The stone's altitude at time
is given by

where
is the acceleration due to gravity. The stone reaches the ground when
:

An exothermic reaction is a chemical reaction that releases heat. It gives net energy to its surroundings. That is, the energy needed to initiate the reaction is less than the energy released. ... Δ H = (energy used in forming product bonds) − (energy released in breaking reactant bonds)
The formula for kinetic energy is
KE = (1/2) (mass) (speed)² .
How you measure the object's mass and speed is up to you.
You'd need different methods for different objects, and in some
cases, you'd need quite a bit of ingenuity.