1. 8.5 Energy Focus 1: Living organisms make compounds which are important sources of energy

2. The sun is the source of energy on Earth Sun Autotrophs make their own food by photosynthesis. Using chlorophyll, they convert light energy to chemical energy. Light energy Photosynthesis 6CO 2(g) + 6H 2 O (l)  C 6 H 12 O 6(aq) + 6O 2(g) This reaction is endothermic, absorbing 2830 kJ/mol of glucose formed. light

3. Carbohydrates C x (H 2 0) y Photosynthesis is often written as the production of glucose (C 6 H 12 O 6 ). However, glucose is only one of the many carbohydrates that are made by plants. Carbohydrates contain the chemical energy that is needed by living things. Common carbohydrates Glucose and fructose (monosaccharides)Glucose and fructose (monosaccharides) Sucrose and maltose (disaccharides)Sucrose and maltose (disaccharides) Starch, cellulose, glycogen (polysaccharides)Starch, cellulose, glycogen (polysaccharides) Respiration - release of chemical energy C 6 H 12 O 6 + O 2  CO 2 + H 2 O + energy Uses for respiration energy Used directly for activitiesUsed directly for activities Converted to protein for growth/repair of tissuesConverted to protein for growth/repair of tissues Stored as fat for energy reservesStored as fat for energy reserves Most is released as heat back to the environmentMost is released as heat back to the environment

4. Fossil Fuels Decaying plants/animals CO 2, H 2 O, nutrients decomposers Energy released Fossil Fuels formed when dead and decaying material was buried before complete decompositionformed when dead and decaying material was buried before complete decomposition Formed over millions of years due to heat and pressure beneath the Earth’s surfaceFormed over millions of years due to heat and pressure beneath the Earth’s surface Energy-rich compounds known as hydrocarbonsEnergy-rich compounds known as hydrocarbons Chemical potential energy is released when burning in oxygenChemical potential energy is released when burning in oxygen Question: Where did the chemical potential energy in fossil fuels originate? Normally when plants and animals die, decomposers (insects, worms and bacteria) help to break down the decaying material into carbon dioxide, water and other minerals.

5. Fossil Fuels CoalCoal –Often formed in swamps and mangroves –Plant material is anaerobically decomposed (i.e. without oxygen) by anaerobic bacteria –As more and more layers of material are deposited, carbon content increases –Temperature and pressure conditions reduce the amount of oxygen (as CO 2 ) and hydrogen (as CH 4 ) –Some impurities are sulphur and other inorganics The sequence of production is:The sequence of production is: Buriedplant Buried plantdebrisPeat (high H 2 0) BrowncoalBlackcoal heatheatheat pressurepressurepressure Increasing carbon content

6. Fossil Fuels PetroleumPetroleum –Mostly formed from the remains of buried aquatic organisms (e.g. plankton) broken down by anaerobic bacteria –They contain a mixture of hydrocarbons commonly known as ‘crude oil’ –Oil is deposited in porous sedimentary rocks and the less dense oil moves upwards unless blocked by impermeable rock –Most of Australia’s oil deposits are found offshore (e.g. the Gippsland Basin (Bass Strait) and the North-West Shelf (WA))

7. Fossil Fuels Natural GasNatural Gas –Mostly the remains of buried aquatic organisms –Often found in a trapped layer just above petroleum deposits –Often contains up to 90% methane –Also contains propane (C 3 H 8 ) and butane (C 4 H 10 ) which are liquefied to produce LPG (liquefied petroleum gas)

8. 8.5 Energy Focus 2: There is a wide variety of carbon compounds

9. Carbon Carbon is in Group IV of the periodic tableCarbon is in Group IV of the periodic table Carbon has 4 valence electronsCarbon has 4 valence electrons Carbon can form 4 covalent bondsCarbon can form 4 covalent bonds Carbon can form single, double and triple bonds with a wide variety of elements forming nearly ten million known compoundsCarbon can form single, double and triple bonds with a wide variety of elements forming nearly ten million known compounds 1s 2 2s 2 2p 2 C12 6

10. Tetravalent Carbon Because carbon has four valence electrons, it forms four bonds with other elements to make up a full valence shell of 8. All valence electrons are involved in bonding. This bonding leads to tetrahedral shapes when all of the bonds involved are single bonds. Hydrocarbons are made up of carbon bonded to hydrogen, but many elements can and do take the place of hydrogen. Common elements that bond to carbon are N, O, S and the halogens (e.g. Cl, F). Carbon atom Hydrogenatoms Methane CH 4 CarbontetrachlorideCCl 4 Carbon tetrachloride CCl 4 Carbon atom Chlorineatoms

11. Carbon Bonding C C HHHH HH C C H H H H C CHH Single bond - ethane Triple bond – ethyne (acetylene) Double bond - ethene C2H6C2H6C2H6C2H6 C2H2C2H2C2H2C2H2 C2H4C2H4C2H4C2H4 StructuralFormulaMolecularFormula

12. Forms of Carbon Allotropes of carbon Allotropes are forms of an element that have different properties. Carbon has four: 1.Amorphous – soot from incomplete burning of hydrocarbons consisting of shapeless particles 2.Graphite – thin sheets of six-sided carbon rings in layers held together by weak intermolecular forces 3.Diamond – crystalline covalent network substance 4.Buckminsterfullerene (‘bucky balls’) – contains 60 carbons with 5 and 6-carbon rings arranged in a structure similar to a soccer ball. http://www.bfi.org/node/351 www.teachmetuition.co.uk Graphite Bucky ball Diamond

13. Carbon Properties & Uses Diamond 3-D network of tetrahedral shapes3-D network of tetrahedral shapes Strong covalent bonds with tightly bound e- resulting in an extremely hard substance (the hardest known) with high mp/bp.Strong covalent bonds with tightly bound e- resulting in an extremely hard substance (the hardest known) with high mp/bp.Uses Glass cuttingGlass cutting Saw bladesSaw blades Dentist’s drillsDentist’s drillsGraphite Thin sheets of 6-carbon ringsThin sheets of 6-carbon rings Layers are held together by weak intermolecular forces leading to a very soft substance that easily turns to a fine slippery powder.Layers are held together by weak intermolecular forces leading to a very soft substance that easily turns to a fine slippery powder. Graphite also conducts electricityGraphite also conducts electricityUses The “lead” in pencilsThe “lead” in pencils A solid lubricant such as in car door catchesA solid lubricant such as in car door catches Electrodes in batteriesElectrodes in batteries

14. 8.5 Energy Focus 3: A variety of carbon compounds are extracted from organic sources

15. Fractional Distillation Recall that crude oil contains a mixture of hydrocarbons ranging from one carbon (C1) up to more than C24. Fractional distillation allows for these components or ‘fractions’ to be separated using a fractionating column. In this process, heat is applied to the bottom of the column and lighter compounds with lower boiling points rise to the top, while heavier compounds remain towards the bottom of the column. Source: http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsoil/oil_and_oilproductsrev5.shtml http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsoil/oil_and_oilproductsrev5.shtml

16. Fractional Distillation Investigation In the laboratory, you will perform a separation of ethanol and water using a fractionating column similar to the one on the right. How will you ensure that the final product is pure ethanol? A fractional distillation setup. Source: http://mypchem.com/myp8/y8sow.htmhttp://mypchem.com/myp8/y8sow.htm

17. Hydrocarbon nomenclature Alkanes - hydrocarbons that contain only single bonds. The table to the right shows the alkane homologous series (a family of compounds that have the same general formula). Formula – C n H 2n+2 Straight-chain alkanes – carbons joined together to form a single chain with no branching. Number of C Alkane1Methane 2Ethane 3Propane 4Butane 5Pentane 6Hexane 7Heptane 8Octane MethaneEthane PropaneButane Structural formulae

18. Hydrocarbon nomenclature Alkenes – hydrocarbons that contain one double bond between two carbon atoms. Formula – C n H 2n Isomers – compounds that have the same molecular formula, but different structure. Alkenes have isomers because the double bond can be in a different location above C4. The location of the double bond is indicated by a numerical prefix counting from the shortest end. Number of C Alkene 1NA 2Ethene 3Propene 4Butene 5Pentene 6Hexene 7Heptene 8Octene CH CH 3 CH 2 CH CH 3 CH 2 CH CH 3 CH CH 2 1-butene1-pentene2-pentene (NOT 3-pentene) Condensed structural formulae

19. Hydrocarbon nomenclature Alkynes – hydrocarbons that contain one triple bond between two carbon atoms. Formula – C n H 2n-2 Number of C Alkyne 1NA 2Ethyne 3Propyne 4Butyne 5Pentyne 6Hexyne 7Heptyne 8Octyne As with alkenes, a numerical prefix indicates the location of the triple bond.

20. Properties of alkanes/alkenes AlkanesAlkenes BP/MP increase with increased number of carbon atoms. C1 to C4 are gases at room temp. C5 to C17 are liquids. All above C18 are solid. BP/MP increase with increased number of carbon atoms. C2 to C4 are gases at room temp. C5 to C15 are liquids. All above C16 are solid. Weak dispersion forces that increase in strength with increasing molecular mass. (More e - = stronger forces) Similar to alkanes. Non-polar due to symmetrical shape and similarity of electronegativity of H and C. Similar to alkanes. Densities increase with increased molecular mass. All are less dense than water. (e.g. float on top) Similar to alkanes.

21. Volatility of hydrocarbons Liquid hydrocarbons such as petrol (C5-C12) tend to readily evaporate. This tendency is known as volatility. In a closed container, a dynamic equilibrium will be achieved between evaporation and condensation (i.e. same rate). Once dynamic equilibrium is established, a constant pressure is exerted on the container known as vapour pressure. Volatile compounds have a tendency to move from liquid to gas VapourLiquid Petrol The weaker the intermolecular forces, the lower the boiling point and the greater the volatility.

22. Safe Storage of alkanes Minimise the quantity of material to be storedMinimise the quantity of material to be stored Store in cool place with good ventilation (flammable cabinet)Store in cool place with good ventilation (flammable cabinet) Avoid inhaling the vapoursAvoid inhaling the vapours Use in a fume hoodUse in a fume hood Keep away from sparks or naked flamesKeep away from sparks or naked flames Store in approved containers (sturdy with narrow neck)Store in approved containers (sturdy with narrow neck) Gas cylinders should be regularly checked and stored outside, under cover in well-ventilated area. They should also be strapped to a permanent structure.Gas cylinders should be regularly checked and stored outside, under cover in well-ventilated area. They should also be strapped to a permanent structure. The weak intermolecular forces in low molecular weight alkanes results in extreme flammability. In addition, some alkanes are carcinogenic, so safe storage of alkanes (and many other hydrocarbons) needs to consider the following: Task: Find specific safety considerations for alkanes C1 to C8. Make a safety poster to put up in a chemical store room.

23. 8.5 Energy Focus 4: Combustion provides another opportunity to examine the conditions under which chemical reactions occur.

24. Chemical Reaction Indicators At least one new substance is formed in a chemical reactionAt least one new substance is formed in a chemical reaction A change in colour (not always a chemical reaction)A change in colour (not always a chemical reaction) A gas is given off (acid + carbonate produces CO 2 )A gas is given off (acid + carbonate produces CO 2 ) Heat is produced or absorbed (combustion)Heat is produced or absorbed (combustion) Light is given off (chemiluminescence)Light is given off (chemiluminescence) A precipitate forms (insoluble ionic compounds)A precipitate forms (insoluble ionic compounds) Acid + Carbonate Burning magnesium Glow Stick - Chemiluminescence Precipitation reaction

25. Combustion is exothermic Combustion is the process of burning Most often, the combustion of a material involves its combination with oxygen gas. Because combustion reactions release energy in the form of heat and light, they are exothermic. Fire in the Penola Forest (SA), Ash Wednesday 1983 Fire in the Penola Forest (SA), Ash Wednesday 1983 (Source: www.austehc.unimelb.edu.au/ fam/1611_image.html) www.austehc.unimelb.edu.au/ fam/1611_image.htmlwww.austehc.unimelb.edu.au/ fam/1611_image.html Example: The combustion of methane (natural gas) in oxygen: CH 4 + 2O 2  CO 2 + 2H 2 O ∆ H rxn = -832 kJ/mol (exothermic)

26. Combustion: why exothermic? Recall that: Energy is required to break bondsEnergy is required to break bonds Energy is released when bonds formEnergy is released when bonds form In the previous example involving the combustion of methane: CH 4 + 2O 2  CO 2 + 2H 2 O + 832kJ And remembering that: ∆H rxn = H (products) – H (reactants) or ∆H rxn = ∑ E (bonds broken) - ∑ E (bonds formed) ∆ H rxn = H (products) – H (reactants) or ∆ H rxn = ∑ E (bonds broken) - ∑ E (bonds formed) This reaction releases more energy than it absorbs resulting in a negative value for ∆Hrxn This reaction releases more energy than it absorbs resulting in a negative value for ∆ Hrxn Breaking bonds Absorbs energy Forming bonds Releases energy H reactants < H products Energy required to break bonds Energy released when bonds formed

27. BondBond energy, kJ/mol C–C347 C=C615 C≡C812 C–O360 C=O798 F–F158 Cl–Cl244 C–H414 H–H436 H–O464 O=O498 AVERAGE BOND ENERGIES OF COMMON BONDS Use these bond energy values to determine the ∆ Hrxn of methane combustion ∆H rxn = ∑ E (bonds broken) - ∑ E (bonds formed) ∆ H rxn = ∑ E (bonds broken) - ∑ E (bonds formed)

28. Combustion of organics Complete combustion METHANE (LAB GAS): CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (g) + 832kJ PROPANE (LPG): C 3 H 8(g) + 5O 2(g)  3CO 2(g) + 4H 2 O (g) + 1560kJ OCTANE (PETROL): C 8 H 18(l) + 25/2O 2(g)  8CO 2(g) + 9H 2 O (g) + 5460kJ CARBON (COKE): C (s) + O 2  CO 2(g) + 393kJ Note that all hydrocarbons produce carbon dioxide (a greenhouse gas) and water. These are the only products when there is sufficient oxygen (i.e. complete combustion). When the oxygen available is insufficient, incomplete combustion results in other pollutants. All exothermic

29. Combustion of organics Incomplete combustion As the quantity of oxygen decreases, combustion of hydrocarbons becomes more and more incomplete, leading to the production of the pollutants carbon monoxide and carbon (soot). Consider the hydrocarbon pentane: Complete:C 5 H 12(l) + 8O 2(g)  5CO 2(g) + 6H 2 O (g) Complete: C 5 H 12(l) + 8O 2(g)  5CO 2(g) + 6H 2 O (g) Incomplete (less O 2 ): C 5 H 12(l) + 6O 2(g)  4CO (g) + CO 2(g) + 6H 2 O (g) Incomplete (even less O 2 ): C 5 H 12(l) + 4O 2(g)  2CO (g) + 3C (s) + 6H 2 O (g) Incomplete combustion products Incomplete combustion of hydrocarbons can produce CO and C.

30. Other Combustion Pollutants Sulfur Dioxide/Trioxide Sulfur is an impurity in fossil fuels, mainly coal (up to 5%). When sulfur burns in air sulfur dioxide is produced: S (s) + O 2(g)  SO 2(g) Sulfur trioxide can be produced by further oxidation of SO 2 2SO 2(g) + O 2(g)  2SO 3(g) These two gases can combine with water in the atmosphere to produce acid rain: SO 2(g) + H 2 O (l)  H 2 SO 3(aq) (sulfurous acid) SO 3(g) + H 2 O (l)  H 2 SO 4(aq) (sulfuric acid) www.robl.w1.com Acid rain effects

31. Other Combustion Pollutants Oxides of Nitrogen (NOx) Nitrogen and oxygen in the air can react at high temperatures that are present inside car engines: N 2(g) + O 2(g)  2NO (g) 2NO (g) + O 2(g)  2NO 2(g) Nitrogen dioxide produces the brown haze known as photochemical smog. This causes respiratory problems in many people. In addition, NO 2 can react with UV light to produce toxic ozone O 3. NO 2(g) + UV  2NO 2(g) + O(g) O (g) + O 2(g)  O 3(g) www.birmingham.gov.uk Photochemical smog

32. Controlling Pollution Use low sulfur coal (Australian coal is relatively low in sulfur)Use low sulfur coal (Australian coal is relatively low in sulfur) Remove sulfur dioxide from effluent gas at power stations (very expensive)Remove sulfur dioxide from effluent gas at power stations (very expensive) Keep car engines tuned properly so that incomplete combustion is minimised.Keep car engines tuned properly so that incomplete combustion is minimised. Catalytic converters mounted to motor vehicle exhaust systems remove unburned hydrocarbons and oxides of nitrogen.Catalytic converters mounted to motor vehicle exhaust systems remove unburned hydrocarbons and oxides of nitrogen. 2NO (g) + 2CO (g)  N 2(g) + 2CO 2(g)

33. Energy changes in chemical reactions Exothermic reactions The enthalpy of the reactants is higher than the products. Reactants ReactantsProducts Endothermic reactions The enthalpy of the reactants is lower than the products. Products Products Reactants Reactants Enthalpy (H) ∆ ∆ H is -ve ∆ ∆ H is +ve Heat released Heat absorbed Reaction progression

34. Activation Energy Reaction progress Reactants Products enthalpy Activationenergy ∆ ∆ H The activation energy is the minimum amount of energy that is required by the reactants for the reaction to proceed to the products.

35. Catalysts Reaction progress Reactants Products enthalpy Activationenergy ∆ ∆ H A catalyst lowers the activation energy of a reaction, but does not change the overall ∆ H of the reaction. Catalysed path Lowered activation energy

36. Ignition Temperature The minimum temperature required by a substance to ignite. In order for a fuel to ignite, a certain amount of heat energy must be applied to the substance and the air that will be involved in the combustion. Not all of the particles must be heated to this temperature to start the reaction. Once the combustion reaction has started, the resulting release of energy provides sufficient temperatures for neighbouring particles to ignite. The higher the activation energy, the higher the ignition temperature Friction on the match head provides enough energy to ignite the sulfur in the match.

37. 8.5 Energy Focus 5: The rate of energy release is affected by factors such as types of reactants

38. Particle Collisions For a reaction to proceed to products, the reactants must collide with one another. Rate of reaction The rate of a reaction is the rate that reactants disappear and products form. As the reaction proceeds and reactants are used up, the rate decreases. Since the reacting particles must collide to react, increasing the rate of collisions increases the rate of reaction. Concentration of reactants Reaction progress  Concentration of products Reaction progress  Disappearance of REACTANTS Formation of PRODUCTS

39. http://boomeria.org/chemlectures/rates/rates.html

40. Rates of Combustion Slow combustion: the stove to the right is designed for the slow burning of fuels such as large pieces of coal or wood. vega.soi.city.ac.uk Fast combustion: the burning of methane in a Bunsen burner is a fast reaction. Explosive combustion: an explosion is the fastest combustion where all of the fuel is ignited almost instantly. encarta.msn.com www1.sedo.energy.wa.gov.au

41. Some Factors Affecting Reaction Rates TemperatureTemperature –Higher temperature means particles have more kinetic energy, leading to an increase in reaction rate. ConcentrationConcentration –Increasing the concentration of the reactants increases the chance of collisions, which increases the rate of reaction. Surface areaSurface area –Fine powders have more surface area than large pieces leading to more collisions. For this reason, fine powders create an explosion hazard when mixed with air CatalystsCatalysts –As previously stated, catalysts lower the activation energy, therefore increasing the rate of reaction

42. Catalysis: Industrial Examples Catalytic Converter: Rhodium and platinum catalysts are coated on a ceramic honeycomb block to remove unburnt hydrocarbons, nitric oxide and carbon monoxide from motor vehicles. www.chem.brown.edu The Haber Process: The production of ammonia from nitrogen and hydrogen gases is catalysed by an iron catalyst. N 2(g) + H 2(g)  NH 3(g) Fe cwx.prenhall.com/.../media_portfolio/14.html

43. 8.5 Energy Compiled by: Robert Slider (2006) Please share this resource with others