GIVE REASONS AND OBSERVATIONS NOTES_CLASS 10

Give reason

  Give appropriate scientific reasons for the following statements:              

1. Zinc oxide can be reduced to zinc by using carbon monoxide, but aluminium oxide cannot be reduced by a reducing agent.

     Aluminium is more reactive than zinc and has a high affinity for oxygen. So Al cannot be reduced by a reducing agent. 

2. Carbon tetrachloride does not conduct electricity.

     Carbon tetrachloride consists of only molecules hence it is a non-electrolyte. As free ions are absent it cannot conduct electricity

3. During electrolysis of molten lead bromide graphite anode is preferred to other electrodes.

     Graphite is inert by nature. It would not react with bromine vapours which evolve at the anode.

4. The electrical conductivity of acetic acid is less in comparison to the electrical      conductivity of dilute sulphuric acid at a given concentration.

or, Conductivity of dilute hydrochloric acid is greater than that of acetic acid

     Acetic acid is a weak acid and sulphuric acid (HCl acid) is a strong acid. Acetic acid undergoes partial ionisation and produces less number of ions whereas sulphuric acid (HCl acid) undergoes complete ionization and produces a large number of ions at a given concentration.  As the number of free ions are less in acetic acid, electrical conductivity is also less.

5. Electrolysis of molten lead bromide is considered to be a redox reaction.

     Lead ions gain electrons from the cathode and thus gets reduced. Br^- ion loses its electron at the anode and gets oxidised. As reduction and oxidation occurs simultaneously at the electrodes, electrolysis is a redox reaction.

6. In the electrolysis of alumina using the Hall Heroult’s Process the electrolyte is covered with powdered coke. 

             Powdered coke protects the graphite rods of the anode from oxidation by oxygen released     at the anode. Powdered coke prevents the loss of heat from the electrolyte.

7. Iron sheets are coated with zinc during galvanization

      Zinc lies higher in the activity series than iron. Thus, zinc is more reactive than iron. So to      prevent rusting iron sheets are coated with zinc.

8. Dilute nitric acid is generally considered a typical acid but not so in its reaction with metals.

     Dilute nitric acid acts as an oxidizing agent. It oxidises the hydrogen to water and itself gets reduced to oxides of nitrogen.    

9.  Concentrated nitric acid appears yellow when it is left standing in a glass bottle.

       Conc. HNO₃ decomposes to form reddish brown gas nitrogen dioxide which dissolves in it and imparts a yellow colour.                                                           

                                                

10.  An all glass apparatus is used in the laboratory preparation of nitric acid.

            HNO₃ vapours are very corrosive in nature. They react with rubber or cork.

11. Methane does not undergo addition reactions, but ethene does.

          Methane is a saturated hydrocarbon. It can only undergo substitution reaction. Ethene is an unsaturated hydrocarbon with a double bond. So it can undergo addition reaction.

12.  Ethyne is more reactive than ethane.

       Ethyne is an unsaturated hydrocarbon with a triple covalent bond hence it is more reactive than ethane.

13. Hydrocarbons are excellent fuels.

       Hydrocarbons on combustion with air produces carbon dioxide and water vapour and            large amount of heat energy. Hence they are good fuels.

14. Sodium Chloride will conduct electricity only in fused or aqueous solution state.

         Na⁺ and Cl^– ions become mobile only on melting or dissolving it in water and only then        can NaCl conduct electricity. 

15. In the electroplating of an article with silver, the electrolyte sodium argentocyanide solution is preferred over silver nitrate solution.

     Addition of sodium argentocyanide helps in getting a uniform and smooth deposit of silver. With silver nitrate alone, the process is rapid and the deposit is uneven.

16. Although copper is a good conductor of electricity, it is a non-electrolyte.

     An electrolyte must have free ions which act as charge carriers whereas in copper the free electrons act as charge carriers.

17. It is preferable to use a number of graphite electrodes as anode instead of a single electrode, during the Hall Heroult Process.                                

The oxygen formed at the anode reacts with the graphite anode. Thus the anode is consumed, hence the use of large number of electrodes ensures continuity of the process.

18. Ionic compounds have a high melting point.

            In ionic compounds oppositely charged ions are held together by strong electrostatic force of attraction. Thus a large amount of energy is required to separate the ions, resulting in high melting point.

19. Inert gases do not form ions.

             Inert gases have stable electronic configuration i.e. 8 electrons in the valence shell (last shell). So they do not lose or gain electrons to form ions.

20. Ionisation potential increases across a period, from left to right.

             Nuclear charge increases and atomic size decreases across a period. Thus the force of attraction on the valence electrons are more resulting in an increase in the ionisation potential.

                                                                  

21. It is necessary to renew the anode periodically.

The O₂ formed at the anode reacts with graphite anode and hence gets consumed.

22. Alkali metals are good reducing agents.

            Alkali metals have low ionization potential. Hence they easily donate their valence electron and act as good reducing agents.

23. Concentrated sulphuric acid is not used for drying ammonia gas.

            Ammonia being basic combines with concentrated sulphuric acid forming ammonium sulphate.

24. Ammonia gas not collected over water.

  NH₃ is highly soluble in water.

25. For the preparation of hydrochloric acid in the laboratory direct absorption of hydrogen chloride gas in water not feasible.

HCl gas dissolves in water at a faster rate than it is produced. Hence back suction occurs which may damage the apparatus.

26. Covalent compounds exist as gases, liquids or soft solids.

       The molecules in a covalent compound are held together by the weak Van der Waal's forces. As this force of attraction is weak they can exist as gases, liquids or soft solids.

State your observations when:   

   (i) Dilute Hydrochloric acid is added to Lead nitrate solution and the mixture is heated.

  A white precipitate soluble on heating is formed.

  (ii) Barium chloride solution is mixed with Sodium sulphate solution.

       A white precipitate is formed which is insoluble in all the mineral acids.

 (iii) Concentrated sulphuric acid is added to Sugar Crystals.

       A black spongy mass is formed.

 (iv) Dilute Hydrochloric acid is added to Copper carbonate.

       A colourless and odourless gas evolves with brisk effervescence which turns lime water   milky.

(v) Dilute Hydrochloric acid is added to Sodium thiosulphate.
   A colourless gas evolved with a suffocating odour which turns potassium dichromate paper from orange to green and yellow particles of Sulphur.

(vi) When ammonium hydroxide solution is added drop by drop and then in excess to each of the following solutions:

     (a) Copper sulphate solution                      (b) zinc sulphate solution.

     (a) On adding NH₄OH solution, the CuSO₄ solution first forms a pale blue precipitate dissolving in excess to form an ink blue solution. 

    (b)With zinc sulphate solution, NH₄OH solution first forms a gelatinous white precipitate          which dissolves in excess to form a colourless solution. 

(vii) Washing Soda Crystals are exposed to the atmosphere.

            Washing soda crystals when exposed to atmosphere lose their water of crystallisation        partially and crumble to form a powder.

 

(viii) The salt ferric chloride is exposed to the atmosphere

            Ferric chloride absorbs moisture when exposed to atmosphere and forms a solution            dissolving in the absorbed moisture.

State one relevant observation for each of the following:

(i) When crystals of copper nitrate are heated in a test tube. 

       Reddish brown gas of nitrogen dioxide gas is evolved and black residue of CuO is formed.

(ii) When the gaseous product obtained by dehydration of ethyl alcohol is passed through             bromine water.

        Brown colour of Bromine solution decolourises.

 (iii) When hydrogen sulphide gas is passed through lead acetate solution.

          Black precipitate of lead sulphide is formed

 (iv) When ammonia gas is burnt in an atmosphere of excess oxygen.

            Burns with a greenish yellow flame. 

 (v)   At the Anode when aqueous copper sulphate solution is electrolysed using copper electrodes. 

     Anode is consumed

State your observation in each of the following cases:

 (i)  When dilute hydrochloric acid is added to sodium carbonate crystals.

        A colourless and odourless  gas evolves with brisk effervescence which turns lime water milky and has no effect on acidified potassium dichromate paper.

           

 (ii) When excess sodium hydroxide is added to calcium nitrate solution.

        A chalky white ppt insoluble in excess sodium hydroxide.

(iii)  At the cathode when acidified aqueous copper sulphate solution is electrolyzed with copper electrodes.

     Reddish brown deposit.

(v)   When calcium hydroxide is heated with ammonium chloride crystals.

        A colourless gas with a pungent odour is evolved which turns moist red litmus paper blue.

 (v)  When moist starch iodide paper is introduced into chlorine gas.

            The paper turns blue black.

State one appropriate observation for each of the following:  _ 2013

(i)  Concentrated sulphuric acid is added drop wise to a crystal of hydrated copper sulphate. 

      The crystals turn from blue to white amorphous  substance.

(ii) Copper sulphide is treated with dilute hydrochloric acid.    

      A colourless gas with a rotten egg smell is produced which turns moist lead acetate paper shiny black.

  (iii) Excess of chlorine gas is reacted with ammonia gas.

          Yellow oily liquid of nitrogen trichloride is produced.

 (iv) A few drops of dilute hydrochloric acid are added to silver nitrate solution, followed by addition of ammonium hydroxide solution.

     Curdy white precipitate is formed which dissolvess in ammonium hydroxide solution to form a colourless solution.

 (v) Electricity is passed through molten lead bromide

     Shiny white metal (lead) is deposited at the cathode and reddish gas (bromine) is collected at the anode.

State two relevant observations for each of the following: 

(i) Ammonium hydroxide solution is added to copper (II) nitrate solution in small quantities and then in excess. 

   A pale blue ppt is formed which with excess of ammonium hydroxide solution forms an inky blue solution.

(ii) Ammonium hydroxide solution is added to zinc nitrate solution in minimum quantities and then in excess.

   A gelatinous white ppt is formed which with excess of Ammonium hydroxide solution forms a clear solution.

(iii) Lead nitrate crystals are heated in a hard glass test tube

   Crackling sound is produced, reddish brown nitrogen dioxide gas is produced and forms a yellow residue of lead oxide which fuses with glass. 

State one relevant observation for each of the following reactions:

(i) Addition of ethyl alcohol to acetic acid in the presence of concentrated Sulphuric acid.

   Fruity smell evolved.

 (ii)  Action of Sodium hydroxide solution on ferrous sulphate solution.

   Dirty green precipitate formed, insoluble in excess NaOH.

(iii) Action of concentrated Sulphuric acid on hydrated copper sulphate

   Blue crystals turn into white amorphous powder.

(iv) Lead nitrate solution is treated with sodium hydroxide solution drop wise till it is in excess.

      Chalky white precipitate soluble in excess of sodium hydroxide solution.

(v) At the anode, when molten lead bromide is electrolyzed using graphite electrodes.

     Reddish brown vapour at the anode.

(vi) Anhydrous calcium chloride is exposed to air for some time.

 Solid CaCl₂ absorbs moisture from atmosphere and turns into solution.

           

(e) Give a chemical test to distinguish between the following pairs of compounds:

 (i) Sodium chloride solution and sodium nitrate solution.

     When silver nitrate solution is added to sodium chloride solution a curdy white precipitate of silver chloride is formed, when the same silver nitrate solution is added to sodium nitrate solution no visible change occurs.               

  (ii) Hydrogen chloride gas and hydrogen sulphide gas. 

     Hydrogen chloride gas gives dense white fumes with ammonia gas, whereas no visible change occurs with hydrogen sulphide and ammonia gas.

            or

     Hydrogen sulphide gas forms a shiny black ppt with lead acetate solution, hydrogen chloride gas forms a white precipitate with lead acetate solution

(iii) Ethene gas and ethane gas. 

     Ethene gas decolourises brown coloured bromine water solution. Ethane retains the brown colour of the bromine water solution.

(iv) Calcium nitrate solution and zinc nitrate solution.

     Calcium nitrate forms a white precipitate with sodium hydroxide solution, in excess the white ppt remains the same. Zinc nitrate solution forms a white ppt with sodium hydroxide solution, with excess it forms a clear solution.   Or On addition of ammonium hydroxide solution to each of the solutions – No precipitate formed with calcium nitrate and white precipitate formed with zinc nitrate which dissolves in excess

(v) Carbon dioxide gas and sulphur dioxide gas.

     Carbon dioxide gas has no change with acidified potassium dichromate solution, whereas sulphur dioxide gas turns orange coloured acidified potassium dichromate solution green 

State the observations at the anode and at the cathode during the electrolysis of:

(i) Fused lead bromide using graphite electrodes.

   Reddish brown fumes at the anode. Silvery grey deposit at the cathode

(ii) Copper sulphate solution using copper electrodes.

Size of the anode decreases or anode gets consumed. Reddish brown deposit of copper at the cathode