Available Answers

1. 1.

   State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include - (a) Colour change (b) Effervescence or gas evolved (c) Evolution or absorption of heat (d) Formation of a precipitate.

   With reference to each of the above indications, state the external indication seen during -

   1. Addition of dilute acid to an active metal
   2. Addition of dilute hydrochlrolic acid to silver nitrate
   3. Addition of water to quicklime
   4. Thermal decomposition of mercury [II] oxide.
2. 2.

   State why a direct combination reaction is called a - 'synthesis reaction'.

3. 3.

   Classify the following reactions into - (a) Direct combination (b) Decomposition (c) Displacement (d) Double composition

   The reactions are -

   1. Zinc hydroxide on heating gives zinc oxide & water
   2. Zinc reacts with copper (II) sulphate to give zinc sulphate & copper
   3. Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide
   4. Molten zinc at high temperatures, burns in air to give zinc oxide.
4. 4.

   Give balanced equations for -

   1. A direct combination reaction involving two elements, one of which is a non-metal
   2. A thermal decomposition reaction involving heat on limestone (calcium carbonate)
   3. An electrolytic decomposition reaction involving a neutral liquid
   4. A displacement reaction involving a metal above hydrogen in the activity series with copper [II] suphate solution
   5. A double decomposition neutralization reaction involving an acid & a base
   6. A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.
5. 5.

   State what is meant by 'reactivity series of the metals'. With reference to - (a) Water (b) Acids explain with suitable examples how the reactivity of the metals could be differentiated.

6. 6.

   A chemical reaction may be 'reversible' in nature. State the meaning of the term in italics. Give a reason why a catalyst is used in certain chemical reactions. Give a balanced equation for the following:
   (a) A reversible catalyst reaction involving -
   (i) nitrogen as one of the reactants
   (ii) sulphur dioxide as one of the reactants