Example A Graphite has the unusual property for a non-metal of being a very good conductor of electricity; diamond does not conduct electricity.

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1 Unit 12.4 arbon ompounds Topic 1: Properties and uses of carbon Topic 1 covers the properties and uses of carbon: State and appearance at room temperature. Allotropes of carbon structure, physical properties and uses. Reaction with oxygen. arbon cycle. arbon as fuel. arbon as an industrial reducing agent. Properties and reactions of carbon At room temperature, carbon is a solid. There are three allotropes of carbon graphite, diamond and buckminsterfullerene (buckyball) which have quite different appearances and properties. Example A Graphite has the unusual property for a non-metal of being a very good conductor of electricity; diamond does not conduct electricity. The two most common allotropes are graphite and diamond. Graphite is the most stable form of carbon. Property Physical state at room temperature Appearance Hardness Properties of carbon Allotrope of carbon Graphite Solid; very high melting point Grey-black; shiny Very soft; used in pencils to leave a trace on paper Diamond Solid; very high melting point lear and dull unless faces cut to reflect light Very hard; hardest naturally occurring substance Electrical conductivity Very good Nil Solubility in water Insoluble Insoluble Reaction with oxygen Both allotropes: Produce carbon dioxide when heated in plenty of oxygen: (s) + O 2 (g) O 2 (g) Produce carbon monoxide when heated in limited supply of oxygen: 2(s) + O 2 (g) 2O(g)

2 172 Unit 12.4 arbon ompounds Structures and uses of carbon allotropes The carbon atom has four electrons available to form covalent bonds. Diamond In diamond, each carbon atom is covalently bonded to four other carbon atoms. This arrangement is continuous, ie a diamond crystal is an extended arrangement of carbon atoms in three dimensions. The very strong, rigid, 3-D network of carbon atoms in diamond makes it the hardest naturally occurring substance. carbon atom strong covalent bond The arrangement of atoms in diamond Diamond is used: For jewellery the hardness of diamond allows faces to be cut into the surface. Example B A skilful jeweller can make the faces of a cut diamond reflect so much light that the diamond sparkles (an uncut diamond has the appearance of dull glass). To cut or grind all other materials. Example Diamond is used to cut glass and as the tips of drills used in mining.

3 Topic 1: Properties and uses of carbon 173 Graphite In graphite, only three electrons per carbon atom are involved in covalent bonding. The fourth electron of each carbon atom bonds very weakly with carbon atoms in layers above and below the layer containing the carbon atom under consideration. The layers can slide over each other (slipperiness) and the loose electron is able to flow through the graphite as an electric charge. Graphite is a good conductor of electricity. strong covalent bonds weak bonds between the layers Structure showing one carbon atom joined to three others, with all atoms in the same plane produces a layer structure Layer of hexagonal rings with a carbon atom at each corner Layer of hexagonal rings with a carbon atom at each corner A piece of natural graphite is dark grey-black, it shines and feels very slippery. The arrangement of carbon atoms in graphite The structure of graphite explains why graphite: Has a slippery texture the layers can slide over each other. Graphite mixed with clay is used in lead pencils. Example D A 7B pencil contains pure graphite and is very soft. A 7H pencil has a large percentage of clay and very little graphite, and is very hard. Between these two extremes, pencils have a range of hardness, with HB in the middle. Is a good conductor of electricity the loose electron is able to flow through the graphite under an electrical potential. Graphite is used as an electrical conductor in electrolysis and electrochemical cells. Example E Graphite is used as the electrodes in the commercial cells to produce aluminium metal. Has a very high melting point. The multiple covalent bonding in the layers of graphite requires a large amount of energy for the network to be broken.

4 174 Unit 12.4 arbon ompounds Example F The cell temperature to produce aluminium metal is 1000 o but the melting point of graphite used for the electrodes is nearly 4000 o. Is stable to most chemicals. Example G Magnesium metal is produced commercially by the electrolysis of molten magnesium chloride. The chlorine that is produced at the graphite anode, although very hot and therefore extremely active, does not react with the graphite anode. Buckminsterfullerene Buckminsterfullerene has a molecular formula of 60 and was discovered in The molecule is hollow and nearly spherical, with 12 pentagonal (five-sided) faces and 20 hexagonal (six-sided) faces. Example H Buckminsterfullerene is named after architect Richard Buckminster Fuller, who designed geodesic domes (light structural frameworks arranged as a set of polygons in the form of a shell). Because of the likeness to the shape and form of a soccer ball, buckminsterfullerene is commonly referred to as soccerene or footballene, or as a buckyball. Structure of buckminsterfullerene Research is developing the use of buckminsterfullerene and related compounds as: Superconductors. Non-metallic magnets.

5 Topic 1: Properties and uses of carbon 175 Unit 12.4 Activity 1A: Physical properties of carbon 1. Explain the meaning of each of the following terms. a. Hard. b. Insoluble. c. Shiny. d. Melting point. 2. Describe the appearance at room temperature of: a. diamond. b. graphite. 3. Explain why: a. diamond is very hard. b. graphite is very soft and slippery. 4. State three uses of carbon based on the physical properties of carbon. Explain the use in terms of the properties of the material. 5. The terms allotrope and isotope are often confused. Define both these terms and give examples of each, using the element carbon. arbon bonding arbon atoms can form strong covalent bonds with other carbon atoms. The consequences of this are: A chain of carbon atoms can be formed, of very great length and strength, using two of the four possible covalent bonds per carbon atom. Molecules with long chains of carbon atoms form the basic structure of all living species. Each carbon atom in the chain has two more bonds it can form with other atoms. Many different elements can have atoms attached to the chain, producing a huge variety of large molecules resulting in a colossal variety of species in the living world. The carbon cycle arbon is continually being recycled in the living world this is known as the carbon cycle. The carbon cycle

6 176 Unit 12.4 arbon ompounds There are two main processes involved in the carbon cycle photosynthesis and respiration. Photosynthesis requires sunlight and the presence of chlorophyll in the leaves of plants to produce glucose and oxygen from carbon dioxide and water, ie: carbon dioxide + water glucose + oxygen. Photosynthesis is an endothermic process it is the energy-absorbing (from the sun) process of the living world. Respiration is an exothermic process it is the energy-supplying process for the living world, ie: glucose + oxygen carbon dioxide + water. Photosynthesis and respiration are the reverse of each other the two processes work together to keep the amount of carbon dioxide in the atmosphere at about 0.03%. Uses of carbon arbon as a fuel arbon is contained in fuels such as coal, coke, petroleum and natural gas. oal is formed from dead vegetation trapped in swamps, then covered with earth and subjected to intense heat and pressure over millions of years. urrent estimates of coal reserves are 1 trillion tonnes ( tonnes), which are predicted to last for approximately 235 years at current usage. oal can be processed into coke a purer form of carbon from which gases and oils in coal have been removed. Petroleum and natural gas are derived from millions of sea organisms (eg shellfish) that, on dying, fell to the seabed and were covered with silt from rivers. Subjected to intense pressure and heat over millions of years, the products are a thick black liquid and gases that are contained below the Earth s surface by a hard, impervious cap rock. Petroleum reserves are estimated at million tonnes. The world is currently using 3252 million tonnes per year it is estimated that the petroleum reserves will last 43 years and natural gas reserves will last 66 years. When fuels containing carbon are burnt, an exothermic chemical reaction occurs, producing carbon dioxide: (s) + O 2 (g) O 2 (g) Billions of tonnes of coal and coke are burnt every year commercially and domestically to produce heat energy. omplete and incomplete combustion If insufficient oxygen is available, the carbon dioxide will react with excess hot carbon to produce poisonous carbon monoxide, O(g). This reaction is called incomplete combustion, ie: O 2 (g) + (s) 2O(g) Some of the heat energy released in the formation of the carbon dioxide is lost in this endothermic reaction.

7 Topic 1: Properties and uses of carbon 177 If more oxygen becomes available, the carbon monoxide formed will burn and carbon dioxide will be produced, again with heat energy being released: 2O(g) + O 2 (g) 2O 2 (g) This reaction is called complete combustion. Example I The charcoal brazier In a charcoal brazier the combustion of carbon proceeds through various stages, depending on the amount of oxygen available. Starting from the bottom, these stages are: formation of carbon dioxide conversion of carbon dioxide to carbon monoxide conversion of carbon monoxide to carbon dioxide release of carbon dioxide to the atmosphere. blue flames lumps of charcoal steel brazier air (oxygen enters) plenty of oxygen little or no oxygen plenty of oxygen ombustion in a charcoal brazier arbon monoxide reacts with oxygen to produce carbon dioxide and heat energy (blue flames are seen): 2O(g) + O 2 (g) 2O 2 (g) Hot carbon reacts with carbon dioxide to produce carbon monoxide, and heat energy is absorbed: (s) + O 2 (g) 2O(g) arbon combines with oxygen to produce carbon dioxide and heat energy: (s) + O 2 (g) O 2 (g) arbon as a reducing agent Some metals are produced by reduction (the removal of oxygen from a compound) of their oxides. In these reactions, carbon is used as the reducing agent.

8 178 Unit 12.4 arbon ompounds Example J Reduction of metal oxides opper(ii) oxide: uo(s) + (s) u(s) + O(g), or 2uO(s) + (s) 2u(s) + O 2 (g) Lead oxide: Iron oxide: Zinc oxide: PbO(s) + (s) Pb(s) + O(g), or 2PbO(s) + (s) 2Pb(s) + O 2 (g) Fe 3 O 4 (s) + 2(s) 3Fe(s) + 2O 2 (g) 2ZnO(s) + (s) 2Zn(s) + O 2 (g) Metal carbides arbon can combine with metals, especially iron, to produce carbides. Example K arbon (as coal or coke) is used in the extraction of iron from its oxide. After cooling, the solid product is not pure iron it contains some iron carbide. If the product contains 2.5 5% carbon, it is a brittle but hard material called cast iron. Today, almost all iron produced contains between 0.5% and 2% carbon and is called steel. Other types of steel can be produced by mixing different metals with the iron to produce, for example, stainless steel (contains chromium and nickel) and acid-resistant steel (contains silicon). Unit 12.4 Activity 1B: arbon in the living world, as a fuel and as a reducing agent 1. For the element carbon, name two sources of the element of commercial value and give some indication of the abundance of the source on planet Earth, eg large, moderate or rare. 2. Supply the missing words to describe the following process (words may be used more than once in the answer the number of letters in the words is shown by the dashes): oal heated in the absence of air produces, gases and oils. is a purer form of the element than coal. 3. omplete the following equations. If the equation is in words, then use words to complete the equation. If the equation is in symbols, then use symbols to complete the equation. The number of letters in the words or symbols is shown by the dashes. a. arbon + oxygen (two words). b. arbon dioxide + water + (photosynthesis). c. ZnO(s) + (s) Zn(s)+ (g). 4. Explain the meaning of the underlined words in the following passage. arbon can be used as a fuel. It will combine with oxygen exothermically to produce carbon dioxide. oal is a mineral (naturally occurring substance that is mined) that is mainly carbon. Other minerals of carbon that are fuels are petroleum and natural gas.