Give balanced equations, wherever possible, or where this is not possible, explain the following by means of examples:
(i) A reaction which gives out heat.
(ii) A reaction which takes place with the help of sunlight.
(iii) A reaction which is brought about by electric current.
(iv) Reversible Reactions.
(v) A reaction with a solid and gas which produces heat.

(i) Coke on heating in air, catches fire and liberates large amount of heat.
C + O₂ → CO₂ + Δ T

(ii) Carbon dioxide and water react in the presence of chlorophyll and sunlight to form glucose and oxygen.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(iii) Molten lead bromide decomposes into lead metal and bromide on the passage of electric current.
PbBr₂ → Pb + Br₂

(iv) The reaction between red hot iron and steam is reversible.


(v) When magnesium burns in air or oxygen it liberates large amount of heat.
2Mg + O₂ → 2MgO + Δ T.