8.5 Energy

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 Photosynthesis 6CO2(g) + 6H2O(l) C6H12O6(aq) + 6O2(g) This reaction is endothermic, absorbing 2830 kJ/mol of glucose formed. light Light energy Autotrophs make their own food by photosynthesis. Using chlorophyll, they convert light energy to chemical energy.

3. Carbohydrates Cx(H20)y Photosynthesis is often written as the production of glucose (C6H12O6). 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) • Sucrose and maltose (disaccharides) • Starch, cellulose, glycogen (polysaccharides) Respiration - release of chemical energy C6H12O6 + O2  CO2 + H2O + energy Uses for respiration energy • Used directly for activities • Converted to protein for growth/repair of tissues • Stored as fat for energy reserves • Most is released as heat back to the environment

4. Energy released Decaying plants/animals CO2, H2O, nutrients decomposers Fossil Fuels 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. • Fossil Fuels • formed when dead and decaying material was buried before complete decomposition • Formed over millions of years due to heat and pressure beneath the Earth’s surface • Energy-rich compounds known as hydrocarbons • Chemical potential energy is released when burning in oxygen • Question: Where did the chemical potential energy in fossil fuels originate?

5. Fossil Fuels • Coal • 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 CO2) and hydrogen (as CH4) • Some impurities are sulphur and other inorganics • The sequence of production is: Buriedplant debris Peat (high H20) Brown coal Black coal heat heat heat pressure pressure pressure Increasing carbon content

6. Fossil Fuels • Petroleum • 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 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 (C3H8) and butane (C4H10) which are liquefied to produce LPG (liquefied petroleum gas)

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

9. C 12 6 Carbon • Carbon is in Group IV of the periodic table • Carbon has 4 valence electrons • Carbon can form 4 covalent bonds • Carbon can form single, double and triple bonds with a wide variety of elements forming nearly ten million known compounds 1s2 2s2 2p2

10. Carbon atom Hydrogen atoms 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). Methane CH4 Carbon atom Chlorine atoms CarbontetrachlorideCCl4

11. H H H H C C H H H H C C H H H H C C Carbon Bonding Structural Formula Molecular Formula Single bond - ethane C2H6 Double bond - ethene C2H4 Triple bond – ethyne (acetylene) C2H2

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

13. Diamond 3-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. Uses Glass cutting Saw blades Dentist’s drills Graphite Thin 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. Graphite also conducts electricity Uses The “lead” in pencils A solid lubricant such as in car door catches Electrodes in batteries Carbon Properties & Uses

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

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? Source:http://mypchem.com/myp8/y8sow.htm A fractional distillation setup.

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 – CnH2n+2 Straight-chain alkanes – carbons joined together to form a single chain with no branching. Structural formulae Propane Butane Methane Ethane

18. CH2 CH2 CH3 CH CH3 CH2 CH2 CH3 CH CH2 CH2 CH CH2 CH Hydrocarbon nomenclature Alkenes– hydrocarbons that contain one double bond between two carbon atoms. Formula – CnH2n 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. Condensed structural formulae 1-butene 1-pentene 2-pentene (NOT 3-pentene)

19. Hydrocarbon nomenclature Alkynes – hydrocarbons that contain one triple bond between two carbon atoms. Formula – CnH2n-2 As with alkenes, a numerical prefix indicates the location of the triple bond.

20. Properties of alkanes/alkenes

21. Vapour Petrol Liquid 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. The weaker the intermolecular forces, the lower the boiling point and the greater the volatility. Volatile compounds have a tendency to move from liquid to gas

22. Safe Storage of alkanes 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: • Minimise the quantity of material to be stored • Store in cool place with good ventilation (flammable cabinet) • Avoid inhaling the vapours • Use in a fume hood • Keep away from sparks or naked flames • 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. Task: Find specific safety considerations for alkanes C1 to C8. Make a safety poster to put up in a chemical store room.