Fuels and Heats of Reactions PDF

**[Fuels and Heats of Reactions]** \*\*Definition: Organic Chemistry is the study of the compounds of carbon.\*\* Carbon is unique among the elements as it can form stable rings and chains of its atoms. Almost all organic compounds are covalent compounds since carbon has the ability to form covale...

**[Fuels and Heats of Reactions]** \*\*Definition: Organic Chemistry is the study of the compounds of carbon.\*\* Carbon is unique among the elements as it can form stable rings and chains of its atoms. Almost all organic compounds are covalent compounds since carbon has the ability to form covalent bonds with itself and other elements. **[Hydrocarbons]** \*\*Definition: A hydrocarbon is a compound that contains carbon and hydrogen only.\*\* Common sources of hydrocarbons are coal, natural gas and petroleum. \*\*Definition: Fossil fuels are fuels that were formed from the remains of plants and animals that lived millions of years ago.\*\* **[The Alkanes]** The alkanes are a family of hydrocarbons in which all the atoms are linked by single bonds. They are said to be saturated compounds. \*\*Definition: A saturated compound is one in which there are only single bonds between the atoms in the molecule.\*\* Name Molecular Formula --------- ------------------- Methane CH~4~ Ethane C~2~H~6~ Propane C~3~H~8~ Butane C~4~H~10~ Pentane C~5~H~12~ Hexane C~6~H~14~ Heptane C~7~H~16~ Octane C~8~H~18~ Nonane C~9~H~20~ Decane C~10~H~22~ A table of chemical formulas Description automatically generated(1) \- All alkanes end in 'ane'.\ - They all follow the general formula C~n~H~2n+2~.\ - The first four members of the alkanes are gases, the next 12 are liquids and the higher members are waxy solids.\ - Soluble in non-polar solvents, e.g., cyclohexane\ - Insoluble in water because the alkanes are non-polar -- they only have Van-der-Waals forces between the molecules. \*\*Definition: 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 having a similar method of preparation and each member differing from the previous member by a (CH~2~) unit.\*\* **[Isomers]** \*\*Definitions: Structural isomers ae compounds with the same molecular formula but different structural formulas.\*\* (2) \- The two compounds here are both made up using 4 carbon atoms and 10 hydrogen atoms\ - However, they are arranged differently. They are different compounds with different properties because of this **[Naming of Alkanes]** \- The rules for naming organic compounds have been drawn up by the International Union of Pure and Applied Chemistry (IUPAC), they are more commonly known as systematic names.\ - In Alkanes, side-chains are called alkyl groups (CH~3~ is methyl, C~3~H~7~ is ethyl)\ - If two identical groups are present in a molecule, they get the prefix 'di-' Give the systematic name of the compound whose structural formula is shown. H H CH~3~ H H H - C -- C -- C -- C -- C -- H H CH~3~ H H H Identify the longest chain of carbon atoms. This is the parent alkane, The compound is named as its derivative of this. Since there are five carbon atoms in the chain, the word pent will be part of the name of this compound. Since this is a saturated hydrocarbon, the name will end in -ane. Number the carbon chain starting from the end that gives the substituents the lowest possible numbers Indicate the type and position of substituents. The name of the compound is 2, 3 dimethylpentane. **[The Alkenes]** Alkenes are a homologous series of hydrocarbons which contain one carbon-carbon double bond. Alkenes are said to be unsaturated. \*\*Definition: An unsaturated compound is one that contains one or more double or triple bonds between the atoms in the molecule.\*\* General formula for alkenes is C~n~H~2n~ +-----------------------+-----------------------+-----------------------+ | Name | Formula | Structural Formula | +=======================+=======================+=======================+ | Ethene | C~2~H~4~ | A diagram of a | | | | chemical formula | | | | Description | | | | automatically | | | | generated | +-----------------------+-----------------------+-----------------------+ | Propene | C~3~H~6~ |  | +-----------------------+-----------------------+-----------------------+ | Butene (isomerism | C~4~H~8~ | A diagram of a | | begins) | | chemical formula | | | | Description | | | | automatically | | | | generated\ | | | | But-1-ene | | | | | | | | \ | | | | But-2-ene\ | | | | A structure of a | | | | chemical formula | | | | Description | | | | automatically | | | | generated | +-----------------------+-----------------------+-----------------------+ Properties of Alkenes\ - Soluble in non-polar solvents, e.g. cyclohexane\ - Insoluble in water because the alkenes are non-polar -- they only have Van-der-Waals forces between the molecules\ - C1-C3 are gases, C4-C14 are liquids, C15 and above are waxy solids -- this is because the larger molecules have stronger Van-der-Waals forces, increasing their boiling and melting points Name the compound whose structural formula is given H CH~3~ H H H H H -- C -- C -- C -- C C -- C -- H H H H H Select the longest chain of carbons containing the c=c double bond. This is named after the parent alkane by changing ane to ene. Six carbons= hexene Number the chain starting at the end nearest to the c=c double bond. Double bond starts at carbon atom 2 Indicate the type and position of substituents\ Methyl group on C-5 = 5-methylhex-2-ene or 5-methyl-2-hexene **[Mandatory Experiment: To prepare ethene and examine its properties]** 1. Pour ethanol into the reaction boiling tube to a depth of about 2 cm. 2. Push in enough glass wool to soak up all of the ethanol. 3. Set up the apparatus, with about 2 g of aluminium oxide halfway along the boiling tube. 4. Heat the catalyst strongly, and occasionally heat the ethanol gently to drive the vapour over the catalyst. 5. Collect a few test tubes of ethene by displacement of water, stoppering the test tubes when they are filled. The first test tube filled can be discarded, as it contains a mixture of air and ethene. 6. When gas bubbles are no longer emerging from the delivery tube, remove the tube from the water, and then turn off the Bunsen burner. 1. Ignite the gas in one of the test tubes. Describe the flame (coloured or clear, smoky or clean). Pour a few drops of limewater into the test tube. Stopper, shake well and record what you see. 2. Add a few drops of diluted bromine water to the second test tube of gas. Stopper and shake well. Record what you see. 3. Add a few drops of acidified potassium manganate(VII) solution to a third test tube of the gas. Stopper immediately and shake well. Record what you see [Results\ ]- Ethene is a colourless gas with a sweet smell\ - Insolube in water but dissolves in some organic solvents\ - Gas burns with a yellow luminous flame which is slightly smoky\ - Limewater turns milky\ - Bromine water turns from red to colourless\ - KMnO~4~ turns from purple to colourless **[The Alkynes]** The alkynes contain a carbon-carbon triple bond. They are highly unsaturated. Named from the corresponding alkanes by changing the ending -ane to -yne.\ General formula for Alkynes is C~n~H~2n-2~ Name Formula Structural Formula -------- ---------- -------------------- Ethyne C~2~H~2~ H -- C C - H \- Ethyne is a gas that is used in oxyacetylene welding and cutting. Ethyne and oxygen burn at temperatures exceeding 3000^o^C. [Properties of Alkynes]\ - Soluble in non-polar solvents, e.g., cyclohexane.\ - Insoluble in water because the alkynes are non-polar -- they only have Van-der-Waals forces between the molecules. **[Mandatory Experiment: To prepare ethyne and examine its properties]** 1. Place 2-3 pieces of calcium dicarbide in a test tube 2. Add water from the teat-pipette, a few drops at a time, until all the calcium dicarbide has reacted. The gas produced is collected in test tubes by displacement of water, the test tubes being stoppered under water when filled with gas. The first test tube filled can be discarded, as it contains a mixture of air and ethyne. [Testing the properties of ethyne] 1. Ignite the gas in one of the test tubes. Describe the flame. Add a few drops of limewater to the test-tube and shake well. Describe what happens. 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. 3. Add a few drops of acidified potassium manganate solution to a test tube of gas, stopper quickly and shake well. Describe what you see. [Results] \- Ethyne burns with a luminous smoky flame and a great deal of soot is formed.\ - The colour of bromine water changes from red to colourless\ - KMnO~4~ changes from purple to colourless **[Aromatic Hydrocarbons]** \*\*Definition: An aliphatic compound is an organic compound that consists of open chains of carbon atoms and closed chain compounds (rings) that resemble them in chemical properties.\*\* A molecule of benzene (benzene ring) C~6~H~6~ \*\*Definition: Aromatic Compounds are compounds that contain a benzene ring structure in their molecules.\*\* \- Benzene is highly toxic and carcinogenic\ - Benzene is very unusual as even though benzene is unsaturated, it is not reactive at all\ - Benzene is stable because electrons in it are delocalised\ - Delocalised means that electrons are being shared by more than two atoms **[Oil refining and Its products]** [Fractional Distillation of Crude Oil] (3) Crude oil is one of the most valuable and useful materials found on the earth. It provides fuel for transport, heating and for the manufacture of a wide range of chemicals. \- Crude oil is, as is it is pumped from under the ground, of little use in itself. It is a thick black liquid with an unpleasant smell.\ - A process called fractional distillation is used to separate crude oil into a number of useful parts\ - Substances are separated as groups of compounds with similar boiling points. The groups are called fractions\ - Crude oil from various sources contains different proportions of the various fractions [Process of Fractional Distillation]\ - Hot, vaporised crude oil enters the bottom of the column\ - The column has a high temperature at the bottom and a low temperature at the top\ - Larger molecules have higher boiling points\ - Large molecules condense and are collected near the bottom of the column\ - Smaller molecules condense and are collected further up the column where it is cooler [Products of Fractional Distillation\ ]- Refinery gases such as methane, ethane, propane and butane can be used by the refinery as a source of fuel. Some are bottled for sale as domestic gas\ - Propane and Butane are easily liquified under pressure so they are often referred to as LPG (Liquefied petroleum gas)\ - As these gases are odourless, sulfur compounds called mercaptans are usually added to give them an unpleasant smell to warn of gas leaks\ - Petrol (light gasoline) is used as a motor fuel\ - Naphtha is useful to the petrochemical industry as source for medicine, plastics and synthetic fibres\ - Kerosene (paraffin oil) is used as domestic heating fuel and as fuel for aircraft\ - Diesel oil (gas oil) used as a fuel in trucks, buses, trains and some cars\ - Lubricating oil is a viscous liquid used as a lubricant to reduce the wear and tear of engines\ - Fuel oil is used in ships, power stations and heating plants\ - Bitumen is used to surface roads and for waterproofing and roofing Residue fractions -- Fractions left over when the more volatile fractions burn off e.g., lubricating oil, fuel oil and bitumen **[Octane Number]** \*\*Definition: The octane number of a fuel is a measure of the tendency of a fuel to resist knocking.\*\* \*\*Definition: Autoignition is premature ignition of the petrol-air mixture before normal ignition of the mixture by a spark takes place.\*\* The smooth running of an engine depends on the explosion occurring in the cylinder at exactly the right time. An early explosion is caused due to the air-petrol exploding as it is compressed rather than the explosion being caused by the spark. This is called knocking. The higher the octane number the better the fuel. There are two reference hydrocarbons in relation to octane number:\ - 2,2,4 - trimethylpentane = 100.\ - Heptane = 0 [Factors affecting Octane Number] \- The shorter the alkane chain, the higher the octane number\ - The more branched the chain, the higher the octane number\ - Cyclic compounds have a higher octane number than straight-chain compounds **[Making Petrol]** Lead used to be added to petrol to reduce engine knocking until it was found to have potential health hazards. The phasing out of lead products meant that new ways had to be found to increase the octane number of petrol [Isomerisation\ ]Changing straight-chain alkanes into their isomers using heat in the presence of a catalyst, this causes the chains to break Pentane (C~5~H~12~) 2-methylbutane (C~5~H~12~)\ Octane number = 62 Octane number = 93 [Catalytic Cracking\ ]\*\*Definition: Catalytic cracking is the breaking down of long-chain hydrocarbon molecules by the action of heat and catalysts into short-chain molecules for which there is greater demand.\*\*\ One of the products will always be an alkene 2,4- dimethylpentane 2-methylbut-1-ene [Dehydrocyclisation\ ]- Involves the use of catalysts to form ring compounds.\ - Hydrogen gas is always lost in this reaction. [Adding oxygenates\ ]- The addition of oxygen-containing compounds to a fuel in order to raise its octane number and make it burn "cleaner" as less carbon monoxide is formed when it is burned\ - Oxygenates include methanol, ethanol and methyl tertiary-butyl ether (MTBE) **[Hydrogen]** There are two methods for manufacturing the fuel Hydrogen gas (H~2~) [Steam Reforming of Natural Gas] \- Reacting methane (natural gas) with steam in the presence of a suitable catalyst\ CH~4~ + H~2~ O 3H~2~ + CO [Electrolysis of Water] \- This can be an expensive way of producing hydrogen due to the cost of electricity\ H~2~O H~2~ + ½ O~2\ ~- Hydrogen is used to produce ammonia (NH~3~)\ - Used to hydrogenate vegetable oils to produce margarine\ - Burned as a fuel because it is environmentally friendly Hydrogen forms an explosive mixture with air so it is difficult to store and transport **[Thermochemistry]** [Exothermic and Endothermic Reactions] \- Any reaction that produces heat is called an exothermic reaction.\ - Any reaction that takes in heat is called an endothermic reaction. The study of heat changes that accompany chemical reactions is called thermochemistry [Heat of Reaction\ ]\*\*Definition: Heat of reaction is the heat change when the numbers of moles of reactants indicated in the balanced equation react completely.\*\* ΔH (delta H) is used to indicate the heat change taking place when a chemical reaction occurs.\ - ΔH: the reaction is exothermic and gets hotter\ +ΔH: the reaction is endothermic and gets cooler [Heat of Combustion\ ]\*\*Definition: The heat of combustion of a substance is the heat change when one mole of the substance is completely burned in excess oxygen.\*\* Heats of combustion are accurately measured using a bomb calorimeter. A calorimeter is any container used for the determination of heat changes. (4)\ - A known mass of a substance whose heat of combustion is being measured is placed in the crucible inside the bomb\ - The bomb is filled with oxygen under pressure\ - The bomb is placed in a known quantity of water contained in the calorimeter\ - The substance is ignited electrically using an ignition coil of wire\ - The rise in temperature of the water is measured. The heat produced is calculated using the formula Heat produced = mass x specific heat capacity x rise in temperature\ - The heat produced when one mole of the substance is completely burned in excess oxygen is then calculated \*\*Definition: The kilogram calorific value of a fuel is the heat energy produced when 1 kg of the fuel is completely burned in oxygen.\*\*\ - Used to compare the efficiencies of various fuels [Bond Energy]\ \*\*Definition: Bond energy is the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other.\*\*\ - When a chemical reaction occurs, bonds are broken and new bonds are formed. - Energy is required to break a bond and energy is released when a bond is formed\ - By calculating how much energy is needed to break the bonds and how much energy is given off by making the new bonds, we can calculate ΔH [Heat of Neutralisation\ ]\*\*Definition: Heat of neutralisation is the heat change when one mole of H^+^ ions from an acid reacts with one mole of OH^-^ ions from a base.\*\*\ - Measured in kj mol^-1^\ - 1 mole of HCl will produce 1 mole of H^+^ ions when in solution **[Mandatory Experiment: To determine the heat of reaction of hydrochloric acid with sodium hydroxide]** 1. Using a graduated cylinder, place 50 cm^3^ of the 1 M hydrochloric acid solution into one of the polystyrene cups. 2. Using a second graduated cylinder, place 50 cm^3^ of the 1 M sodium hydroxide solution into the second polystyrene cup. 3. Measure the temperature of the hydrochloric acid solution. 4. Using a second thermometer, measure the temperature of the sodium hydroxide solution. 5. When both solutions are at the same temperature, quickly add the base to the acid, stirring well. Take care to avoid any loss of liquid due to splashing. 6. Place a lid on the polystyrene cup and with continuous swirling record the maximum temperature reached. 7. Summarise your results as follows: Temperature of HCl solution before mixing = ^o^C Temperature of NaOH solution before mixing = ^o^C Highest temperature reached after mixing = ^o^C Temperature rise = ^o^C 8. Now use the equation **heat liberated = m c ∆T** (where m = mass in kg of the solution, c is the specific heat capacity of the solution, and ∆T is the temperature rise) to calculate the heat liberated. 9. Calculate the heat of reaction, i.e., the heat liberated when one mole of acid reacts fully. Because the reaction is exothermic, your answer should be given a negative sign. [Heat of Formation\ ]\*\*Definition: The heat of formation of a compound is the heat change that takes place when one mole of a compound in its standard state is formed from its elements in their standard states.\*\* The heat of formation of water is -285.8 kj mol^-1^. This may be represented as follows:\ H~2(g)~ + 1/2 O~2(g)~ H~2~O~(l)~ ΔH = -285.8 kj mol^-1^ [Hess's Law\ ]\*\*Definition: Hess's Law states that if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage.\*\* Hess's Law may be used to calculate the unknown heat of reaction. Calculate the heat of formation of methane from the following data:\ (a) C~(s)~ + O~2(g)~ CO~2(g)~ ΔH = -393 kJ mol^-1\ ^(b) H~2(g)~ + ½ O~2(g)~ H~2~O ΔH = -286 kJ mol^-1^\ (c) CH~4(g)~ + 2O~2(g)~ CO~2(g)~ + 2H~2~O~(g)~ ΔH = -879 kJ mol^-1^ Write down the equation you require\ C~(s)~ + 2H~2(g)~ CH~4~ ΔH = ? Introduce C. \[C in in a above\]\ C~(s)~ + O~2(g)~ CO~2(g)~ ΔH = -393 kJ mol^-1^ Introduce 2H~2~. We have H~2~ in (b) above, therefore, multiply (b) x 2\ 2H~2(g)~ + O~2(g)~ 2H~2~O~(g)~ ΔH = -572 kJ mol^-^ Get the left hand side of the required equation by adding (a)\ C~(s)~ + O~2(g)~ CO~2(g)~ ΔH = -393 kJ mol^-1\ ^[2H~2(g)~ + O~2(g)~ 2H~2~O~(g)~ ΔH = -572 kJ mol^-^]^1^\ C~(s)~ + 2H~2(g)~ + 2O~2(g)~ CO~2(g)~ + 2H~2~O~(g)~ ΔH = -965 kJ mol^-1^ Introduce the CH~4~. We have CH~4~ on the left hand side of equation (c) We need it on the right hand side so reverse the equation\ CO~2(g)~ + 2H~2~O~(g)~ CH~4(g)~ + 2O~2(g)~ ΔH = +879 kJ mol^-1^ Now add (c) reversed to (d)\ C~(s)~ + 2H~2(g)~ + 2O~2(g)~ CO~2(g)~ + 2H~2~O~(g)~ ΔH = -965 kJ mol^-1\ ^[CO2(g) + 2H2O(g) CH4(g) + 2O2(g) ΔH = +879 kJ mol-1\ ]C~(s)~ + 2H~2(g)~ + ~~2O~2(g~~~~)~ + ~~CO~2(g)~~~ + ~~2H~2~O~(g)~~~ ~~CO~2(g)~~~ + ~~2H~2~O~(g~~~~)~ + CH~4~(g) + ~~2O~2(g)~~~ = ΔH -86 kJ mol^-1^ C~(s)~ + 2H~2(g)~ CH~(4)(g)~ CH~4(g)~ ΔH = -86 kJ mol^-1^ ΔH = -86 kJ mol^-1^ ^\ ^ **[Exam Questions]** [2014 -- HL -- Section B -- Question 6 ] 6\. The fuel in camping gas cylinders, like the one pictured on the right, is a liquefied mixture of propane, butane, and another compound which is a structural isomer of butane.\ (a) Name the homologous series to which propane and butane belong. Alkane\ Draw the structural formula of propane. CH~3~CH~2~CH~3~\ (b) Propane and butane have boiling points of --42.1 ºC and --0.5 ºC, respectively. Explain why propane has a lower boiling point than butane.\ - Propane is smaller with fewer electrons therefore weaker intermolecular forces between molecules.\ (c) (i) What is meant by saying that compounds are structural isomers?\ Compounds having the same molecular formula but different structural formulas.\ (ii) Draw the structural formula of the isomer of butane.\ \ (d) Define heat of combustion. Heat change when 1 mole of a substance is burned completely in excess oxygen\ Write the balanced equation for the complete combustion of butane in an adequate supply of oxygen.\ C~4~H1~0~ + 13/2 O~2~ → 4CO~2~ + 5H~2~O\ Calculate the heat of combustion of butane, given that the heats of formation of carbon dioxide, water and butane are --393.5, --285.8 and --125.7 kJ mol--1, respectively. 4C + 4O~2~ → 4CO~2~ ΔH = --1574 kJ\ 5H + 5/2 O~2~ → 5H~2~O ΔH = --1429 kJ\ C~4~H~10~ → 4C + 5H~2~ ΔH = 125.7 kJ\ C~4~H~10~ + 13/2 O~2~ → 4CO~2~ + 5H~2~O ΔH= --2877.3 kJ mol--1 [2011 -- HL -- Section B -- Question 6] Over 20% of the crude oil refined at Whitegate in Cork Harbour in 2010 was imported from Libya. Libyan crude oil is particularly valued because of its rich light gasoline and naphtha content. Social unrest in the Middle East early in 2011 has again highlighted Ireland's heavy dependence on oil as an energy source. Unstable supplies, the high cost of importing quality crude oil, and environmental issues focus attention on alternative energy sources including fuels, other than fossil fuels, e.g., hydrogen, and diesel derived from vegetable oil.\ (a) What is the nature of the chemicals that make up the bulk of crude oil? Hydrocarbons\ (b) Unprocessed crude oil, obtained by drilling on land or at sea, is not generally useful. Describe with the aid of a labelled diagram how crude oil is separated into useful substances in an oil refinery.\ - Crude oil introduced at bottom of fractionating column.\ - Vapour moves up through the series of trays.\ - Fractions come off through outlets Depending on their boiling points.\ Give the major use for the light gasoline and naphtha fractions of crude oil. Petrol\ (c) What is catalytic cracking?\ Splitting of long chain molecules to give short chain molecules\ Why is it carried out in oil refining? To give more useful products\ (d) Hydrogen gas can be obtained industrially by the reaction between natural gas and water in the form of steam (steam reforming).\ (i) Describe another method by which large quantities of hydrogen can be obtained from water. Electrolysis\ (ii) State one disadvantage of using hydrogen as a fuel. Difficult to store and transport **[References]** 1. Aplustopper.com 2. Slideplayer.com 3. Shalom-education.com 4. Toppr.com