CARBON DIOXIDE. B] Heating metal carbonates and bicarbonates Metallic carbonates and bicarbonates decompose on strong heating to produce CO2 (a)

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Rosemary Shields
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1 Molecular formula CO2 Molecular mass 44 CARBON DIOXIDE DISCOVERY CO2 was discovered by Van Helmont in 1630 by burning charcoal in air. In 1775, Joseph Black prepared it by action of dilute acids on metals. Antoine Lavoisier studied the chemical composition of CO2 and called it acid carbonique. OCCURRENCE In nature, CO2 is present in both free state and combined state:- FREE STATE COMBINED STATE i. CO2 exists in earth's atmosphere as a trace gas at i. CO2 is present in metallic carbonates and a concentration of about 0.03%-0.04% by volume bicarbonates in earth s crust and sea shells ii. The main minerals containing CO2 are ii. CO2 is present in rocks of volcanic regions a. Dolomite: (MgCO3.CaCO3) b. Limestone: CaCO3 iii. CO2 is present as a dissolved gas in natural iii. CO2 is also present as carbonates in the water. It is fairly soluble in water. exoskeletons and endoskeletons of various animals PREPARATION OF CO2 CO2 can be prepared by the following methods:- A] Burning carbon or its compounds The burning of carbon (charcoal/coke) or carbon compounds produces CO2 (a) (b) B] Heating metal carbonates and bicarbonates Metallic carbonates and bicarbonates decompose on strong heating to produce CO2 (a) (b) C] Action of dilute acids on metal carbonates and bicarbonates Metallic carbonates and bicarbonates react with dilute acids to produce CO2 (a) ( 1 )

2 (b) (c) (d) D] LABORATORY PREPARATION OF CO2 CO2 is prepared in the laboratory by the action of dilute acid on metal carbonate. Reagents Used Chemical Reaction Procedure Observation Collection of CO2 1. Marble chips (CaCO3) it is cheap and easily available 2. Dil.HCl reacts with CaCO3 to form CaCl2. (H2SO4 is not used as it forms CaSO4 which is an insoluble salt and covers the marble chips which stops the reaction) CaCO3 + H2SO4 CaSO4 + H2O + CO2 CaCO3 + 2HCl CaCl2 + H2O + CO2 1. In a Woulfe s bottle, some marble chips are placed at the bottom. 2. From one mouth of the bottle, a thistle funnel is introduced from which dil.hcl is added to the marble chips such that the mouth of the thistle funnel is immersed in the acid. 3. From the other mouth of the bottle, a bent delivery tube is fitted which opens in a gas jar. 1. When dil.hcl is added to marble chips, a vigorous reaction takes place and brisk effervescence of CO2 is seen. 1. Few bubbles of CO2are allowed to escape first as they may contain impurities. 2. The gas is then collected through the delivery tube into a gas jar by upward displacement of air, as CO2 is heavier than air. (CO2 is not collected over H2O as it is slightly soluble in H2O) ( 2 )

3 PHYSICAL PROPERTIES OF CARBON DIOXIDE Colourless, odourless gas with a faint acidic taste. 1.5 times heavier than air. Soluble in water. Solubility increases with increases in pressure. It can be liquefied under pressure. When liquid carbon dioxide is cooled to -78 o C under normal pressure, it forms snow-white solid called dry ice. Dry ice is a sublimable substance and has a cooling effect. It is a non-poisonous gas but a person may die in an atmosphere of carbon dioxide due to lack of oxygen. CHEMICAL PROPERTIES OF CARBON DIOXIDE Carbon dioxide is neither combustible nor a supporter of combustion. Metals like sodium, potassium and magnesium continue to burn in presence of carbon dioxide. It turns moist blue litmus paper red i.e. it is acidic in nature. It dissolves in water to give carbonic acid. CO2 + H2O H2CO3 It reacts with alkalis to produce salt and water. 2NaOH + CO2 Na2CO3 + H2O 2KOH + CO2 K2CO3 + H2O Caustic soda (NaOH) and Caustic potash (KOH) are thus used to remove CO2 from atmosphere as they can absorb large amount of CO2. KOH has a higher absorbing capacity than NaOH. When excess of CO2 is passes through alkalis, a soluble bicarbonate is obtained. Na2CO3 + CO2 + H2O 2NaHCO3 K2CO3 + CO2 + H2O 2KHCO3 CO2 turns lime water milky this is a reaction between lime water [Ca(OH)2] solution and CO2. CO2 reacts with Ca(OH)2 to form insoluble CaCO3. Ca(OH)2 + CO2 CaCO3 + H2O When excess of CO2 is passed through the solution, the milkiness disappears as soluble Ca(HCO3)2 is formed. CaCO3 + H2O + CO2 Ca(HCO3)2 Metals like Mg, burn in the presence of CO2 to give magnesium oxide and free carbon (black). 2Mg + CO2 2MgO + C CO2 reacts with non-metals like carbon (in the form of red hot coke) to produce carbon monoxide, a highly poisonous gas. CO2 + C 2CO TESTS FOR CARBON DIOXIDE CO2 is a non-supporter of combustion a burning candle extinguishes if introduced in a gas jar containing CO2. ( 3 )

4 CO2 turns lime water milky this reaction is an excellent test for CO2. USES OF CARBON DIOXIDE Used in the manufacture of aerated drinks. The gas is dissolved in water under pressure to give soda water. When the pressure is released, the bottled gas escapes with a brisk effervescence that adds fizz to the drink. Common soft drinks are formed by dissolving carbon dioxide in a sugar solution. Used in refrigeration and preservation of food stuffs. Solid carbon dioxide is used as a coolant and a refrigerant in ships for preserving food articles like fruit, vegetable, meat etc. if grains are kept in an atmosphere of carbon dioxide they remain in good condition for a long period of time, without allowing insect attacks. Used in hospitals. Carbogen is used in artificial respiration. It is given to patients suffering from gas poisoning, pneumonia, drowning etc. Used in the manufacture of fertilizers. Urea is an important nitrogenous fertilizer. It is prepared when carbon dioxide and ammonia are heated at 200 C under high pressure. Used in preparation of chemicals. Example washing soda (sodium carbonate), baking soda (sodium bicarbonate). Used in baking industry. Carbon dioxide is used to make the dough rise and become light. Baking powder is a mixture of baking soda and potassium hydrogen tartrate. When it is added to the dough, the ingredients of baking soda react to release carbon dioxide. As the gas rises through the dough, spaces are formed, thus making the dough porous and the cakes and bread light, soft and spongy. Used in photosynthesis. Photosynthesis helps in maintaining the balance of carbon dioxide and oxygen in the atmosphere. Used in extinguishing a fire. Since carbon dioxide neither burns nor does it help in burning and also being heavier than air, it insulates the burning substance by cutting of the supply of oxygen. Fire extinguisher: Fire extinguishers are devices in which carbon dioxide is produced in different forms to be used as an extinguishing agent. The common types of fire extinguisher are (1) Soda acid fire extinguisher It contains sodium bicarbonate and sulphuric acid in separate chambers. The metallic cylinder is filled with two thirds with saturated solution of sodium bicarbonate. When the apparatus has to be used the cylinder is inverted and made to hit the floor. As a result, the glass bottle breaks and the two chemicals react to produce carbon dioxide gas. The gas comes out through the nozzle with a great force. It spreads over the fire and cuts off the supply of oxygen and hence the fire is extinguished. 2NaHCO3 + H2SO4 Na2SO4 + 2H2O + 2CO2 (2) Foam type fire extinguisher It contains aluminium sulphate and sodium bicarbonate in separate chambers. When needed, the two chemicals are made to mix to react to produce carbon dioxide and aluminium hydroxide which comes out in the form of foam through the nozzle. It is used to extinguish oil fed fires because the foam covers the oil as well as cuts off air supply to the fire. 6NaHCO3 + Al2(SO4)3 3Na2SO4 + 2 Al(OH)3 + 6CO2 (3) Liquid carbon dioxide fire extinguisher It is a modern and improvised type of extinguisher. Liquid carbon dioxide is stored in a steel cylinder under pressure. On opening the valve of the cylinder pressure falls and liquid carbon dioxide solidifies into white snow which is dry ice. ( 4 )

It can be put out both oil fed fires and electrical fires. Why the soda acid and the foam types of extinguisher cannot be used for fighting electrical fires?

As a result, it might generate an electric shock leading to short circuit and another fire.

5 It can be put out both oil fed fires and electrical fires. Why the soda acid and the foam types of extinguisher cannot be used for fighting electrical fires? In both, the solutions are prepared in water which conducts electricity. As a result, it might generate an electric shock leading to short circuit and another fire. Foam based extinguisher Soda acid extinguisher CARBON CYCLE The process by which the amount of carbon dioxide is maintained in the atmosphere is called Carbon Cycle. The amount of carbon dioxide in the atmosphere remains almost constant because of the addition of CO2 to the atmosphere and removal of CO2 from it. ( 5 )

6 Addition and removal of CO2 in atmosphere CO2 is added and removed from the atmosphere by various methods: - Addition of CO2 to atmosphere by Removal of CO2 from atmosphere by i. Respiration CO2 is released during respiration by human beings, animals and plants i. Photosynthesis CO2 is removed from the atmosphere by plants for ii. Combustion of fuels burning of fuels emits photosynthesis CO2 iii. Decay of organic matter decomposition of dead animals, plants and plant products results in release of CO2 iv. Volcanic eruptions volcanic activity adds to the CO2 in atmosphere v. Industrial activities cement and metal industries release substantial amount of CO2 in the atmosphere vi. Sea water CO2 is also given out by sea water if the percentage of the gas in the water increases ( 6 ) ii. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Dissolution of CO2 in water when the percentage of CO2 in the atmosphere increases GREENHOUSE EFFECT AND GLOBAL WARMING [A] Greenhouse Effect The earth receives heat energy from the sun in form of radiations. Some of these radiations are absorbed and the rest are reflected from the earth s surface. From the reflected radiations, some of them are trapped by CO2 present in air which make the earth warm. This is greenhouse effect or also known as blanket effect. Definition The trapping of the earth s radiated energy by CO2 present in the air to keep the earth warm is called greenhouse effect. Why is it called greenhouse effect? In colder regions, this principle of trapping the heat radiations is applied to grow plants in houses made of glass walls. The heat radiations from the sun can enter the house due to glass and get trapped in the house. Thus, keeping the house warm inside for plant growth. This effect is also responsible for heating up of a closed car parked under the sun for a very long time. [B] Global Warming Due to air pollution, the amount of CO2 in the air has increased. Also, the release of gases like nitrous oxide(n2o), ozone(o2), chlorofluorocarbons(cfcs), methane(ch4), etc. which can trap the heat radiations has also increased. These gases create a thermal blanket around the earth trapping more heat on increasing the greenhouse effect. This causes further warming of the earth known as global warming. Effects of Global Warming: - Rise in temperature The earth s temperature has risen by 0.5 o C in the past 100 years. Melting of glaciers The Gangotri glacier in the Himalayas has started melting. Melting of ice in the Polar Regions causes floods in coastal regions and islands. Ecological imbalance Global warming results in change in climatic patterns. It can also disturbs the rainfall pattern and indirectly affect forest and agriculture.

7 Climate change may also cause extinction of species. It may also result in change in the pattern of crop cultivation. INCREASED LEVELS OF CO2 IN AIR CO2 levels in air have increased due to various human activities like: Deforestation Plants help in removal of CO2 from air by photosynthesis, but deforestation contributes to accumulation of CO2 in atmosphere. Air-pollution burning of fuels, use of chemical weapons, emissions from industries lead to air pollution and in turn increase the CO2 levels in air. BALANCED CO2 LEVELS IN ATMOSPHERE The percentage of CO2 in atmosphere must be balanced to avoid the serious consequences of global warming and greenhouse effect. This can be achieved by: Growing more trees and plants. Using smokeless sources of energy like solar energy, biogas, etc. Using filters in chimneys of factories and power houses. Reducing consumption of fossil fuel. International treaties and agreements between the countries of the world to reduce emissions of greenhouse gases. E.g.: Kyoto Protocol, Montreal Protocol, etc ( 7 )

CHEMISTRY- AIR AND ATMOSPHERE CLASS VII

CHEMISTRY- AIR AND ATMOSPHERE CLASS VII Q1. Explain the following terms. 1. Catalyst: A catalyst is a chemical substance that alters the rate of a chemical reaction without undergoing any chemical change