Fuels and Heat of Reactions PDF

R. Gallagher www.theconicalflask.ie Fuels and Heat of Reactions Hydrocarbons Hydrocarbons are defined as compounds containing only carbon and hydrogen. - Three classes of hydrocarbons: Alkan...

R. Gallagher www.theconicalflask.ie Fuels and Heat of Reactions Hydrocarbons Hydrocarbons are defined as compounds containing only carbon and hydrogen. - Three classes of hydrocarbons: Alkanes, Alkenes, Alkynes. Alkanes: - Consist of only single carbon-carbon bonds. - Compounds with only single carbon-carbon bonds are referred to as saturated. - All carbons are tetrahedral. - Boiling points: Low (Van Der Waals) but increase with molecular size due to increasing number of Van Der Waals forces. - The alkane family have many similar properties so we call them a homologous series. - Application: Fuel. A homologous series is defined as: A series of compounds of similar chemical properties Showing gradations in physical properties Having a general formula for its members Each member differing from the previous member by a CH2 unit. Alkenes: - Consist of one double carbon-carbon bond. Classified as unsaturated. - Boiling points: Low (Van Der Waals) but increase with molecular size due to increasing number of Van Der Waals forces. - Also a homologous series. - The C=C bond is planar. - Application: Ethene is used to ripen bananas. 39 R. Gallagher www.theconicalflask.ie Alkynes: - Consist of a triple bond. - Highly unsaturated. - Ethyne is the only alkyne you need to study. - Application: Welding and cutting (acetylene torches). Isomers Structural isomers are compounds with the same molecular formula but different structural formulas. - Alkanes: Remove a CH3 (methyl group) from end of molecule and place in the middle. - Alkenes: Place the double bond on a different carbon - Note: Watch the videos on The Conical Flask for a more in depth explanation. Aromatic compounds - Aliphatic compounds: Carbon atoms are joined together in straight chains. - Aromatic compounds: Contain a benzene ring in their structure. - Aromatic compounds have a fruity smell. Oil refining and its products - Fractional distillation is a process in which crude oil is separated into useful parts (fractions) by heating and separating due to their boiling points. - Smaller hydrocarbons: Low boiling points - separate at the top fractions - Larger hydrocarbons: High boiling points-separate at the bottom fractions. Refinery gases: methane, ethane, propane and butane. - Mercaptans (sulfur compound) are added for safety to provide a smell. - Liquefied Petroleum Gas (LPG): Propane and butane 40 R. Gallagher www.theconicalflask.ie Octane number The octane number of a fuel is a measure of the tendency of the fuel to resist knocking. - Auto ignition is a premature ignition of the petrol air mixture. The early ignition causes "knocking" in the engine and loss of power. - The higher the octane number the better the fuel. - Two reference hydrocarbons in relation to octane number: 2,2,4-trimethylpentane: 100. Heptane: 0 Factors that increase the octane number: - Short carbon-carbon chains. - Branching i.e. lots of alkyl groups. - Cyclic compounds. - Note: Tetraethyl lead increases the octane number but is not used anymore because it is toxic to humans and a catalytic converter poison. Increasing the octane number: 1. Isomerisation 2. Catalytic cracking 3. Dehydrocylisation 4. Adding oxygenates 1. Isomerisation: The changing of long straight chained alkane into its isomers - more branched. 2. Catalytic cracking: Splitting long chained hydrocarbons into short chain molecules using a catalyst. Note: An alkene is always produced. 3. Dehydrocyclisation: Forming of ring compounds, hydrogen gas is always given off. 4. Adding oxygenates: Adding methanol, MTBE, and ethanol are added. 41 R. Gallagher www.theconicalflask.ie Note: Adding oxygenates has an added advantage of less pollution. Hydrogen - Advantage: Hydrogen gas produces a lot of energy. It also burns cleaner than hydrocarbons. - Two methods to producing hydrogen. 1. Steam reforming a natural gas 2. Electrolysis of water. Steam reforming of a natural gas - Methane + steam = hydrogen. - CH4 + H2O 3H2 + CO Electrolysis of water Water + electricity = hydrogen H2O H2 + ½ O2 Problems with Hydrogen: - Difficult to store & transport - explosive - Not easily liquefied Physical properties of hydrocarbons 1. Boiling points: Van Der Waals resulting in low boiling points. Boiling points increase with molecular mass due to the increased number of Van Der Waal forces. 2. Solubility: Non-polar covalent resulting in only soluble in organic solvents (not polar substances e.g.water). - The rule of “like dissolves like” applies more with every CH, the molecule becomes more and more non-polar. 42 R. Gallagher www.theconicalflask.ie Heat of reaction: The heat change when a reaction takes place according to a balanced equation. - Exothermic reaction = ∆H negative (products have less energy than the reactants) - Endothermic reaction = ∆H positive (products have more energy than the reactants). Bomb calorimeter: Device used to measure the heats of combustion of fuels and foods. Bond energy: The average amount of energy to break bonds in the gaseous state. Heat of combustion: The heat change when 1 mole of a substance is burned completely in excess oxygen. Heat of formation: Heat energy when 1 mole of a compound is formed from its elements in their standard ground states. Hess’s law: The heat change for a reaction depends only on the initial and final states. Law of conservation of energy: Energy cannot be created or destroyed, but only changed from one form to another. Thermochemistry calculations - See The Conical Flask videos. 43 R. Gallagher www.theconicalflask.ie Sigma and Pi bonding - Covalent bonding can be represented in terms of Sigma and Pi bonding. Sigma: Head-on overlap of orbitals Pi: Sideways overlap of p-orbitals - Single bond: Sharing 1 pair of electrons (1 sigma bond) - Double bond: Sharing 2 pairs of electrons (1 sigma and 1 pi bond) - Triple bond: Sharing 3 pairs of electrons (1 sigma and 2 pi bonds) Bonding in benzene - Aromatic due to benzene ring. - Benzene is highly carcinogenic (cancer causing). - Should be reactive but actually very stable due to delocalisation - it doesn’t turn bromine water colourless. - Delocalisation is the sharing of electrons among several bonds. Represented by a circle inside a hexagon. Benzene Sigma and Pi bonds: - 6 pi electrons delocalised: Each double bond has 2 pi electrons - 12 sigma bonds electrons: 6 carbon to carbon single bonds & 6 carbon to hydrogen bonds. 44