AQA A Level Chemistry Year 1 – Alkanes Questions and Answers

AQA A Level Chemistry Year 1 – Alkanes Questions and Answers What is a source of hydrocarbons? Crude oil (petroleum) What is crude oil (petroleum) ? A mixture consisting mainly of alkane hydrocarbons which can be separated by fractional distillation How might blockage of the air vents in a combustion engine affect its performance? Less energy given out by the fuel Engine less powerful/efficient What is fractional distillation? The continual evaporation and condensation of a mixture causing the different components to separate because they have different boiling points Explain fractional distillation (4 steps) 1. Crude oil is heated to a vapour which passes into the column 2. The column is hotter at the bottom and cooler at the top 3. Vapours rise up the column and cool, condensing when they meet their boiling point 4. Short chain alkanes are collected near the top as they have lower boiling points Give a use of alkanes Fuels What is an alkane? A saturated hydrocarbon What is cracking? The proceds where long chain hydrocarbon molecules are broken into shorter chain molecules which are in high demand What bond is broken during cracking? C-C Why are high temperatures needed in both thermal and catalytic cracking? The C-C bond is very strong Requires high amount of energy to break Why is it necessary to crack hydrocarbons? To produce short chain hydrocarbons which are in high demand What are the conditions for thermal cracking? 1200K 70 atm 1 second No catalyst What are the products of thermal cracking? High % of alkenes with some short chain alkanes What are the conditions for catalytic cracking? 800-1000K 1-2 atm 2-4 seconds Catalyst of aluminosilicate zeolite What is aluminosilicate zeolite? An acidic mineral Catalyst with a honeycomb structure What is aluminosilicate zeolite made of? Aluminium oxide Silicon dioxide What are the products of catalytic cracking? Mainly aromatic hydrocarbons and motor fuels What is the difference between complete and incomplete combustiom? Complete combustion : plenty of O2, products are CO2 and H2O Incomplete combustion : Limited O2, products are CO or C and H2O How does CO form? When carbon in a fuel compound burns in limited oxygen How do nitrogen oxides form? Nitrogen in the air reacts with oxygen at the very high temperatures of combustion engine How does SO2 form? Sulphur impurities in fuel reacts with O2 when hydrocarbons combust How does a catalytic converter work? Gases pass over the catalyst and react with each other to produce less polluting products Give the two reactions which may happen in a catalytic converter Hydrocarbon + NOx -- N2 + CO2 + H2O 2NO + 2CO -- 2CO2 + N2 Methane building up in mines has an explosion risk Explain why the risk is lower when methane concentration is 15% For an explosive methane must react with oxygen Not enough oxygen Describe flue gas desulphurisation Gases passed through a slurry containing calcium oxide/carbonate. Neutralise the acidic sulphur dioxide to form calcium sulphite Why does CaSO3 need to be oxidised ? Forms CaSO4 Commercially useful What is the equation for the oxidation of CaSO3? CaSO3 + [O] -- CaSO4 Give the two equations which can occur in flue gas desulphurisation CaO + SO2 -- CaSO3 CaCO3 + SO2 -- CaSO3 + CO2 Describe the structure of a zeolite and suggest why it is important Honeycomb Large surface area What environmental problems can NOx and SO2 form? Acid rain Destroys fish and vegetation Kills fish Corrodes buildings Explain the principles of fractional distillation Different alkanes have different boiling points depending on their size Column is cooler at the top Shorter chain alkanes have lower boiling points Shorter chain alkanes condense at the top of the column What are the environmental problems of unburnt hydrocarbons? React with NOx Form ground level ozone - respiratory problems Photochemical smog (NOx and SO2 and unburnt hydrocarbons react with sunlight) What is the problem with photochemical smog? Health problems Lung diseases What is the environmental problem of CO2? Global warming Greenhouse effect