What is the purpose of the initial heating?
It starts the reaction. It provides activation energy (Compare with the energy a child must provide to climb up to a toboggan !).
The reaction liberates heat. How is such a reaction called?
The reaction is exothermic.
Two elementary substances combine to give a compound. How do we call that type of reaction?
It is called synthesis.
Write the equation of the reaction !
$4Na$ $+$ $O_2$ $\longrightarrow$ $2Na_2O$.