Developing Fuels

A summary of Developing Fuels

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  • Created by: R_Hall
  • Created on: 27-12-12 17:08

3.3 Structural Isomerism

  • Isomers are two molecules which have the same formula, but different arrangements of atoms, with different properties. The occurrence of isomers is called isomerism
  • Structural isomers have atoms bonded in different orders
  • Stereoisomers have the same order of bonding, but the atoms are arranged in space differently
  • Structural isomerism can be split into 3 areas- chain isomerism, position isomerism and functional group isomerism
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4.1 Energy out, Energy In

  • A reaction that gives out energy and heats the surroundings is exothermic, and a reaction that takes in energy and cools the surroundings is endothermic
  • An enthalpy change (ΔH) is an energy change under constant pressure
  • Exothermic reactions have -ΔH value, energy is lost to the surroundings. Endothermic reactions have +ΔH value, energy is gained from surroundings
  • Standard conditions are temperature of 298K (25C), 1 mole concentration solution and 1 atmosphere pressure, with reactants in the standard states
  • Enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states, under standard conditions
  • Enthalpy of combustion is the enthalpy change when 1 mole of a fuel is burnt completely in oxygen, under standard conditions
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4.1 Energy Out, Energy In cont.

  • Enthalpy changes can be calculated by measuring the temperature change of water when it is heated (or cooled) by the reaction. We also need to know the mass and specific heat capacity of the water
  • Energy transferred= c x m x ΔT
  • c is specific heat capacity (J), m is mass (g) and ΔT is the temperature change (°C)
  • In order to calculate the enthalpy change from the energy transfer, you need to convert the energy transfer for volume reactants used into the energy transfer for 1 mole
  • In enthalpy experiments, it is difficult to avoid energy from being transferred to the surroundings ("heat loss"). Because of this, we have to assume that there is no other energy loss.
  • We ignore the heat transferred to the calorimeter (used to insulate the reaction) and energy transferred to any solids in the calorimeter
  • Hess's Law- the enthalpy change for any chemical reaction is independent of the route by which the chemical reaction takes place
  • So long as your starting a finishing points are the same, the enthalpy change will also be the same
  • There is a direct route and an indirect route for the enthalpy change for the reaction below, both routes end up at the same point
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4.2 Where Does The Energy Come From?

  • A chemical bond is an electrical attraction between atoms or ions.
  • When you break a bond, you have to overcome these attractive forces. The quantity of energy needed to break a bond in a molecule is the bond dissociation enthalpy or bond enthalpy
  • The bigger the enthalpy value, the stronger the bond
  • The shorter the bond, the stronger the bond-stronger attraction between atoms, so more energy needed to break them apart
  • Double bonds require more energy to break than single bonds, and triple bonds need more than double bonds
  • Bond enthalpies given are often averages from several compounds, so the results of calculations are not always precise
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4.3 Entropy and the Direction of Change

  • Entropy is a measure of the number of ways that molecules can be arranged.
  • The higher the entropy value, the greater the disorder (the greater of number of ways the molecules can be arranged)
  • Gases have higher entropies than liquids; liquids have higher entropies than solids. Gas molecules are arranged at random, whereas in solids the molecules are normally regular and crystalline
  • Gases > Liquids > Solids
  • Larger molecules have higher entropies than small molecules
  • The entropy of the products will be greater than the reactants if solids-liquids or liquids-gases OR there are more moles of product than reactant
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10.5 What is a Catalyst

  • A catalyst is a substance which speeds up a chemical reaction but can be recovered chemically unchanged at the end. The process is called catalysis
  • If reactants and catalyst are in the same state- homogeneous catalysis. If the reactants and the catalyst are in different physical states- heterogeneous catalysis
  • How heterogeneous catalysts work
  • The reactants are adsorbed (form bonds) onto the surface of the catalyst. The bonds within the reactants are weakened and break, causing new bonds to form to form the products. The product diffuses away from the catalyst surface
  • A catalyst needs a large surface area in order to increase contact with reactants
  • Catalysts can be poisoned so they no longer work. The "poison" molecules are adsorbed more strongly to the catalyst surface than the reactants, so the catalyst becomes inactive as the poison molecule block the active sites
  • It is sometimes possible to clean or regenerate the surface of a catalyst
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12.1 Alkanes

  • Hydrocarbons contain carbon and hydrogen only. Compounds with benzene ring are aromatic, compounds without are aliphatic
  • Alkanes are saturated compounds. They contain the maximum number of hydrogen atoms as all the bonds between carbons are single
  • Homologous series- a series of compounds related to each other eg. the alcohol, alkanes
  • Alkanes- CnHn+2
  • Cycloalkanes- CnH2n
  • The first four alkanes are gases, 5-16 are colourless liquids and the rest are white waxy solids
  • Alkanes mix well with each other but not with water (alkanes contain non-polar molecules whereas water does)
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13.2 Alcohols and Ethers

  • All alcohols have the hydroxyl functional group -OH
  • Alcohols with more than one hydroxyl group are known as polyhydric alcohols
  • Alcohol molecules are polar  because of the polarised O-H bond (like water). There is an attractive force between molecules due to hydrogen bonds.
  • The boiling points of alcohols are higher than those of corresponding alkanes with similar relative molecular mass because the hydrogen bonds need to be broken as well as the normal bonds, which requires more energy
  • The polarity means that alcohols and water mix
  • Ethers are derived from alkanes by substituting an alkoxy group (-OR). The longer hydrocarbon chain is chosen as the parent alkane
  • Ether molecules are only slightly polar and the attractive forces between molecules are relatively weak. No H-O bonds, so no hydrogen bonds.
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CS Hydrocarbons

  • Crude oil is a mix of many hydrocarbons
  • At the refinery, it is heated to vaporise it then the vapour passes through a distillation column. The less volatile hydrocarbons produced condense on the lower trays in the column (hotter). This is fractional distillation
  • Isomerisation- Where straight chain alkanes are broken down through heating with a catalyst to make branched chains. It is used to increase the octane number of petrol
  • Reforming- Where straight chain alkanes are converted into ring compounds (to cycloalkane, then aromatic hydrocarbons). Platinum is the catalyst (finely dispersed on aluminium oxide- platforming). Increases the octane number
  • Cracking- Where long-chain alkanes (cannot be used in petrol) are broken down to smaller-chain alkanes (often branched). Heavy oils (eg. gas oil) are heated in the presence of a catalyst to produce petrol. Called catalytic cracking. Increases octane number
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CS Octane Number and Volatility

  • In cars, petrol vapour mixes with air and is ignited
  • Petrol companies produce different blends of petrol for the seasons. In winter and cold countries, the petrol contains more volatile components (smaller alkanes), so it is easier to vaporise
  • Octane number is a measure of how likely a fuel is to auto-ignite, and cause a problem called knocking which damages the engine
  • Auto-ignition means that the mixture of air and petrol ignites prior to a spark being applied, it occurs due to high compression which causes heat
  • The higher the octane number (up to 100), the lower the tendency to auto-ignite
  • Shorter alkanes have higher octane numbers, so are less likely to auto-ignite. Adding more branched alkanes and anti-knocking additives (eg. lead) also raise the octane number
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CS Catalysts

  • Zeolites contain a network of pores and channels which provide a large surface area. They are widely used in industry as catalysts and molecular sieves (to sort out molecules by size and shape
  • Catalysts (made of platinum or rhodium) speed up reactions in exhaust systems that convert pollutants to less harmful substances. These catalysts are used in catalytic converters
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CS Pollutants

  • Evaporative emissions- hydrocarbon fumes caused by evaporation of petrol on warm day
  • Secondary pollutants- not formed directly by burning petrol, but by reacting with other things in the atmosphere
  • Carbon dioxide- Formed from combustion of hydrocarbons in petrol. Contributes to the greenhouse effect, too much can cause global warming
  • Carbon monoxide- Comes from incomplete combustion of hydrocarbons in petrol. Contributes to the greenhouse effect and is TOXIC to humans and animals
  • Unburnt hydrocarbons- From unburnt petrol. Makes up photochemical smog
  • NOx- Combination of N and O from air, formed when a spark is passed into engines. Are TOXIC, cause acid rain, damage the ozone layer and make up photochemical smog
  • SOx- From combustion of sulphur or sulphur compounds in petrol. Are TOXIC and cause acid rain
  • Particulates- Caused by incomplete combustion of carbon in petrol. Affect people with respiratory problems and cause buildings to turn black
  • Photochemical smogs are a mixture of different pollutants which cause haziness and reduced visibility in the air close to the ground
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CS Reducing Emissions

  • In order to reduce emissions, you can change the car technology, or change the fuel
  • Oxygenates are petrol additives which contain O (alcohols or ethers). They increase the octane number and reduce CO emissions. However, they are soluble, and can pollute water supplies
  • The stoichiometric ratio is the ratio of air to fuel
  • If you have less air, it is called a "rich" mixture. A "lean" mixture contains more air, and produces less NOx and CO (but more CxHy). If a fuel is too lean, the engine misfires and emissions increase. Lean fuels give better fuel economy
  • Catalysts can decrease certain emissions
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CS Other Fuels

  • Methanol- Burns cleanly so less CO due to incomplete combustion. Has a high octane number and does not released aromatic hydrocarbons into the atmosphere. However, does not mix with petrol (need co solvent), absorbs water and produces less energy per litre of fuel
  • Hydrogen- Plentiful supply, can be distributed as methane and can be made from electrolysis of water. However, it is hard to store the large volumes needed in gaseous form , must be stored as a liquid in tanks. The hydrogen economy would use H as a way of storing and distributing energy
  • Ethanol- Petrol additive with a high octane number. Produces less pollution (les CO, SO2 and NOx) and contributes fewer hydrocarbons to photochemical smog. However, overall the energy efficiency is questioned
  • Biodiesel- Biodegradable and produces lower emissions (less CO, CxHy, SOx and particulates, but more NOx) than regular diesel. Made from renewable sources (plants and animals) and is carbon neutral.  However, NOx emissions are higher, and is made from oils and fats which can be eaten
  • LPG- Cheaper than petrol, very safe, lead free and produces less emissions. However, less energy efficient and is difficult to supply as it is a gas (needs to be pressurized)
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