CARBON

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CONTENTS Carbon: The king of Elements Allotropes of carbon Compounds of carbon

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UNIT 1 CARBON-THE KING OF ELEMENTS

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Carbon (from Latin carbo 'coal') is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up about 0.025 percent of Earth's crust.[15] Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years.Carbon is one of the few elements known since antiquity.

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1.1What is Carbon? • • • •

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Carbon is an element represented by C, it belongs to the 14th period in the periodic table. The atomic number of carbon is 6 and the atomic mass is 12.01gmol-1. Carbon is a nonmetal and tetravalent i.e has 4 electrons in the valence shell. According to the data, it is the seventeenth most abundant element found on earth. It is found in the minerals of most metals in the form of carbonates. Carbon is the 2nd most abundant element in the human body.

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1.2 Existence of Carbon •

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Carbon has three naturally occurring isotopes namely C-12, C-13 and C14. C-12 is the most abundant whereas C-14 is radioactive and used for carbon dating. Atomic carbon is short-lived but it is stabilised by allotropic forms. Carbon has three allotropes namely diamond, graphite and amorphous form. The properties of these allotropes are different due to the different arrangements of C atoms inside them. The most common oxidation state of carbon is +4. It can also form +2 and 0 oxidation states in inorganic compounds and carbonyls.

1.3 Properties of Carbon • •

The electronic configuration of C is 1s2 2s2 2p2. So it is tetravalent and can form four covalent bonds. Catenation is the special feature of carbon by which it can form longchain compounds. C-C bonds are non-polar and stronger. 6

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Carbon products are obtained by heating coal, natural gas or wood or bone at elevated temperatures in the presence of insufficient oxygen to allow combustion. Carbon reacts with oxygen to form carbon oxides at elevated temperatures and can steal oxygen from metal oxides to leave only the elemental metal. Most forms of carbon are comparatively unreactive. Most of the fuel is made up of carbon and its compounds. When carbon burns in the presence of oxygen, it produces heat and light. This process of burning carbon and its compounds to release energy is known as combustion. Carbon mainly takes part in the four main types of reaction such as oxidation reactions, addition reactions, substitution reactions and combustion reactions.

1.4 Uses of Carbon • •

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One of the most amazing properties of carbon is its ability to make long carbon chains and rings. This property of carbon is known as catenation. Carbon has many special abilities amongst one unique ability is that carbon forms pπ-pπ bonds which are nothing but double or triple bonds with itself and with other electronegative atoms like oxygen and nitrogen. Just because of these two properties of carbon i.e catenation and multiple bond formation, it has a number of allotropic forms.

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Carbon

Symbol

C

Atomic Number

6

Atomic Mass

12.011

Discovered

Lavoisier proposed carbon in 1789

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1.6. Carbon Cycle Carbon cycle is the process where carbon compounds are interchanged among the biosphere, geosphere, pedosphere, hydrosphere, and atmosphere of the earth.

1.7. Carbon Cycle Steps Following are the major steps involved in the process of the carbon cycle: 1. Carbon present in the atmosphere is absorbed by plants for photosynthesis. 2. These plants are then consumed by animals and carbon gets bioaccumulated into their bodies. 3. These animals and plants eventually die, and upon decomposing, carbon is released back into the atmosphere. 4. Some of the carbon that is not released back into the atmosphere eventually become fossil fuels. 5. These fossil fuels are then used for man-made activities, which pump more carbon back into the atmosphere. Also Read : CARBON CYCLE

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1.7 .Carbon Cycle Diagram The carbon cycle diagram below elaborates the flow of carbon along different paths.

Carbon Cycle diagram showing the flow of carbon, its sources and paths.

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1.8. Carbon Cycle on Land Carbon in the atmosphere is present in the form of carbon dioxide. Carbon enters the atmosphere through natural processes such as respiration and industrial applications such as burning fossil fuels. The process of photosynthesis involves the absorption of CO2 by plants to produce carbohydrates. The equation is as follows: CO2 + H2O + energy → (CH2O)n +O2 Carbon compounds are passed along the food chain from the producers to consumers. The majority of the carbon exists in the body in the form of carbon dioxide through respiration. The role of decomposers is to eat the dead organism and return the carbon from their body back into the atmosphere. The equation for this process is: (CH2O)n +O2 → CO2 + H2O

1.9.Oceanic Carbon Cycle This is essentially a carbon cycle but in the sea. Ecologically, oceans take in more carbon than it gives out. Hence, it is called a “carbon sink.” Marine animals convert carbon to calcium carbonate and this forms the raw building materials require to create hard shells, similar to the ones found in clams and oysters. When organisms with calcium carbonate shells die, their body decomposes, leaving behind their hard shells. These accumulate on the seafloor and are eventually broken down by the waves and compacted under enormous pressure, forming limestone. When these limestone rocks are exposed to air, they get weathered and the carbon is released back into the atmosphere as carbon dioxide.

1.10. Importance of Carbon Cycle Even though carbon dioxide is found in small traces in the atmosphere, it plays a vital role in balancing the energy and traps the long-wave radiations from the sun. Therefore, it acts like a blanket over the planet. If the carbon cycle is disturbed it will result in serious consequences such as climatic changes and global warming. Carbon is an integral component of every life form on earth. From proteins and lipids to even our DNA. Furthermore, all known life on earth is based on

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carbon. Hence, the carbon cycle, along with the nitrogen cycle and oxygen cycle, plays a vital role in the existence of life on earth.

EXTENDED ACTIVITIES 1.Write a note on carbon 2.Discuss about presence of carbon in daily life

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UNIT 2 ALLOTROPES OF CARBON

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Allotropes of Carbon The phenomenon by which an element can exist in more than one physical state is called allotropy. The allotropes of carbon can be categorized into two: • •

Amorphous Carbon Allotropes Crystalline Carbon Allotropes

2.1 What are Allotropes of Carbon? Carbon with atomic number 6 and represented by the symbol ‘C’ in the periodic table is one of the most influential elements we see around us. Carbon is one of the elements which shows allotropy. The allotropes of carbon can be either amorphous or crystalline (Diamond, Graphite). Carbon due to its capability of having variable oxidation states or coordination number makes carbon one of the few elements to have multiple numbers of allotropic forms. Carbon’s ability to catenate is another contributing factor. Thus, it leads to the formation of various allotropes of carbon.

2.2How many Carbon Allotropes are there? •

Diamond: It is extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. This allotrope of carbon is a poor electrical conductor and an excellent thermal conductor. • Lonsdaleite: These are also called hexagonal diamond. • Graphene: It is the basic structural element of other allotropes, nanotubes, charcoal, and fullerenes. • Q-carbon: These carbon allotropes are ferromagnetic, tough, and brilliant crystal structure that is harder and brighter than diamonds. • Graphite: It is a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices (graphene), which are then layered in sheets. • Linear acetylenic carbon (Carbyne) • Amorphous carbon • Fullerenes, including Buckminsterfullerene, also known as “buckyballs”, such as C60. • Carbon nanotubes: Allotropes of carbon with a cylindrical nanostructure. Let us now take a look into the more widely known allotropes of carbon: 14

2.3 Graphite It is also a pure form of carbon. This allotrope of carbon is composed of flat two-dimensional layers of carbon atoms which are arranged hexagonally. It is a soft, black and slippery solid. This property of graphite persists because it cleaves easily between the layers. In each layer, each C atom is linked to three C atoms via a C-C covalent bond. Each carbon here is sp2 hybridized. The fourth bond is formed as a pi bond. Since the π-electrons are delocalized, they are mobile and can conduct electricity. Graphite is of two forms: α and ß. In α form, the layers are arranged in the sequence of ABAB with the third layer exactly above the first layer. In the ß form, the layers are arranged as ABCABC.

2.4 Properties of Graphite: •

Since the layers are stacked over each other, this carbon allotrope can act as a lubricant. • It also has metallic lustre which helps in the conduction of electricity. It is a very good conductor of both heat and electricity • One of the most important properties of graphite is that it is used as a dry lubricant for machines at high temperature where we cannot use oil. • Graphite is used to make crucibles which have the property that they are inert to dilute acids as well as to alkalis. Note: In comparison to diamond, Graphite is thermodynamically more stable.

2.5 Structure of Carbon Allotrope (Graphite): Graphite has a unique honeycomb layered structure. Each layer is composed of planar hexagonal rings of carbon atoms in which carbon-carbon bond length within the layer is 141.5 picometers. Out of four carbon atoms three forms sigma bonds whereas the fourth carbon forms pi-bond. The layers in graphite are held together by Vander Waal forces. 15

VIDEO LINK: STRUCTURE OF GRAPHITE

2.6 Diamond It is the purest crystalline allotrope of carbon. It has a number of carbons, linked together tetrahedrally. Each tetrahedral unit consists of carbon bonded to four carbon atoms which are in turn bonded to other carbons. This gives rise to an allotrope of carbon having a three-dimensional arrangement of C-atoms. ⇒ Also Read: CHEMICAL BONDING Each carbon is sp3 hybridized and forms covalent bonds with four other carbon atoms at the corners of the tetrahedral structure.

2.7 Do you know why a Diamond is Hard? 16

It is hard because breaking a diamond crystal involves rupturing many strong covalent bonds. Breaking covalent bonds is no easy task. This property makes this carbon allotrope the hardest element on earth.

2.8 Physical Properties of Diamond • • • • • • • •

It is extremely hard It has a very high melting point It has a high relative density It is transparent to X-rays It has a high value of the refractive index It is a bad conductor of electricity It is a good conductor of heat It is insoluble in all solvents

VIDEO LINK :STRUCTURE OF DIAMOND

EXTENDED ACTIVITIES 1.Mention different allotropes of carbon 17

2.List out characteristics of allotropes of carbon

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UNIT 3 COMPOUNDS OF CARBON

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Carbon is very reactive. It forms a huge number of compounds with many other elements.

3.1 Carbondioxide Carbon dioxide is a colourless and non-flammable gas at normal temperature and pressure. Although much less abundant than nitrogen and oxygen in Earth's atmosphere, carbon dioxide is an important constituent of our planet's air. A molecule of carbon dioxide (CO2) is made up of one carbon atom and two oxygen atoms.

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Production On industrial scale it is produced by burning coke in excess of air. 2CO + O2 → 2CO2 ΔH = 394 kJ mol-1 Calcination of lime produces carbon dioxide as by product. CaCO3 → CaO + CO2 Carbon dioxide is prepared in laboratory by the action of dilute hydrochloric acid on metal carbonates. CaCO3 + 2HCl → CaCl2 + H2O + CO2

VIDEO LINK: PREPARATION OF CARBONDIOXIDE

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Properties It is a colourless, nonflammable gas and is heavier than air. Its critical temperature is 31⁰ C and can be readily liquefied. Carbon dioxide is a very stable compound. Even at 3100 K only 76 % decomposes to form carbon monoxide and oxygen. At still higher temperature it decomposes into carbon and oxygen.

Reducing behaviour: At elevated temperatures, it acts as a strong reducing agent. For example, CO2 + Mg → 2MgO + C

Water gas equilibrium: The equilibrium involved in the reaction between carbon dioxide and hydrogen, has many industrial applications and is called water gas equilibrium. CO2 + H2 ↔ CO + H2O Water gas Acidic behaviour: The aqueous solution of carbon dioxide is slightly acidic as it forms carbonic acid. CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

Uses of carbon dioxide 22

· Carbon dioxide is used to produce an inert atmosphere for chemical processing. ·

Biologically, it is important for photosynthesis.

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It is also used as fire extinguisher and as a propellent gas.

· It is used in the production of carbonated beverages and in the production of foam.

Greenhouse Effect “Greenhouse effect is the process by which radiations from the sun are absorbed by the greenhouse gases and not reflected back into space. This insulates the surface of the earth and prevents it from freezing.”

Greenhouse Gases “Greenhouse gases are the gases that absorb the infrared radiations and create a greenhouse effect. For eg., carbondioxide and chlorofluorocarbons.”

What is Global Warming? Global warming is the phenomenon of a gradual increase in the temperature near the earth’s surface. This phenomenon has been observed over the past one or two centuries. This change has disturbed the climatic pattern of the earth. However, the concept of global warming is quite controversial but the scientists have provided relevant data in support of the fact that the temperature of the earth is rising constantly. There are several causes of global warming, which have a negative effect on humans, plants and animals. These causes may be natural or might be the outcome of human activities. In order to curb the issues, it is very important to understand the negative impacts of global warming.

Man-made Causes of Global Warming Deforestation Plants are the main source of oxygen. They take in carbon dioxide and release oxygen thereby maintaining environmental balance. Forests are being depleted

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for many domestic and commercial purposes. This has led to an environmental imbalance, thereby giving rise to global warming. Use of Vehicles The use of vehicles, even for a very short distance results in various gaseous emissions. Vehicles burn fossil fuels which emit a large amount of carbon dioxide and other toxins into the atmosphere resulting in a temperature increase. Chlorofluorocarbon With the excessive use of air conditioners and refrigerators, humans have been adding CFCs into the environment which affects the atmospheric ozone layer. The ozone layer protects the earth surface from the harmful ultraviolet rays emitted by the sun. The CFCs have led to ozone layer depletion making way for the ultraviolet rays, thereby increasing the temperature of the earth. Industrial Development With the advent of industrialization, the temperature of the earth has been increasing rapidly. The harmful emissions from the factories add to the increasing temperature of the earth. In 2013, the Intergovernmental Panel for Climate Change reported that the increase in the global temperature between 1880 and 2012 has been 0.9 degrees Celsius. The increase is 1.1 degrees Celsius when compared to the pre-industrial mean temperature.

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VIDEO LINK: GLOBAL WARMING

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3.2 CARBONMONOXIDE

Carbon monoxide is a colourless gas and is represented as CO. It has one carbon atom which is covalently bonded to one oxygen atom. It is also known as Carbonic oxide or Carbon oxide. It is a toxic gas. It is odourless and has no taste. It is a poisonous gas that when inhaled causes asphyxiation and damage to the Central Nervous System. When it combines with haemoglobin it forms Carboxyhemoglobin. It is widely used as industrial fuel for various operations.

Properties of Carbon monoxide – CO CO

Carbon monoxide

Molecular Weight/ Molar Mass

28.010 g/mol

Density

789 kg/m3

Boiling Point

−191.5 °C

Melting Point

−205.02 °C

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CO Uses • • • • • •

It is used in the production of methanol It is used in the packaging of fresh meat products such as fish, beef, etc It is used as a strong reducing agent It is used in infrared lasers It is used in beverages and food like jam and cola to acidify them It is used to remove rust from the surface to metals.

EXTENDED ACTIVITIES Discuss the consequence of global warming What measures can be taken to avoid the production of carbon monoxide

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LEARNING OUTCOMES • Describe the peculiarities of carbon. • Describe the structure, carbon cycle, allotropes, global warming • Explain carbon dioxide and carbon monoxide

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