Fuels and Heats of Reaction

Transcription

1 Fuels and Heats of Reaction Organic chemistry involves the study of the structure, properties, composition, reactions, and preparation of chemical compounds consisting primarily of carbon and hydrogen, which may contain any number of other elements, including nitrogen, oxygen, the halogens as well as phosphorus, silicon and sulfur. Does not include CO; CO 2 ; Carbonates and Hydogencarbonates Hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon bonded covalently. Example Methane (CH 4 ) Hydrocarbons are referred to as consisting of a "backbone" or "skeleton" composed entirely of carbon and hydrogen and other bonded compounds, and lack a functional group that generally facilitates combustion. So : C 2 H 2 OH (ethanol) is not a hydrocarbon Classifications Saturated hydrocarbons (alkanes) are the most simple of the hydrocarbon species and are composed entirely of single bonds and are saturated with hydrogen Saturated hydrocarbons (alkanes) are the basis of petroleum fuels and are either found as linear or branched species of unlimited number. The general formula for saturated hydrocarbons is C n H 2n+2 (assuming non-cyclic structures). Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with one double bond are called alkenes, with the formula C n H 2n (assuming non-cyclic structures). Those containing triple bonds are called alkynes, with general formula C n H 2n-2. Cycloalkanes are hydrocarbons containing one or more carbon rings to which hydrogen atoms are attached. The general formula for a saturated hydrocarbon containing one ring is C n H 2n Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring. Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene). Sources Fossil Fuels ; Crude Oil ; Natural gas ; From Dead Marine Animals ; Coal ; Dead Wood

2 Uses The predominant use of hydrocarbons is as a combustible fuel source. Mixtures of volatile hydrocarbons are now used in preference to the chlorofluorocarbons as a propellant for aerosol sprays, due to chlorofluorocarbons impact on the ozone layer. Where do we find them in our lives Methane [1C] and ethane [2C] are gaseous at ambient temperatures and cannot be readily liquified by pressure alone. Propane [3C] is easily liquified, and exists in 'propane bottles' mostly as a liquid. Butane [4C] is so easily liquified that it provides a safe, volatile fuel for small pocket lighters. Pentane [5C] is a clear liquid at room temperature, commonly used in chemistry and industry as a powerful nearly odorless solvent. Hexane [6C] is also a widely used non-polar, non-aromatic solvent, as well as a significant fraction of common gasoline. The [6C] through [10C] alkanes, alkenes and isomeric cycloalkanes are the top components of gasoline, naptha, jet fuel and specialized industrial solvent mixtures. At the opposite extreme from [1C] methane lie the heavy tars that remain as the lowest fraction in a crude oil refining retort. They are collected and widely utilized as roofing compounds, pavement composition, wood preservatives (the creosote series) and as extremely high viscosity sheer-resisting liquids. Aliphatic & Aromatic Hydrocarbons Aliphatic Hydrocarbons with straight chains or Rings (not Benzene) Aromatic Hydrocarbons with Benzene type Rings Aliphatic Hydrocarbons Organic Chemistry is the study of carbon compounds (except CO 2, CO ; Carbonates and hydrogencarbonates)

3 Carbon has a facility to form chains and rings and there are more organic compounds than compounds of all the other elements put together So we classify them Aliphatic hydrocarbons are straight chain and branched chain hydrocarbons as well as those that contain rings...but not benzene

4 Ones that do contain benzene rings are called aromatic hydrocarbons Homologous Series We Simplify the study of organic compounds by grouping into them into families called homologous series Each family has a general formula that tells us the ratio of atoms in a given compound within the family Alkanes have the formula Formulas for each member of the series can be found by changing the value of n Alkenes Unsaturated chemical compound containing at least one carbon-to-carbon double bond. e.g. ethylene (C 2 H 4 ) Alkynes Each contain a carbon to carbon triple bond traditionally known as acetylenes or the acetylene series Definition A homologous series is a family of organic compounds with the same general formula. Similar chemical properties, and successive members differing by CH 2 Naming System The International Union of Pure and Applied Chemistry (IUPAC) Each Hydrocarbon has 2 parts Root Suffix Root longest carbon chain

5 First ten Roots No. Carbon Atoms Root No. Carbon Atoms Root 1 Meth- 6 Hex- 2 Eth- 7 Hept- 3 Prop- 8 Oct- 4 But- 9 Non- 5 Pent- 10 Dec- Suffix Depends on the homologous series Alkanes end with -ane Alkene end with -ene Alkyne end with yne Alkanes Aliphatic Hydrocarbons These do not contain ring structures

6 No. Carbon atoms Name Formula No. Carbon atoms Name Formula 1 Methane CH 4 5 Pentane C 5 H 12 2 Ethane C 2 H 6 6 Hexane C 6 H 14 3 Propane C 3 H 8 7 Heptane C 7 H 16 4 Butane C 4 H 10 8 Octane C 8 H 18 Formula shown is the molecular formula No information on the way the atoms are bonded or arranged Structural Formula gives us this information Single bonds Double Bonds = Triple Bonds =

7 Each carbon atoms has four separate single bonds and so the valency is said to be satisfied Straight chain Alkane molecues with 2 or more carbons are in fact zig zag shaped Alkanes are single bonded saturated molecules Are these the same molecule? Answer NO

8 They have the same number of carbons and the same number of hydrogen but they are constructed differently and so behave differently Structural Isomers Compounds that have the same molecular formula but a different structural formula Naming structural isomers is a little harder than straight forward hydrocarbons...here s how it goes No. Carbon atoms in the longest straight chain...gives the root 2. Groups attached...gives the prefix 3. Positions on the chain where the groups attach...gives the numbered prefix Naming structural isomers of Butane Naming structural isomers of Butane There are two : Butane itself and Methylpropane No. Carbon atoms in the longest straight chain is 3 therefore its a propane derivative The only Group attached is a Methyl group (CH 3 ) therefore its a methylpropane The Positions on the chain where the group is attached is at the second carbon so the name is : 2 - methylpropane Pentane Abbreviations We can write the formulas in a shortened version Atoms joined to the main chain are written immediately after the carbon to which they are joined CH 4 Grps like (CH 3 ) are written in brackets after the carbon CH 3 CH(CH 3 )CH 3

9 Physical Properties of Alkanes Alkane Boiling Point (K) State Methane (CH 4 ) 111 Gas Ethane (C 2 H 6 ) 144 Gas Propane (C 3 H 8 ) 231 Gas Butane (C 4 H 10 ) 237 Gas Pentane (C 5 H 12 ) 309 Liquid Hexane (C 6 H 14 ) 342 Liquid Heptane (C 7 H 16 ) 371 Liquid Octane (C 8 H 18 ) 399 Liquid Alkanes with more than 15 carbons are waxy solids Alkanes are non polar and therefore insoluble in H 2 O Soluble in non - Polar Solvents such as cyclohexane & methyl benzene Alkenes Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) Containing a carbon-carbon double bond. The first two are: Ethene C 2 H 4 and PropeneC 3 H 6 You can work out the formula of any of them using: C n H 2n

10 For example, The carbon to carbon double bond is the functional group or reactive portion of the molecule Molecules containing a double (or even triple) bond are unsaturated No. Carbons Name Mol Formula 2 Ethene C 2 H 4 3 Propene C 3 H 6 4 Butene C 4 H 8 Isomerism in the alkenes Structural isomerism All the alkenes with 4 or more carbon atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula. Example - Butene Use the models to build Butene Possibility 1 But 1 ene CH 2 =CHCH 2 CH 3

11 Possibility 2 But 2 ene CH 3 CH=CHCH 3 Possibility 3 2- Methyl - Propene CH2=C(CH 3 )CH 3 Physical Properties of alkenes Alkene Boiling Point (K) State Ethene (C 2 H 4 ) 168 Gas Propene (C 3 H 6 ) 225 Gas

12 Butene (C 4 H 8 ) 267 Gas Higher Alkenes are Liquid at normal temp Alkenes are non polar or only slightly polar & therefore are not very soluble in water Soluble in polar solvents such as cyclohexane and methylbenzene Alkynes Form the homologus series of Aliphatic hydrocarbons with the formula C n H 2n-2 Contain a C to C triple bond Ethyene C 2 H 2 Alkynes are unsaturated Physical Properties of alkynes Because they have low or even zero polarity they are virtually insoluble in water Soluble in non - Polar Solvents such as cyclohexane & methyl benzene Chemical Rxs of Ethyne Like most hydrocarbons Ethyne burns in air to form CO 2 and Water Vapour CO 2 can be detected using Limewater and ethyne decolourises bromine water and acidified KMnO 4 (potassium manganate(vii) solution)

13 Experiment Preparation and properties of ethyne Theory Calcium dicarbide reacts with water producing ethyne and calcium hydroxide: The gas usually contains hydrogen sulfide and phosphine The impurities, which cause an unpleasant odour, can be removed by bubbling the gas through acidified copper sulfate solution. Properties Ignite the gas in one of the test tubes. Describe the flame (Smoky,sooty). The smoky, sooty flame indicates that combustion is incomplete due to insufficient oxygen, and particles of carbon are evident. Add a few drops of limewater to the test-tube and shake well. Describe what happens. The carbon dioxide turns limewater milky indicating the presence of CO 2. Add a few drops of a solution of bromine water to a test tube of gas, stopper quickly and shake well. Describe what you see. Bromine Decolourises which indicates un-saturation.

14 Add a few drops of acidified potassium manganate solution to a test tube of gas, stopper quickly and shake well. Describe what you see.(decolourises) This also indicates unsaturation. Uses of Ethyne Commonly known as acetylene gas Burns in air with sooty flame because of carbon Mix with pure oxygen and the flame is clear and 3000 o C Useful for welding Aromatic Hydrocarbons Unsaturated molecules have a double or triple bond Special type of unsaturated compound called aromatic hydrocarbons Most important type is Benzene Benzene C 6 H 6 A carbon at each corner and a hydrogen attached to each carbon Each C-C bond varies between single and double Another Aromatic Hydrocarbon Cyclohexane (C 6 H 12 ) Also have six carbons in a ring but properties are very different All Carbon bonds are single

15 All aromatic hydrocarbons contain a benzene ring eg. Methylbenzene Hydrogen replaced by a methyl grp An organic Solvent In Ethylbenzene the hydrogen is replaced by C 2 H 5 Physical Properties of Aromatic Hydrocarbons Name Formula Boiling Pt (K) State Benzene C 6 H Liquid Mehtylbenzene C 6 H 5 CH Liquid Ethylbenzene C 6 H 5 CH 2 CH Liquid Aromatic Hydrocarbons of Lower relative molecular mass are liquid at room temp and those of higher relative molecular mass are solids at room temp Exothermic and Endothermic Reactions All chemical reactions involves bonds broken and made Energy is required to do this Either given out when a bond is made or used up when a bond is broken The energy change is usually heat related

16 If the amount of heat produced in forming bonds is greater than the energy used in breaking them then there is heat to spare and this is called an exothermic reaction Demonstration of an exothermic reaction If the amount of heat produced in forming bonds is less than the energy used in breaking them then there is no heat to spare and this is called an endothermic reaction Demonstration of an endothermic reaction An important fact of Alkanes (most hydrocarbons) is that they burn in air or oxygen Sufficient oxygen and they burn completely, give off heat and are therefore exothermic Alkanes are very suitable as fuels. Petrol is complex but mostly alkanes

17 Heat of Reaction The heat of reaction, ΔH, of a chemical reaction is the heat in Kilojoules released or absorbed when the number of moles of the reactants indicated in the balanced equation describing the reaction react completely 1 mole of hydrogen gas burned in oxygen to form water results in 242KJ of energy as heat Rx is exothermic and the energy is lost to the surroundings therefore the ΔH is negative ΔH = -242 KJ mol -1 Why is ΔH negative? If 2 moles of hydrogen gas burned in oxygen to form water results in 484KJ of energy as heat. Rx is exothermic and the energy is lost to the surroundings therefore the ΔH is negative. ΔH = -484 KJ mol -1 What this shows us is the relationship between the balanced equation and the heat as the amount of heat is directly affected by the number of moles of reactants The reverse is also true

18 1 mole of water broken and oxygen and hydrogen as produced. This Rx required 286KJ of energy to happen Rx is therefore endothermic and the ΔH is positive. ΔH = 286 KJ mol -1 Example cm 3 Nitric (HNO 3 ) react with 100cm 3 of 1 M Potassium Hydroxide (KOH) the temperature rises by 6.7Kelvins Question: Calc the heat of Rx described by the equation 100cm 3 Nitric (HNO 3 ) react with 100cm 3 of 1 M Potassium Hydroxide (KOH) the temperature rises by 6.7Kelvins Formula Heat Change = mc ΔH Where M = mass in Kg of the reactants C = specified heat capacity ΔH = Rise in temperature Assumptions Because the Solutions are dilute the density (m) and specific heat capacity (c) is taken as = water Density of 100cm 3 = 0.1 Kg There is 200cm 3 of reactants so density is = 0.2 Kg NOTE: Specific heat capacity of water = 4.2 KJ Kg -1 K -1 Formula

19 Therefore mc ΔH 0.2 x 4.2 x 6.7 = Kj No of moles of Nitric in 100cm 3 of 1M Nitric acid solution is calculated as follows No. of moles = volume in litres x molarity = 0.1 x 1 = 0.1 moles As 0.1 moles produces KJ of heat when reacted with KOH In the balanced equation (1 mole of each was used) so the ΔH = Kjmol -1 and as the rx is exothermic that number becomes moll -1 Bond Energy The energy of bonds can vary depending on the molecular environment = 425 KJ mol -1 needed for first C-H bond break in CH 4 but only = 335KJ mol -1 need for last three. Therefore we use average bond energies for handiness Heat of Combustion Heat of combustion of a substance is the heat change in kilojoules when 1 mole of the substance is completely burned in excess oxygen Called heat of combustion rather than heat of reaction because it is a specific reaction Example 1 mole of Hydrogen gas burned completely in in excess O 2 Water vapour is formed with 242KJ of heat energy released Write an equation for the heat of combustion Answer Balanced equation 242 KJ of heat released therefore Rx is exothermic ΔH = -242 KJ mol -1

20 The equation describes the complete combustion of 1 mole of Hydrogen Measuring Heats of combustion Use a Bomb Calorimeter Fuel Formula Heat of Combustion / KJ mol -1 Methane CH Propane C 3 H Hydrogen H Petrol (Octane) C Reminder There are not any school level gas law problems that I am aware of that use the Celsius temperature directly in the calculation. You can convert between Celsius and Kelvin like this: Kelvin = Celsius Often, the value of is used instead of We use 273 Kilogram calorific Values The amount of heat produced per unit mass is an important quantity It lets us know how much of any fuel type is necessary to produce a given amount of heat Kilogram Calorific Value of a fuel is defined as the heat energy produced by 1 KG of fuel Fuel KG calorific Value / KJ Kg -1 Coal 30,000 Kerosene 48,000 Fuel Oil 45,000 Wood 27,000 Ethanol 30,000

21 Heat of formation Definition The heat of formation is the heat change in kilojoules, when 1 mole of a substance is formed from its elements in their standard states When we study the formation of a compound then the heat of Rx is better known as the heat of formation Standard State is an element at 298K and Pa The standard state of hydrogen at 298K and Pa is H 2(g) Worked Example 1 mole H 2 O (L) formed from its standard states 286 KJ heat energy released Write an equation for the heat of formation 286 KJ of energy produced therefore Rx is exothermic and ΔH = -286 Kj mol -1 Heat of formation = -286 KJ mol -1 The Equation is : Law of conservation of energy Law of conservation of energy states that energy cannot be created or destroyed, but only changed from one form to another Some heat changes cannot be measured directly in the Lab But they can be calculated from the known heat changes of other reactions

22 Can be done because of the Law of conservation of energy...energy cannot be created or destroyed only changed Hess s law (of constant heat summation ) This is a Modified law of conservation of energy The heat change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which the reaction may occur Illustrated by looking at the formation of Sulfur Trioxide Reaction takes one step but can take two The total heat change is the same for 2 steps as it is for 1

23 Problems using Hess s Law Given equation The heat of Rx equals the sum of the heats of formation of the products of the Rx less the sum of heats of formation of the reactants of the Rx ΔH r = ΔH f [products] - ΔH f [reactants] Note: heat of formation of an element is 0 Kj mol -1 Example In this example all the heats of formation are given and we have to calculate a heat of combustion Calculate the heat of combustion of Methane Given that the heats of formation of Methane Gas (-74.4KJ mol -1 ), Carbon Dioxide gas (-393.5KJ mol -1 )and water liquid(-285.8kj mol -1 ) Answer ΔH =

24 ΔH f [products] - ΔH f [reactants] Explanation of formula Heat of Rx equals the sum of the heats of formation of the products of the Rx less the sum of heats of formation of the reactants of the Rx ΔH c (CH 4 ) = ΔH f [CO2] + 2 x ΔH f [H 2 O] - ΔH f [CH 4 ] - 2 x ΔH f [O 2 ] heats of formation of Methane Gas (-74.4KJ mol -1 ), Carbon Dioxide gas (-393.5KJ mol -1 )and water liquid (-285.8KJ mol -1 ) Insert information into formula ΔH c (CH 4 ) = (-393.5KJ mol -1 ) + 2 x (-285.8KJ mol -1 ) - (-74.4KJ mol -1 ) 2 x (0 KJ mol -1 ) ΔH c (CH 4 ) = KJ mol -1 Another Example In this example all the heat of combustion are given and all but one of the heats of formation which we have to calculate Heat of Combustion of ethanol = mol -1 Heats of formation of Carbon dioxide = mol -1 ; liquid water = mol -1 Calculate the heat of formation of ethanol

25 Answer ΔH c (C 2 H 5 OH) = 2 x ΔH f [CO2] + 3 x ΔH f [H 2 O] - ΔH f [C 2 H 5 OH] - 3 x ΔH f [O 2 ] Heat of Combustion of ethanol = mol -1 ; heats of formation of ethanol (? KJ mol -1 ), Carbon Dioxide gas (-393.5KJ mol -1 )and water liquid (-285.8KJ mol -1 ) mol -1 = 2 x (-393.5KJ mol -1 ) + 3 x (-285.8KJ mol -1 ) ΔH f (C 2 H 5 OH) 3 x (0 mol -1 ) KJ mol -1 = ΔH f (C 2 H 5 OH) 0 ΔH f (C 2 H 5 OH) = ΔH f (C 2 H 5 OH) = KJ mol -1 Oil Refining and its products Fractional Distillation Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them

26 Fractional Distillation of Crude Oil Fraction Boiling Range Carbon atoms per Molecule Typical uses Refinery Gas <300K 1-4 LPG Light Gasoline K 5 8 Petrol Naphtha K 8-10 Petrol & organic chemicals Kerosene K Central Heating Fuel, Jet Fuel Gas Oil K Diesel Fuel Fuel oil & Residue >625K >19 Power station fuel, bitumen, lubricating oil, greases Cracking Cracking hydrocarbons is when you break long chain hydrocarbons up into short ones Necessary in naphtha fraction Natural Gas 2 nd most important source of Alkanes after crude Oil Natural Gas is a mixture (85% CH 4 (methane)); 10% C 2 H 6 (ethane)); smaller amounts of Propane and Butane as well as trace amounts of CO 2, N 2, O 2, He Use: Odourless Domestic Fuel Mixed with smelly mercaptans to make it safe

27 Petrol and the Internal Combustion engine Petrol is a complex blend of hydrocarbons (100 compounds) Petrol and the Internal Combustion engine What happens in an engine Stroke 1 Petrol and air in Stroke 2 Compressed and ignited. Gases expand and push the piston Stroke 3 Power stroke crankshaft turns due to expanding gases Stroke 4 Exhaust stroke removes waste gases Knocking Auto Ignition When gases are compressed they heat up and this can cause auto ignition Meant to happen in a diesel engine where there is no spark plug but bad for a petrol engine...means the fuel is ignited before the engine is ready Octane Numbers Octane number is the Measure of the tendency of a fuel to auto ignite Lower the octane number the more likely That s why high octane fuels are better for your car The Octane Scale is based on Heptane (C 7 H 16 ) which has a very high tendency to ignite (Octane number 0) and 2,2,4-trimethylpentane which has a very low tendency to ignite (Octane number 100) A mix of both with 95% 2,2,4-trimethylpentane is said to have an Octane number of 95 Petrol with the same tendency to ignite as this mix is also given an octane number of 95 Additives Used to prevent knocking Example: Lead Tetraethyllead used since 1920 s Banned in ireland since 2000 Causes problems for the environment and produces poisons when used with a catalytic converter

28 Example: Oxygenates Alcohols and ethers prevent knocking by raising the octane number MTBE (methyl tertiary butyl ether) octane number = 118 High Octane Compounds Using a mix of high octane compounds also reduces knocking Compounds with certain structural features have High Octane numbers Branching Short chains Rings Low octane compounds can be turned into high octane compounds by Isomerisation Dehydrocyclisation Cracking All involve Catalysts Isomerisation Long chain alkanes heated in the presence of a catalyst will break the chain and it reforms as a branched compound Example Pentane (Oct 62) changes to 2,methyl butane (oct 93)

29 Dehydrocyclisation Involves a two step process 1. The formation of a ring compound Hexane (Oct 25) goes to cyclohexane (Oct 83) plus hydrogen Dehydrocyclisation 2. The process continues another step until an aromatic compound is formed (benzene) Octane number >100 and 3 more H 2 molecules are eliminated Catalytic Cracking Involves heavy oils (Kerosene) and gas oil heated to high temp The long straight chains break into smaller ones some saturated and some unsaturated Unsaturated products are used in polymer manufacture (plastics) Saturated molecules are usually branched alkanes and suitable for petrol Hydrogen Colourless, odourless gas, almost insoluble in water. Discovered by Henry Cavendish in 1776 Hydrogen is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane, Uses Manufacture of Ammonia Hydrogenation of Oils to make saturated fats Manufacture of Hydrochloric acid Potential as a fuel source Used to propel rockets into space By product is harmless Water Produced on large scale by Steam Reforming or electrolysis

30 Steam Reforming Methane is first reacted with steam The mixture is reacted with more steam and CO is oxidised to CO 2 making more hydrogen in the process Over 70% of methane is converted to Hydrogen Electrolysis of Water Expensive because lots of electricity required also water is not an electrolyte (will not conduct electricity) so Sulfuric acid is added Need to use inert electrodes such as carbon or platinum