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Alkanes

Alkanes - Fractional Distillation, Cracking and Combustion.

QuestionAnswer
What is an Alkane? An Alkane is a Saturated Hydrocarbon with only C-C and C-H Single bonds.
What is the general Formula for an Alkane? CnH(2n+2)
Describe and explain the change in Boiling/Melting points of Alkanes: As chain length increases, the boiling point increases as larger molecules have greater van der Waals forces between them. More Branching reduces the Melting points as the molecules cannot pack together well, reducing van der Waals forces.
Describe and explain the solubility of Alkanes in Water: Alkanes are Insoluble in water because they are Non-Polar molecules and Water is only an effective solvent for Polar molecules.
Describe the reactivity of Alkanes: They will burn in oxygen to produce CO2 and Water and they will react with Halogens under specific circumstances. Otherwise they are unreactive.
What is different between Fractional Distillation and normal distillation? Fractional Distillation produces many different parts (or fractions) each with different boiling points, Distillation only separates the volatile and nonvolatile products.
Cracking: Cracking is the process of breaking a long Hydrocarbon chain into shorter Hydrocarbon chains, some of which can be Alkenes.
Why is cracking useful: Typically, the shorter Hydrocarbon chains are more economically desirable because they are more volatile, less viscous and have lower boiling points.
Name the two types of Cracking: Thermal Cracking and Catalytic Cracking.
Thermal Cracking: Is the process of breaking a Hydrocarbon chain under very high temp and very high pressure. When the chain is broken, each carbon gains an electron from the covalent bond, forming Free Radicals then forming Alkanes and Alkenes (useful for polymers).
Name the conditions required for Thermal Cracking: Temp of 700K-1200K and Pressure of up to 7000kPa.
Name the two products from Thermal Cracking: Alkanes and high percentage of Alkenes.
Outline the stages of the reaction involving the Hydrocarbon Chain in thermal Cracking: Long Hydrocarbon chain ---> Free Radical Intermediates ---> Alkanes + Alkenes.
Catalytic Cracking: Takes place at lower temp (700K) and lower pressure and uses a ZEOLITE catalyst. Products are mostly Motor fuels, Aromatic compounds, cycloalkanes and Branched Alkanes.
What are the main products of Catalytic Cracking: Motor Fuels, Aromatic Compounds, Branched Alkanes and Cycloalkanes.
What are the conditions required for Catalytic Cracking: High temps (720k) but lower than Thermal Cracking, relatively low pressure and the presence of a ZEOLITE catalyst.
What are the main economic reasons for Cracking long hydrocarbon chains: To produce shorter hydrocarbon chains that are more valuable, eg Motor Fuels, Aromatic compounds and Alkenes for Polymers.
What are the use of Alkenes produced from Thermal Cracking? To make Polymers, e.g. Ethene --> Polyethene.
What are the two types of Combustion of Alkanes: Complete (in excess Oxygen) and incomplete (in limited or very limited Oxygen).
Write the products of Complete Combustion: H2O and CO2.
Write the two different lots of products formed from incomplete combustion: H2O and CO (Limited Oxygen) and H2O and C (Soot).
List the pollutants produced by burning Alkanes: CO, CO2, NO(x), SO2, H2O, Unburnt Hydrocarbons.
Name the Greenhouse Gases produced from Burning Alkanes: Carbon Dioxide, Water Vapour, Unburnt Hydrocarbons.
Name the compounds that contribute to Acid Rain: Nitric Acid and Sulfuric Acid. (formed from NOx and SO2).
How can the Sulfur containing impurities be dealt with? (Flue Gases). Calcium Carbonate Scrubbers or Calcium Oxide in Flues to make Gypsum (CaSO4).
How are Nitrogen Oxides formed? From large heat causing the Oxygen in the air and Nitrogen to bond.
How can Carbon Monoxide and Nitrogen Oxide be dealt with? Catalytic Converters, these honeycomb structures that are layered with Platinum create a large surface area and allow for the NO and CO to react together to produce N2 and CO2.
How can the Unburnt Hydrocarbons that are produced from burning Alkanes be dealt with? The Catalytic Converter will allow for the Hydrocarbon and Nitrogen Oxide to react together to form N2, CO2 and H2O.
What is a special name given to the products: CO2, Unburnt Hydrocarbons (Methane etc) and H2O? These are called greenhouse gases and they contribute to the greenhouse effect which can increase the rate of Global Warming.
Created by: mjwilson1988