Natural Rubber

A polymer is a large, long-chain molecule formed by joining together thousands of small m on o m er molecules. Polymer can be classified into two groups which are natural polymers and synthetic polymers. Nat ural rubber is one of the example of natural polymers. The monomer unit of natural rubber is isoprene. Its IUPAC name is 2-methylbuta-1,3-diene.

An addition polymerization joins thousands of isoprene unit together to form poly(isoprene) or natural rubber.

The milky fluid obtained from tapped rubber trees is called latex. It consists of an aqueous suspension of colloidal rubber particles. Each rubber particle is made up of rubber polymers covered by a layer of protein membrane. Negative charges are found on the surface of the membrane, making each rubber particle negative charged. The negative-charged rubber particles repel each other, preventing themselves from combining and coagulating.

Acids such as methanoic acid(formic acid) are added to make the latex coagulate. Hydrogen ions from the acid neutralize the negative charges on the surface of the membrane. A neutral rubber particle is formed. When these neutral particles collide with each other, their outer membrane layers break up. The rubber polymers are set free.

The rubber polymers start to coagulate by combining together to form large lumps of rubber polymers which then precipitate out of the latex solution. Latex can still coagulate if acids are not added. Normally, the latex will coagulate if left overnight. Bacteria from the air slowly attack the protein on the membrane to produce lactic acid. Ionisation of the lactic acid produces hydrogen ions. The hydrogen ions neutralize the negative charges to form neutral rubber particles, allowing coagulation to occur. Alkalis such as solution are added to latex to prevent coagulation. The hydroxide ions from alkali neutralize hydrogen ions produced by lactic acid as a result of bacterial attack on protein. Because there are no hydrogen ions to neutralize the negative charges on the rubber particles, they remain negatively charged and hence cannot combine and coagulate.

Vulcanisation is a manufacturing process discovered by Charles Goodyear in 1839 to convert raw rubber into a tough useful product. In this year, about 1-3% by weight of sulphur is added to raw rubber and the mixture is carefully heated. Sulphur atoms form cross-links between adjacent chains of rubber polymers at the carbon-carbon double bonds. The number of sulphur atoms in the cross-links us usually one to four.

The cross-linking improve the properties of raw rubber, making vulcanized rubber

(a) a tougher material that is more resistant to oxidation.

(b) more elastic as the cross-linked chains can revert back to their original positions.

(c) more heat resistant which means the vulcanized rubber is less soft and sticky on warming.

(d) less soluble in organic solvent.

The Manufacturing Process of Sulphuric Acid ( Contact Process)

The manufacturing of sulphuric acid is one of the most important chemical inductries at the present time. Sulphuric acid, H₂SO₄ is a non-volatile diprotic acid. Concentrated sulphuric acid is a viscous colourless liquid. Sulphuric acid is an important chemical used to make other manufactures substances. The uses of sulphuric acid are to manufacture fertilizers, detergents, pesticides, synthetic fibre, paint pigments, as an electrolyte in lead acid accumulators, to remove metal oxides from metal surfaces before electroplating and etc.

Sulphuric acid is manufactured by the Contact process in industry. The raw materials used in the Contact process are sulphur or sulphide mineral), air and water. The Contact process involves three steps:

sulphur -> sulphur dioxide -> sulphur trioxide -> sulphuric acid

Step I: Production of sulphur dioxide gas, SO_2.

This is done by two methods.
(a) Burning of sulphur in air.

S + O₂ --> SO₂

(b) Burning of metal sulphide such as zinc sulphide or iron(III) sulphide in air.

2ZnS + 3O₂ --> 2SO₂ + 2ZnO

Step II: Conversion of sulphur dioxide to sulohur trioxide, SO₃

(a) Pure and dry sulphur dioxide with excess dry oxygen(from air) are passed

through a converter.

(b) A high percentage of sulphur dioxide is converted into sulphur trioxide under the following conditions:

(i) The presence of vanadium(V) oxide,V₂O₅ as a catalyst.

(ii) A temperature of 450’C – 550’C.

(iii) A pressure of one atmosphere.

2SO₂ + O₂ <--> 2SO₃

Step III: Production of sulphuric acid

(a) The sulphur trioxide produced is dissolved in concentrated sulphuric acid to produce oleum, H₂S₂O₇, a viscous liquid.

SO₃ + H₂SO₄ --> H₂S₂O₇

(b) Oleum is then diluted with an equal volume of water to produce concentrated sulphuric acid(98%).

H₂S₂O₇ + H₂O --> 2H₂SO₄

Sulphur trioxide is not dissolved directly in water to produce sulphuric acid.

SO₃ + H₂O --> H₂SO₄

This is because:

(a) the solubility of sulphur trioxide in water is low.

(b) The process releases a large amount of heat which will vapourise sulphuric acid to form fumes.

The sulphur dioxide gas is dried and purified before the production of sulphur trioxide gas.This is to remove:

(a) water vapour ( the reaction of water with sulphur trioxide will produce heat that will vapourise the acid)

(b) Contaminants such as arsenic compounds( found in the sulphur or sulphide mineral) which will poison the catalyst and make it ineffective.

The conversion of sulphur dioxide into sulphur trioxide is a reversible process. This means that not all the sulphur dioxide can be concerted into sulphur trioxide. The optimum temperature of 450’C – 550’C, the pressure of 1 atm and the use of V2O5 as catalyst are needed to ensure that a high percentage of sulphur trioxide is formed. Under these conditions, the conversion efficiency is about 98%.

Dry Cells

Chemical cells can be divided in to two main types, namely primary cells and secondary cells.
Primary cells are not rechargeable and can be used only once. They have to be disposed off once they are exhausted or fully discharged. Examples of primary cells are dry cells, alkaline cells and mercury cells. Secondary cells are rechargeable when they are exhausted and can be used again and again. Secondary cells can be recharged by passing an electric current through them in the opposite direction to the current flow during discharged or used. Examples of secondary cells include lead-acid accumulator, nickel-cadmium cell and lithium-ion cells.

A dry cell consists of a carbon rod(cathode) and a metal container made of zinc(anode). The electrolyte is ammonium chloride in the from of a paste. The carbon rod is surrounded by a mixture of carbon powder and manganese (iv) oxide, which is again surrounded by ammonium and zinc chloride paste. Carbon powder is a good electric conductor and it reduces the resistance in the cell. Carbon in powder form also increases the surface area of the carbon electrode.

When the dry cell is in use, the zinc metal releases electrons and is ionised to form Zn²⁺ ions. Thus, zinc acts as the reducing agent. At the anode:

Zn(s) --> Zn²⁺(aq) + 2e^-

Electrons flow from the zinc metal container through the external circuit to the carbon rod where NH₄⁺ ions receive electrons to produce ammonia gas and hydrogen gas. Thus, ammonium ions act as the oxidising agent. At the cathode:

2NH₄⁺(aq)+ 2e^- --> 2NH₃(aq)+ H₂(g)

The hydrogen produced surrounds the carbon rod and acts as an insulator, reducing the effiency of the cell. This effect is called polarization. The function of manganese(iv) oxide, MnO₂ is to reduce cell polarisation. Manganese(iv) oxide oxidises the hydrogen gas and minimises the formation of gas bubbles at the carbon rod when the cell is in use.

2MnO₂ + H₂ --> MnO₃ + H₂O

Therefore, the overall reaction is:

Anode(-) : Zn(s) --> Zn²⁺(aq) + 2e^-
Cathode(+) : 2NH₄⁺(aq)+ 2e^- --> 2NH₃(aq)+ H₂(g)
Overall : Zn(s) + 2NH₄⁺(aq) --> Zn²⁺(aq) + 2NH₃(aq)+ H₂(g)

When the cell produces electric current, zinc metal dissolves. When the zinc metal container is perforated and the electrolyte leaks out, the dry cell cannot be used anymore.

Rusting as a Redox Reaction

When metals are exposed to their environment, they undergo corrosion. For example, after some time, a shiny aluminium pot will lose its shine, silverware will tarnish and an iron structure will rust.

Corrosion of metal is a redox reaction in which a metal is oxidised naturally to its ions, resulting in partial or complete destruction of the metal. During corrosion, the metal atoms lose electrons to form positive ions.

M ---> Mⁿ⁺ + n^e-

Rusting is the corrosion of iron. It is the most common corrosion of metal around. For iron to rust, oxygen and water must be present. Rusting is a redox reaction whereby oxygen acts as the oxidising agent and iron act as the reducing agent.

The surface of iron at the middle of the water drerves as the anode, the electrode at which oxidation occurs. The iron atom here lose electrons to form iron(ii) ions.

Oxidation half equation :

Fe(s) --> Fe^{2+(aq) + 2e-}

The electrons flow to the edge of the water droplet, where the is plenty of dissolved oxygen. The iron surface there serves as the cathode, The electrode at which reduction occurs. oxygen gains the electrons and is reduced to hydroxide ions.

Reduction half-equation:

O₂(g) + 2H₂O(l) + 4e^- --> 4OH^-(aq)

The iron(ii) ions produced combine with the hydroxide ions to form iron(ii) hydroxide.

Fe²⁺(aq) + 2OH^-(aq) --> Fe(OH)₂(s)

Thus, the overall redox reaction is as follow:

At the anode: 2Fe(s) --> 2Fe²⁺(aq) + 4e^-
At the cathode: O₂(g) + 2H₂O(l) + 4e^- --> 4OH^-(aq)
Overall equation: 2Fe(s) +O₂(g) + 2H₂O --> 2Fe(OH)₂(s)

The iron(ii) hydroxide is then further oxidised by oxygen to form hydrated iron(iii) oxide, Fe₂O₃.xH₂O whereby x varies. The hydrates come in various shades of brown and ornage and together make up what In the presence of acids and salts, rusting occurs faster. These substances increase hte electrical conductivity of water, making water a better electrolyte. is commonly know as rust.For example:

(a) iron structures such as bridges, fences and cars at coastal areas rust faster due to the presence of salts in the coastal breeze.

(b) iron structures in industrial areas rust quickly as these areas have air polluted with acidic gases such as sulphur dioxide and nitrogen oxides.

The Effectiveness of the Cleansing Action of Soap and Detergent

Although soap is a good cleaning agent, its effectiveness is reduced when used in hard water. Hard water contain a great amount of calcium and magnesium ions. These ions react with the soap to form an insoluble precipitate known as soap scum.

2CH₃(CH₂)₁₆COO^-Na⁺(aq)(Sodium sterate) + Ca²⁺(aq)(Calcium ions) -->
[CH₃(CH₂)₁₆COO]₂Ca(s) (Insoluble calcium sterate)(scum)+ 2Na⁺(aq)(Sodium ions)

2CH₃(CH₂)₁₆COO^-Na⁺(aq)(Sodium sterate) + Mg²⁺(aq)(Magnesium ions) -->
[CH₃(CH₂)₁₆COO]₂Mg(s) (Insoluble magnesium sterate)(scum)+ 2Na⁺(aq)(Sodium ions)

Formation of soap scum reduces the amount of soap available for cleaning, thus causes wastage of soap. Soaps are only suitable for use in soft water. Soft water is water that contain little or no calcium and magnesium ions. Soaps do not form scum with soft water. The effectiveness of the cleansing action of soap is also reduced when used in acidic water.

(a) The hydrogen ions in acidic water react with the soap to form long-chain fatty acids.
(b) Long-chain fatty acids are insoluble in water due to their high relative molecular masses. This reduces the amount of soap available for cleaning. The effectiveness of the cleansing action of soap is thus reduced.

CH₃(CH₂)₁₆COO^-Na⁺(aq)(Sodium sterate) + H⁺(aq)(Hydrogen ions) -->
CH₃(CH₂)₁₆COOH(s) (Insoluble steratic acid) + Na⁺(aq)(Sodium ions)

Detergent do not form scum with hard water. They form soluble substances with calcium and magnesium salts.

2CH₃(CH₂)₁₁OSO₃^-Na⁺(aq)(Sodium dodecyl sulphate) + Ca²⁺(aq)(Calcium ions) -->
[CH₃(CH₂)₁₁OSO₃^-]₂Ca²⁺(aq)( Soleble calcium dodecyl sulphate) + 2Na⁺(aq)(Sodium ions)

2CH₃(CH₂)₁₁OSO₃^-Na⁺(aq)(Sodium dodecyl sulphate) + Mg²⁺(aq)(Magnesium ions) -->
[CH₃(CH₂)₁₁OSO₃^-]₂Mg²⁺(aq)( Soluble magnesium dodecyl sulphate) + 2Na⁺(aq)(Sodium ions)

This means detergent can still perform its cleansing action in hard water. Thus, detergent is more effective than soap in hard water. Detergent do not form precipitate in acidic water as well. Therefore, their cleansing action is not affected.

CH₃(CH₂)₁₁OSO₃^-Na⁺(aq)(Sodium dodecyl sulphate) + H⁺(aq)(Hydrogen ions) -->
CH₃(CH₂)₁₁OSO₃^-H⁺(aq)( Soluble dodecyl sulphonic acid) + Na⁺(aq)(Sodium ions)