CBSE Class 10 Answered

When iron and sulphur are heated together, they combine to form a single product, iron sulphide.

                             2Fe_(s)     +     S_(s)         →               FeS _(s)

                             Iron         Sulphur                 Iron sulphide

When lead dioxide reacts with sulphur dioxide, both these compounds combine to form lead sulphate.

                     PbO_2(s)      +         SO_2(s)         →             PbSO_4(s)

                      Lead dioxide   Sulphur dioxide       Lead sulphate

When mercuric oxide is heated in a crucible, the orange-red powder begins to darken and a silver mirror begins to deposit on the cooler parts of the crucible. This is mercury. If we hold a glowing splint near the crucible, it can relight. This shows that the gas evolved during the reaction is oxygen.

                           2HgO_(s)    →       2Hg_(s)     +    O₂ ↑

                    Mercuric oxide           Mercury     Oxygen

 Electrolysis of water produces two volumes of hydrogen gas and one volume of oxygen gas on passing an electric current through acidulated water.

                            2H₂O_(l) →  2H_{2 (g)} + O_2(g)

During thermal decomposition a chemical compound breaks into simpler compounds. The simpler compounds do not reunite to form the original compound on cooling.

Example:

                     2 KClO₃  → 2KCl  +  3O₂

                     2NaNO₃  →   2NaNO₂  +   O₂

Example:

Photosynthesis in plants.

               6CO₂  +  6H₂O → C₆H₁₂O₂

Example:

Silver chloride turns grey in the sunlight. This is due to the decomposition of silver chloride into silver and chlorine by sunlight.

                 2AgCl  →  2 Ag  +   Cl₂

Example:

Fused potassium chloride on passing electric current through it breaks (decomposes) into its charged particles (ions).

                   KCl  →  K⁺  +  Cl^-

The insoluble solid formed during the double displacement reactions is called a precipitate. Reactions in which a precipitate is formed as one of the products are also called precipitation reactions.

 Sodium sulphate reacts with barium chloride to form barium sulphate and sodium chloride solution.

                    Na₂SO_{4 (aq)}   +   BaCl₂      →             BaSO_4(s)    +      2NaCl_(aq)

                     Sodium            Barium                      Barium              Sodium

The reaction between an acid and a base to form a salt and water is called as neutralisation reaction.

                   NaOH_(aq)   +    HCl_(aq)        →                  NaCl_(aq)   +    H₂O_(l)

                   Sodium        Hydrochloric                        Sodium         Water

                  hydroxide           acid                               chloride

Oxidation reactions

Hydrogen sulphide reacts with chlorine gas to form hydrogen chloride and sulphur. In this reaction, the chlorine acts as an oxidising agent and removes hydrogen from hydrogen sulphide to form sulphur.

                     H₂S         +       Cl₂      →              2HCl        +          S

                 Hydrogen      Chlorine                    Hydrogen          Sulphur

                  sulphide                                        chloride   

 Ferrous chloride reacts with chlorine gas to form ferric chloride. In this reaction, ferrous chloride gains the electronegative radical chlorine and is therefore oxidised to ferric chloride.

                        2FeCl₂    +     Cl₂        →                 2FeCl₃

                     Ferrous     Chlorine                           Ferric

                     chloride                                         chloride   

Reduction reactions

Ferric oxide reacts with carbon monoxide to form iron and carbon dioxide. In this reaction, carbon monoxide acts as a reducing agent and removes oxygen from ferric oxide to form iron.

          Fe₂O₃          +          3CO          →              2Fe       +        3CO₂↑

   Ferric oxide          Carbon monoxide                   Iron        Carbon dioxide

Chlorine gas reacts with hydrogen sulphide to form hydrogen chloride and sulphur. In this reaction, hydrogen sulphide acts as a reducing agent and supplies hydrogen to chlorine to form hydrogen chloride.

                    H₂S       +      Cl₂      →                  2HCl        +        S

                Hydrogen    Chlorine                        Hydrogen           Sulphur

Consider the redox reaction between hydrogen sulphide and chlorine.

In the reaction between hydrogen sulphide and chlorine, the products formed are hydrogen chloride and sulphur.

                       H₂S       +      Cl₂      →                  2HCl        +        S

                   Hydrogen        Chlorine                  Hydrogen             Sulphur

                    sulphide                                       chloride   

 When an atom or a group of atoms loses electrons, oxidation takes place. In this reaction, hydrogen sulphide is oxidised to sulphur.

                                                    H₂S        →            S

                                       S^-2    -    2 e^-       →              S

                                Sulphide ion                           Sulphur

 Since, hydrogen sulphide loses electrons; it is called a reducing agent.

 When an atom or a group of atoms gains electrons, reduction takes place. In this reaction, chlorine is reduced to hydrogen chloride.

                                                   Cl₂      →               HCl

                                     2Cl   +   2 e  ^̶     →                 2Cl  ^̶

                                  Chlorine                            Chloride ion

 Since, chlorine accepts electrons it is called an oxidising agent.

Zinc displaces copper in copper sulphate to form zinc sulphate.

       Zn_(s)   +     CuSO_4(aq)          →         ZnSO_4(aq)     +       Cu_(s)

      Zinc      Copper sulphate                 Zinc sulphate       Copper

 Chlorine, being more electronegative than potassium displaces it from potassium bromide to form potassium chloride and bromine gas.

            Cl₂         +       2KBr       →       2KCl          +       Br₂

        Chlorine        Potassium             Potassium        Bromine

Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

             NaOH_(aq)      +         HCl_(aq)     →              NaCl_(aq)         +        H₂O_(l)

             Sodium            Hydrochloric                     Sodium                    Water

           hydroxide                 Acid                          chloride