UNIT SUBJECT TOPIC SUB-TOPIC CLASS SIZE
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2 CHEMISTRY NITROGEN AND ITS COMPOUNDS AMMONIA S.3 50 Students
Introduction Nitrogen occurs as combined gas in air. It forms 78% by volume of air. It exists as a diatomic molecule N2. It occurs naturally in compounds such as proteins, nitrates and ammonium salts. LABORATORY PREPARATION OF NITROGEN
Apparatus – delivery tubes, furnace, beaker, trough, Bunsen burner, 2 wash bottles bo ttles Chemicals – Air, water, sodium hydroxide Procedure Either: Nitrogen is prepared from the air by removing oxygen and carbon dioxide. Water is used to push air through sodium hydroxide solution (caustic soda solution) which removes carbon dioxide. 2 NaOHaq + CO2g Na2 CO3 + H2O (1) The remaining gas is passed over heated copper turnings to remove Oxygen. 2Cus + 02 (g) 2 CuO(s) Nitrogen is collected over water as it is insoluble in water.
OR: Nitrogen can also be prepared by b heating a mixture of ammonium chloride and sodium nitrite. NaNO2(s) + NH4 Cl(s) NH4 NO NO2(s) + NaCls
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Nitrogen is formed by decomposition of ammonium nitrite NH4 NO2(s) N2(g) + 2H2O(i) INDUSTRIAL PREPARATION (LARGE SCALE) Industrially, Nitrogen is prepared by fractional distillation; Oxygen is obtained at the same time. Air is purified by removing gas from it. Then carbondioxide is removed and lastly water vapour. The remaining air is removed and compressed. The liquidified air is allowed to evaporate. Because Nitrogen has a lower boiling point, it evaporates first leaving liquid oxygen. PROPERTIES OF NITROGEN Physical - it is colourless gas without smell - it is a reactive gas - it does not burn / doesn’t support combustion - it is neither acidic nor basic
Chemical properties 1. Nitrogen is inert unlike Oxygen, it reacts under special conditions for example - It reacts with some metals at very high temperatures forming nitrides e.g. Calcium and magnesium. 3Mg(s) + N2(g) Mg3 N2(S) 3Ca + N2(g) Ca3 N2(s) 2. It reacts with hydrogen to form ammonia (Haber’s process) N2(g) + 3H2g
⇋ 2NH3 (g)
AMMONIA (MH3) Ammonia is a covalent compound. It has a molecular mass of 17. LABORATORY PREPARATION OF AMMONIA
Apparatus – Round bottomed flask, clump, Bunsen burner, delivery tube, trough, curd board, wire gauze, lime water. Chemicals – Calcium hydroxide, Ammonium chloride, Calcium oxide. Procedure Ammonia is prepared by heating a mixture of calcium hydroxide and ammonium chloride. Ca(OH)2 (S) + 2NH4Cl (S) CaCl2 (S) + 2H2O (I) + 2NH3 (g) The tube in which ammonia is generated is fixed in a slanting position to prevent the water formed from running back and crack the whole tube. - Concentrated sulphuric acid and anhydrous calcium chloride are not used to dry ammonia because they react with it. Ammonia is collected by upward delivery as it is lighter than air.
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HABER PROCESS (INDUSTRIAL PREPARATION)
Apparatus – Compressing chamber, catalytic chamber, cooling chamber, delivery tube. Chemicals – Finely divided iron, Alminium Oxide, Water, Hydrogen, Nitrogen. Procedure: It is manufactured by reacting Nitrogen and hydrogen in the presence of finely divided iron as a catalyst at temperatures 350ºC - 400ºC at a pressure of about 350 atmospheres. N2(g) + 3H2(g)
⇋
2NH3(g)
Alminium Oxide is added to the catalyst to improve its performance. It makes it more porous and this provides a high surface area to the reaction.
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The reaction is reversible hence it is not possible to convert all the reactants into ammonia. To separate ammonia from the mixture is cooled, only ammonia liquidfies and it is separated. The uncombined Nitrogen and hydrogen are recycled. Another way of separation is to pass the mixture into wa ter. Only ammonia dissolves.
FACTORS AFFECTING THE REACTION (PROCESS)
1. Pressure High pressure causes a better yield of ammonia because it favours the formation of the smaller products. It also increases the speed of reaction because the reacting molecules collide more often. 2. Temperature At low temperatures the yield at equilibrium of Ammonia is higher but the reaction is slow. At high temperatures the yield of ammonia is low but the reaction is fast, a temperature of about 500ºC is used. The yield is good but the reaction is still too slow. A catalyst is therefore necessary to speed up the reaction. 3. Catalyst A catalyst speeds the reaction but does not affect the equilibrium. The catalyst should be finely divided because reaction occurs only at the surface. Properties of Ammonia 1. Ammonia is a colorless gas with a characteristic pungent smell. 2. It turns dump / wet red litmus paper blue. It is the only common alkaline gas. 3. It forms dense white fumes with hydrogen chloride gas NH3(g) + HCl(g) NH4Cl(s)
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Ammonia diffuses faster and white dense fumes will be formed near hydrogen chloride gas – the white dense fume is ammonia chloride. 4. Ammonia is very soluble in water. The great solubility of ammonia can be demonstrated using the fountain experiment. Procedure: A largely dry, round bottomed flask with ammonia is used. The mouth of the flask is placed under water and a red litmus paper in a trough of gas jar. It is clamped firmly in position. The spring clip at the end of the long glass tube is opened. Water slowly rises up the tube until one drop is at the jet at the top. The drop dissolves so much ammonia that there is a partial vaccum in the flask. Water is sucked rapidly up the tube and enters the flask as a fountain. The litmus paper turns blue.
5. Ammonia burns in a lot of air (oxygen). The flame is yellow green. 4NH3(g) + 3O2(g) 6H2O(i) + 2N2(g)
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Nb: Glass wool is to spread out the oxygen in order to bring it into greater contact with ammonia. In presence of a catalyst ammonia reacts with oxygen to form nitrogen monoxide. The monoxide is easily oxidized to dioxide hence if a hot platinum or copper wire is suspended in a beaker of concentrated ammonia and oxygen is bubbled through the solution, redish brown fumes are seen. The fumes later turn white. The brown fumes are due to nitrogen dioxide which turn white as ammonium nitrate is formed. 4NH3 (g) + 5O2(g) 4NO(g) + 6H2O(g) 2NO(g) + O2(g) 2NO2(g) NO2(g) + O2(g) + 2H2O(l) 4HNO3(g) HNO3 (g) + NH3(g) NH4 NO3(s)
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6. Ammonia reduces heated copper II oxide to copper i.e. copper turns from black to brown. 3 CuO (s) + 2NH3 (g) 3 Cu(s) + 3H2O (l) +N2 (g)
7. Ammonia burns in chlorine forming mist of hydrogen chloride gas. In excess ammonia, dens white fumes of ammonia chloride are formed. 2 NH3(g) + 3 Cl2(g) N2(g) + 6HCl(g) HCl(g) + NH3(g) NH4 Cl(s) 8. Ammonia solution (Ammonium hydroxide) contains hydroxyl ions with metal ions precipitates of the hydroxides are formed. Hence a blue precipitate forms 7
when aqueous ammonia is added to copper II sulphate solution. The precipitate dissolves in excess ammonia forming a deep blue solution. Cu(aq)²+ + 2OH-(aq) Cu(OH)2(s) Cu(OH)2(s) + 4NH3(aq) Cu[NH3]2+4 + 2OH-(aq) Tetra Ammine Copper II ions (deep blue solution) Iron(II) is (Fe ) forms a dirty green precipitate with ammonia insoluble in excess Iron(III) is (Fe 3+) forms a brown precipitate insoluble in excess. 2+
Uses of ammonia 1. It is used in the manufacture of fertilizers e.g. Ammonium sulphate. 2. It is used in softening water. 3. It is used in making nitric acid. 4. It is used in making plastics. 5. Ammonium chloride is used in dry cells. 6. It is used in making explosives. Test for Ammonia It is the only common alkaline gas known. It changes the dump / wet litmus paper blue. Ammonia forms dense fumes of ammonium chloride when brought into contact with fumes of hydrogen chloride from concentrated hydrochloric acid. Subject : Chemistry Topic : Nitrogen and its compounds Sub-topic : Ammonia Class : S.3 Size : 50 students In groups of 10 students Time: 4 doubles a week Course content and outline A student is required to know ammonia, its state – covalent compound Its monocular mass (17) Its formula NH3 Its preparation on small scale (in the laboratory) The reagents used (calcium hydroxide and ammonium chloride) Relevant equation for ammonia formation Precautions used in its preparation e.g. the tube in which ammonia is generated is fixed in a slanting position to prevent the water from running back and crack the whole tube. Concentrated sulphuric acid and an hydrous calcium chloride are not used to dry ammonia because they react with it. Its method of collection – by upward delivery – as it is lighter than air. Diagramic illustration
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Its preparation on large (industrial scale) Haber process The reagents used e.g. N2 & H2, catalyst finely divided iron, temp 350ºc 400ºc, pressure about 350 atmospheres -
N2(g) + 3H2(g) ⇋ 2NH3(g) To explain the factors affecting the reaction in ammonia formation. Pressure, temperature, catalyst, To explain the properties of ammonia, using diagrams where possible, both physical and chemical properties. To explain the uses of ammonia.
Organization advice Student experiments can be done in groups. Each group should be able to carry out assigned experiments perfectly. Ensure that the group is small (5-6), manageable and maximum discipline must be observed. When grouping them, choose a leader for each group, he/she must be of average IQ (Intelligent quotient). Practice team teaching whenever possible. Make prior arrangements to produce the necessary resource materials. Try out the experiment before giving them to students. This will help you (the teacher) to get solutions to any short comings. Improvise where necessary. Relate the experiments to daily life. Organize project work on Haber process. Bear in mind the safety precautions Get into the minds of students and think about the questions they are likely to ask Teach students to learn to make genuine conclusions from the experiments carried out Time should be set and followed
References 1. Holderness and Lambert, (1987), A new certificate Chemistry, 6th edition, pages 187. 2. Wilford L. DR, (1984), Chemistry for first Examinations, pages 211-217. 3. Sinclair M and Afolayan S. A., (1990), “O” level Chemistry for East Africa.
Sample lesson plan Topic: Nitrogen and its compound Sub topic: Ammonia
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Step I II III IV V
Time 10 min
Teacher’s activity Teacher greets students and takes roll call. 5 min Revision of Ask questions on previous work previous work covered. 50 min Topic and notes Teacher dictates notes, draws relevant diagrams. 15 min Tasks Teacher gives students class work / home work 5 min End Teacher collects the students’ tasks.
Exercise 1. 2. 3. 4. 5.
6.
7. 8. 9.
Content Introduction
Student activity Respond when they are called. Answer questions asked. Take notes attentively. Do the tasks (class work / home work). Students hand in their tasks written either in their books or papers.
What is a catalyst? What is a covalent compound? Give any precaution taken during the manufacture of ammonia. How is ammonia gas collected? Give a reason to your answer. Give and explain the factors affecting the reaction during the manufacture of ammonia. Perform a simple experiment to show that Ammonia (i) forms dense white fumes with hydrogen chloride. (ii) is very soluble in water (iii) burns in a lot of air (oxygen) (iv) reduces heated Copper(II) oxide to copper Give the industrial uses of ammonia. Perform a simple test how to detect the ammonia gas using the litmus paper. Ammonia reacts with lend(II) oxide according to this equation 2MH3(g) + 3PbO(s) N2(g) + 3Pb(s) + 3H2O(i) Calculate the volume of ammonia at room temperature that would be required to completely react with 2.5g of lend(II) oxide.
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