chains, making it less susceptible to oxidative degradation by oxygen and ozone.