Sample Answers/Solutions - Study of Compounds

Transcription

1 Sample Answers/Solutions - Study of Compounds Hydrogen Chloride 1. It is a gas at room temperature. 2. Refer to pg.128 right side section If the reaction is carried out above 200 o C, then the by-product formed is Na 2 SO 4 which forms a hard crust which sticks to the flask and is difficult to remove. 4. HCl reacts with most other drying agents like KOH, CaO etc. since they are bases. So you need a drying agent which is an acid so that it won t react with HCl. 5. Since it is heavier than air, HCl can be collected by the upward displacement of air. HCl is highly soluble in water, so it cannot be collected over water. 6. The funnel arrangement is used to prevent the back suction into the flask caused by the rapid dissolution of HCl gas in water. The funnel is kept just touching the surface of the water. Due to the large cross-sectional area of the funnel as compared to a glass tube, the water will not rise much in the funnel. And as soon as some water starts rising in the funnel, it loses contact with the water surface, since the funnel is kept just touching the surface. This causes the water which had risen to fall back. So the water does not rise beyond a certain height in the funnel. 7. Take two jars A and B, A filled with HCl and B with air. Invert jar A on top of B, leave them for some time and then keep it upright again. Test both flasks for HCl, by bringing a glass rod dipped in ammonia solution near their mouths. Now jar B would give dense white fumes, showing that the HCl had descended from A to B when we inverted it. 8. Write balanced chemical equations for the reaction of dilute HCl with the following substances. (a) Na 2 S 2 O 3 + 2HCl 2NaCl + H 2 O + SO 2 + S (b) F es + 2HCl F ecl 2 + H 2 S (c) Na 2 CO 3 + 2HCl 2NaCl + H 2 O + CO 2 (d) Na 2 SO 3 + 2HCl 2NaCl + H 2 O + SO 2 (e) Zn + 2HCl ZnCl 2 + H 2 (f) CaO + 2HCl CaCl 2 + H 2 O (g) AgNO 3 + HCl AgCl + HNO 3 9. A glass rod dipped in ammonia solution will give dense white fumes if HCl gas is present. This happens because the ammonia gas escaping from its solution reacts with the HCl gas to form solid ammonium chloride which can be seen as dense white fumes. NH 3 + HCl NH 4 Cl 10. (a) MnO 2 + 4HCl MnCl 2 + 2H 2 O + Cl 2 P bo 2 + 4HCl P bcl 2 + 2H 2 O + Cl 2 1

2 (b) Mn : O : -2-2 H : Cl : -1-1,0 So, Mn is getting reduced while Cl is getting oxidised. It s very similar with the other case also. 11. (a) They are below hydrogen in the reactivity series, so they cannot displace hydrogen from acids. Other metals like copper, silver, mercury also don t react with acids. (b) Zinc is a metal which does react with acids, since it is more reactive than hydrogen. Zn + 2HCl ZnCl 2 + H 2 Zn : 0 +2 H : +1 0 Cl : -1-1 So, zinc is getting oxidised while hydrogen is getting reduced. The one that is getting reduced itself, to allow something else to be oxidised is called an oxidising agent. So here, hydrogen is the oxidising agent. (c) Au + 3[Cl] AuCl (i) manufacture of drugs, dyes and paints. (ii) cleaning of metal surfaces. Ammonia 1. It is a gas at room temperature. 2. Refer to pg.139 left side- both methods under Laboratory Preparation. 3. CaO is basic like ammonia, so it doesn t react with ammonia. 4. Ammonia is collected by the downward displacement of air, since it is lighter than air. 5. (a) nitrogen, hydrogen (b) 3N 2 + H 2 2NH 3. The reaction requires high temperature (about 450 o C), high pressure (about atm) and finely divided iron as catalyst. 6. Ammonia dissolves in water to generate ammonium and hydroxyl ions. NH 3 +H 2 O NH + 4 +OH. [We will consider the formation of ammonium ions in more detail when we do Chemical Bonding.] 7. (a) (i) 4NH 3 + 3O 2 2N 2 + 6H 2 O (ii) NH 3 + 5O 2 4NO + 6H 2 O (b) (i) NH 3 + 3Cl 2 NCl 3 + 3HCl (ii) 8NH 3 + 3Cl 2 6NH 4 Cl + N 2 (c) 3CuO + 2NH 3 3Cu + 3H 2 O + N 2 3P bo + 2NH 3 3P b + 3H 2 O + N 2 (d) 4NH 3 + 3O 2 2N 2 + 6H 2 O N: -3 0 O: 0-2 NH 3 + 5O 2 4NO + 6H 2 O N:

3 O: 0-2 NH 3 + 3Cl 2 NCl 3 + 3HCl N: Cl: 0-1 8NH 3 + 3Cl 2 6NH 4 Cl + N 2 N: -3-3,0 Cl: 0-1 3CuO + 2NH 3 3Cu + 3H 2 O + N 2 Cu: +2 0 N: -3 0 In all the above reactions, the nitrogen of ammonia is getting oxidised while reducing some other element. So, ammonia acts as a reducing agent in all these reactions. 8. The test for ammonia is to show a glass rod dipped in conc. HCl. Dense white fumes confirm the presence of ammonia. This is because ammonia reacts with hydrogen chloride fumes to produce ammonium chloride. NH 3 + HCl NH 4 Cl 9. Uses- manufacture of fertilizers, manufacture of nitric acid. Nitric Acid 1. Refer to pg.150 right side. 2. If the reaction is carried out above 200 o C, then the by-product formed is Na 2 SO 4 which forms a hard crust which sticks to the flask and is difficult to remove. Also, nitric acid decomposes faster at higher temperatures. 3. The vapours of nitric acid are highly corrosive and destroy materials like cork and rubber. So the apparatus is made completely of glass. 4. Even at room temperature nitric acid slowly decomposes to give nitrogen dioxide, which escape as brown fumes or remain dissolved in nitric acid to give it a brown colour. 5. (a) ammonia (b) Step 1: Catalytic conversion of ammonia to nitric oxide. Carried out at o C in the presence of platinum as catalyst. 4NH 3 + 3O 2 2N 2 + 6H 2 O Step2: Oxidation of nitric oxide to nitrogen dioxide 2NO + O 2 2NO 2 Step3: Oxidation and absorption of nitrogen dioxide 4NO 2 + O 2 + 2H 2 O 4HNO 3 6. (a) Na 2 S 2 O 3 + 2HNO 3 2NaNO 3 + H 2 O + SO 2 + S (b) F es + 2HNO 3 F e(no 3 ) 2 + H 2 S (c) Na 2 CO 3 + 2HNO 3 2NaNO 3 + H 2 O + CO 2 3

4 (d) Na 2 SO 3 + 2HNO 3 2NaNO 3 + H 2 O + SO 2 (e) Zn + 2HNO 3 Zn(NO 3 ) 2 + H 2 (f) CaO + 2HNO 3 Ca(NO 3 ) 2 + H 2 O 7. (a) C + 4HNO 3 CO 2 + 4NO 2 + 2H 2 O C: 0 +4 N: (b) S + 6HNO 3 H 2 SO 4 + 6NO 2 + H 2 O S: 0 +6 N: (c) Cu + 4HNO 3 Cu(NO 3 ) 2 + 2H 2 O + NO 2 Cu: 0 +2 N: +5 +5,+4 (d) Zn + 4HNO 3 Zn(NO 3 ) 2 + 2H 2 O + NO 2 Zn: 0 +2 N: +5 +5,+4 In the above reactions, nitrogen from nitric acid gets reduced, thereby oxidising carbon, sulphur, copper and zinc respectively. So nitric acid acts as an oxidising agent. 8. Zn + 4HNO 3 Zn(NO 3 ) 2 + 2H 2 O + NO 2 Zn: 0 +2 N: +5 +5,+4 Nitrogen from nitric acid is getting reduced for zinc to be oxidised. Zn + 2HCl ZnCl 2 + H 2 Zn: 0 +2 H: +1 0 Cl: -1-1 Here, hydrogen from the acid is getting reduced for zinc to be oxidised. It is a simple displacement reaction, in which zinc being more active than hydrogen, forces hydrogen to accept its electrons. The first reaction, on the other hand, takes place due the tendency of nitrogen which is in the +5 oxidation state, to go to a lower oxidation state. 9. production of explosives, etching of metals. Sulphuric Acid 1. (a) Sulphur, or iron pyrites (b) Step 1: production of sulphur dioxide S + O 2 SO 2 Step 2: oxidation of sulphur dioxide, in the presence of vanadium pentoxide as catalyst, at o C and atmospheric pressure. 2SO 2 + O 2 SO 3 Step 3: absorption of sulphur trioxide. SO 3 + H 2 SO 4 H 2 S 2 O 7 4

5 Step 4: dilution of oleum (H 2 S 2 O 7 ) H 2 S 2 O 7 + H 2 O 2H 2 SO 4 2. Dissolution of sulphuric acid in water is a highly exothermic process, which could give rise to temperatures as high as 120 o C. If water is added to the acid, as it can be suddenly converted into steam causing dangerous splashing of the acid. On the other hand, if the acid is added to water, it will ensure that only a small amount of acid is dissolving at a time, making the process and the heat release more controlled. 3. (a) Na 2 S 2 O 3 + H 2 SO 4 Na 2 SO 4 + H 2 O + SO 2 + S (b) F es + H 2 SO 4 F eso 4 + H 2 S (c) Na 2 CO 3 + H 2 SO 4 Na 2 SO 4 + H 2 O + CO 2 (d) Na 2 SO 3 + H 2 SO 4 Na 2 SO 4 + H 2 O + SO 2 (e) Zn + H 2 SO 4 ZnSO 4 + H 2 (f) CaO + H 2 SO 4 CaSO 4 + H 2 O 4. (a) C 1 2H 2 2O 1 1 (b) CuSO 4.5H 2 O conc. H2SO4 12C + 11H 2 O conc. H2SO4 CuSO 4 + 5H 2 O 5. (a) NaCl + H 2 SO 4 NaHSO 4 + HCl (b) KNO 3 + H 2 SO 4 KHSO 4 + HNO 3 In both the reactions, sulphuric acid acts as a less volatile acid forcing out a more volatile acid from its salt. 6. (a) C + 2H 2 SO 4 CO 2 + 2SO 2 + 2H 2 O C: 0 +4 S: (b) S + 2H 2 SO 4 3SO 2 + H 2 O S: 0,+6 +4 (c) Cu + 2H 2 SO 4 CuSO 4 + 2H 2 O + SO 2 Cu: 0 +2 S: +6 +6,+4 (d) Zn + 2H 2 SO 4 ZnSO 4 + 2H 2 O + SO 2 Zn: 0 +2 S: +6 +6,+4 In the above reactions, sulphur from sulphuric acid gets reduced, thereby oxidising carbon, sulphur, copper and zinc respectively. So sulphuric acid acts as an oxidising agent. 7. Zn + 2H 2 SO 4 ZnSO 4 + 2H 2 O + SO 2 Zn: 0 +2 S: +6 +6,+4 Sulphur from sulphuric acid is getting reduced for zinc to be oxidised. Zn + H 2 SO 4 ZnSO 4 + H 2 Zn: 0 +2 H: +1 0 S:

6 Here, hydrogen from the acid is getting reduced for zinc to be oxidised. It is a simple displacement reaction, in which zinc being more active than hydrogen, forces hydrogen to accept its electrons. The first reaction, on the other hand, takes place due the tendency of sulphur which is in the +6 oxidation state, to go to a lower oxidation state. 8. Uses- lead storage batteries (rechargeable UPS/car batteries), manufacture of dyes/drugs/fertilisers/artificial fibres etc. 6