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The Manufacture of Alkenes by Cracking

1. Alkenes are obtained by the cracking of petroleum fractions, such as the naphtha fraction.

2. Cracking is the process in which large hydrocarbon molecules are broken down into smaller molecules.

3. When an alkane molecule is cracked, a mixture of short-chain alkanes, alkenes and hydrogen are formed.
Example:
When the alkane C13H28 in naphtha is cracked, it is broken up into smaller alkane and alkene molecules

3. When an alkane molecule is cracked, a mixture of short-chain alkanes, alkenes and hydrogen
are formed.

Example:
When the alkane C13H28 in naphtha is cracked, it is broken up into smaller alkane and alkene molecules

4. The conditions used for cracking
(a) In industries, cracking is carried out by passing the petroleum fraction over the catalyst at high temperature. The process is called catalytic cracking.
(b) The conditions for catalytic cracking are:
• Temperature: 600°C
• Catalyst: Silicon(IV) oxide, SiO2, or aluminium oxide, A/203

5. In the school laboratory, catalytic cracking can be carried out using the apparatus as shown in the following diagram.

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Key definitions – Alcohols and Carboxylic Acids

1. Alcohols are organic compounds that contain the hydroxyl (—OH) group.

2. Fermentation is the process of converting sugars into ethanol and carbon dioxide, catalysed by the enzymes in yeast.

3. Carboxylic acids are organic compounds that contain the carboxyl (—COOH) group.

4. Esters are organic compounds produced from the reactions between carboxylic acids and alcohols.

5. Esterification is the reaction between a carboxylic acid and an alcohol to produce an ester and water.

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Macromolecules – IMPORTANT DEFINITIONS

1. A macromolecule is a large molecule in which there is a very large number of atoms joined together by covalent bonds.

2. A polymer is a large molecule made up of many repeat units joined together by covalent bonds.

3. A monomer is the starting material for making a polymer.

4. Polymerisation is the process of joining up of many small molecules (monomers) to form a long-chain molecule (polymer).

5. Addition polymerisation is a reaction in which monomers join together to form a large molecule (polymer) as the only product.

6. Condensation polymerisation is a reaction in which monomers join together to form polymers with the elimination of small molecules.

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Macromolecules

1. A macromolecule is any long-chain molecule that contains hundreds or thousands of atoms, joined together by covalent bonds.

2. A macromolecule is formed by linking together many small repeating units known as monomers.

3. The process of joining together a large number of small molecules (monomers) to form a macromolecule is called polymerisation. The macromolecule formed is called a polymer.

5. Synthetic macromolecules, such as plastics, are now manufactured on a large scale.

6. Different macromolecules have different units and different linkages.

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Addition Polymerisation

1. Addition polymerisation occurs when monomer units join together without losing any molecules or atoms.

2. Poly(ethene)
(a) At high pressure and temperature and in the presence of a catalyst, ethene undergoes addition polymerisation to form a long-chain molecule, poly(ethene).

(b) Ethene is called the monomer and poly(ethene) is called the polymer.

(c) Uses of poly(ethene)
Poly(ethene) is widely used as plastic bags, plastic bottles and cling films for wrapping food.

Condensation Polymerisation

1. In condensation polymerisation, small molecules such as water, are eliminated when monomers join together to form a polymer.

2. For condensation polymerisation to occur, each monomer must have two reactive groups. The polymer chain is formed by the removal of water molecules.

3. There are two main groups of condensation polymers: polyamides and polyesters.

4. Nylon: a polyamide
(a) Nylon is made from two different monomers:
• Dicarboxylic acid (a molecule with two -COOH groups)
• Diamine (a molecule with two -NH2 groups)
(b) When the two monomers combine, the polymer, nylon, is formed and water molecules are eliminated.

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Condensation Polymerisation

1. In condensation polymerisation, small molecules such as water, are eliminated when monomers join together to form a polymer.

2. For condensation polymerisation to occur, each monomer must have two reactive groups. The polymer chain is formed by the removal of water molecules.
Example:
Dicarboxylic acid (two -COOH groups) + diamine (two -NH2 groups)
HOOC – X – COOH + NH2 – Y – NH2 polyamide + H2O
dicarboxylic acid diamine (e.g. nylon) water

3. There are two main groups of condensation polymers: polyamides and polyesters.

4. Nylon: a polyamide
(a) Nylon is made from two different monomers:
• Dicarboxylic acid (a molecule with two -COOH groups)
• Diamine (a molecule with two -NH2 groups)
(b) When the two monomers combine, the polymer, nylon, is formed and water molecules are eliminated.

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EXPERIMENTAL CHEMISTRY – Do you Know?

1. Experimental Design

Burettes and pipettes are both used to measure accurate volumes of liquids because they have been accurately calibrated.

Pipettes are calibrated to measure fixed volumes such as 10.0 cm3, 25.0 cm3. Burettes measure up to an accuracy of 0.1 cm3.

2. Uses of filtration

Insoluble salts such as copper(II) oxide, lead(II) iodide are separated from water by filtration.
The filter paper has tiny holes (called pores) that enable the particles of liquid (e.g. water, ink dyes, dissolved sodium chloride) to pass through, retaining behind the larger solid particles (e.g. sand, copper(II) oxide).

Solutions such as aqueous sodium chloride can be collected as a filtrate as the sodium and chloride ions are small enough to pass through the pores of the filter paper.

3. Crystallisation

A saturated solution is one that contains the maximum amount of solute that can possibly dissolve in it at a given temperature. A hot solution can dissolve more solute than a cold one. Hence, on cooling, the bulk of the solute is obtained as crystals.

A hot saturated solution gives large crystals when cooled slowly e.g. cooling at room temperature gives larger crystals compared to freezing. This is because in freezing, particles in a saturated solution have a shorter period of time to pack closer together to form a larger crystal.

Crystals can be re-crystallised (that is dissolved again in the same solvent, then repeating the entire crystallisation process) in order to obtain purer crystals.

After crystallisation, the crystals can be weighed and the percentage purity of the impure salt can be calculated using:

Purity = (Mass of salt obtained/ Initial mass of impure salt )x 100%

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EXPERIMENTAL CHEMISTRY – Do you Know?

Use of distillation

To obtain pure water from sea water

The porcelain boiling stones are used to smoothen the boiling.
The first few drops of liquid are discarded to make sure that any possible impure liquid that may have a boiling point slightly lower than that of the required liquid is not collected.
The running tap must not be turned off before the flame is extinguished to avoid breakage of the condenser if overheated.
The thermometer shows a constant temperature during the distillation process when pure solvent is being collected at the boiling point of the solvent.

Fractional distillation

Separation of a mixture of miscible liquids with different boiling points can be done through fractional distillation.

Uses of fractional distillation

1. Separation of liquid air into oxygen, nitrogen and other useful gases.
2. Separation of crude oil into petrol, kerosene and other useful components of crude oil.
3, Separation of femented liquor into ethanol and water

Do you Know?
The pure liquid that has the lowest boiling point will distil off first. This is because the glass beads in the fractionating column condense the liquids with the higher boiling points back into the flask, allowing the pure liquid with the lowest boiling point to vapourise and distil off as the first distillate

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Elements, Compounds and Mixtures

Do you know?
All metals exist as atoms. Most non-metals exist as molecules

Do you know?
Many non-metallic elements exist as molecules. For example, hydrogen and oxygen exist as H2, and 02 molecules respectively

Do you know?
Tap water is not a pure compound. It is a mixture that contains water and dissolved substances such as chlorine and other minerals.

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2015 O Level Chemistry Schedule

S3	1.5 hrs	SAT	9pm – 10.30pm
S3	1.5 hrs	SAT	12.30pm – 2pm
S3	1.5 hrs	SUN	12.30pm – 2pm
S4	2 hrs	FRI	5pm – 7pm
S4	1.5 hrs	SAT	12.30pm – 2pm
S4	1.5 hrs	SUN	9am – 10.30pm

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Kinetic Particle Theory – Concise Notes

1.1 State of Matter

Solids:
• Have a fixed shape, fixed volume; cannot be compressed.
• Particles vibrate and rotate about fixed positions.

Liquids:
• Have no fixed shape but have fixed volumes; cannot be compressed.
• Particles move.

Gases:
• Have no fixed shape, no fixed volume; can be compressed easily.
• Particles move about rapidly.

1.2 Kinetic Particle Theory and the Changes of State
• The kinetic particle theory states that (a) all matter is made up of tiny particles, and (b) all particles are in constant, random motion.
• Particles have kinetic energy.
• When matter is heated or cooled, heat energy is taken in or given out. This causes the kinetic energy of the particles to change, leading to a change of state.

1.3 Diffusion
• Diffusion provides evidence that the particles in gases and liquids are constantly moving.
• Examples of diffusion include: the spreading of the smell of perfume, the spreading of bromine in a gas jar of air, and the spreading of potassium manganate(VII) in water.
• The lower the molecular mass of the particles, the faster the rate of diffusion.
• The higher the temperature, the faster the rate of diffusion.

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