Organic Chemistry Flashcards
How do the physical properties of alkanes and alkenes change on going down the series (increase in carbon atoms)?
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Their melting and boiling points increase.
- Reason: Alkanes are covalent compounds and have simple molecular structures. Alkanes with more C atoms have a bigger relative molecular mass (Mr). Larger amount of energy is needed to overcome the stronger intermolecular forces of attraction in alkanes with more C atoms.
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They become more viscous (flow less easily).
- Reason: As the molecular masses of alkanes increase, more energy is needed to overcome stronger intermolecular forces in these bigger alkanes.
- They become denser.
- They become less flammable (the fractions burn less easily & burn with a more smoky flame).
State the conditions for Hydrogenation to convert alkenes to alkanes
Conditions: 150 °C, nickel catalyst
State conditions for Bromination to alkenes
Condition: Room temperature (25 °C)
State the conditions of Hydration of alkenes to alcohols
Condition: 300°C, 60 atm, phosphoric(V) acid (H3PO4) as catalyst
State how polymerisation of ethene works
At high temperature and pressure and in the presence of catalyst, the carbon-carbon double (C=C) bonds of the ethene molecules break to form carbon-crbon single bonds (C-C). Many ethene molecules join to form long-chain molecule called poly(ethene).
What is cracking?
Cracking is a process where the larger hydrocarbons molecules, usually alkanes are broken down into smaller hydrocarbon molecules by heat and in the presence of catalyst.
Conditions for catalytic cracking
- high temperature, 600°C;
- pressure of about 1 atm
- presence of finely divided aluminium oxide catalyst, Al2O3 or silicon dioxide / silicon(IV) oxide, SiO2 **
Why is cracking important?
- Cracking is used to produce smaller chain hydrocarbons such as petrol which are in great demand as fuels for cars. (e.g. naphtha cracks to petrol)
- Cracking is used to produce short-chain alkenes such as ethene is produced. Ethene is very useful starting material for making ethanol and plastics.
- Hydrogen produced can be used as a fuel. Hydrogen can be used in the production of ammonia.
What happens to the chemical structure of the oil as the margarine is formed?
In vegetable oil, carbon-carbon double bond (C=C) breaks to carbon – carbon single bond. Hydrogen atoms are added to the carbon atoms to form a solid product, margarine.
State the similarities and differences of Alkanes and Alkenes
- Both hydrocarbons and contain a number of carbon atoms
- Both are flammable. On complete combustion, they form carbon dioxide and water
- Both can react aqueous bromine
- Alkane contains carbon-carbon single covalent bonds while alkene contains carbon-carbon double covalent bonds
- Alkanes are generally unreactive while alkenes are very reactive
State the conditions for the fermentation of Glucose
Conditions: presence of yeast, 37°C, absence of oxygen
Why should there a temperature of 37°C, presence of yeast and absence of oxygen for fermentation of glucose?
- Yeast contains biological enzyme that converts glucose to ethanol and carbon dioxide.
- To prevent the oxidation of ethanol to ethanoic acid.
- This is the optimal temperature (37°C) for yeast to function.
- At a higher temperature yeast will be killed and no fermentation of glucose will take place. At a lower temperature, the rate of the reaction will be too slow.
State the conditions for oxidation of alcohols
acidified aqeous potassium manganate(VII)/potassium dichromate (sulfuric acid), warm the mixture
Conditions for esterification
Conditions: concentrated sulfuric acid catalyst, warming
What are isomers?
Isomers are compounds that have the same molecular formula but different structural formula.
State the uses of poly(ethene)
Uses of poly(ethene): to make plastic bags & clingfilm
What are some uses of polyamides and polyesters?
- Clothes made re shrink-proof and crease-proof. They are also easier to wash and dry.
- Synthetic fibres are usually strong and can be drawn into long thin strands without breaking. Thus, they can be used to make fishing lines.
- The tent and sleeping bag are made from Terylene. Unlike natural fibres, synthetic fibres do not shrink when exposed to water.
State the Differences of additional polymerisation and codensation polymerisation
- formed from monomers with C=C bonds while the otherfrom monomers with 2 different functional groups
- C = C bond in monomers break, the monomers then join together, with no elimination of product while condensation polymers are formed with the elimination of small molecule such as water.
- mass of addition polymers formed is equal to the total mass of all the monomers used. Mass of condensation polymers formed is less than the total mass of all the monomers. Loss in mass is due to the elimination
of small molecule
How is polymers recycled physically and chemically?
Physically
* After pre-treatment, small pieces of plastics such as poly(ethene) are melted, cooled, pulled into long, thin strands, and cut into pellets. These pellets can then be made into new products
Chemically
* - Plastic waste, such as poly(ethene) waste, can undergo cracking to form short chains of alkanes and alkenes.
- short-chain alkanes can be used as fuel,
- short-chain alkenes can be used to make other useful chemicals.
Define depolymerisation
Depolymerisation is a process in which polymers are broken down into their monomers (using water in the presence of an acid catalyst).
Define addition polymerisation
Addition polymerisation occurs when unsaturated monomers join together without losing any molecules or atoms to form addition polymers.
How can alkenes and alkanes be distinguished?
- Test to distinguish between alkene and alkane:
- Test: Add aqueous / liquid bromine to an alkene and alkane separately.
- Observations: Alkene (unsaturated hydrocarbon): Reddish-brown aqueous bromine turns colourless. Alkane (saturated hydrocarbon): Reddish-brown aqueous bromine remains unchanged / reddish-brown.
Why is plastic recycled?
Plastic is non-biodegradable and won’t decompose through natural processes
State an environmental issue that might arise from recycling plastics.
Breakdown of plastic waste through cracking requires high temperatures that is obtained through the burning of fossil fuel. Burning of fossil fuels
produce greenhouse gases that causes global warming.
