How chemical energy change to mechanical energyI can`t understand

Chemical reactions produce heat. So do nuclear reactions. Heat is a different form of energy. So the question now becomes ‘how can we turn thermal energy into mechanical force’. Steam is an obvious mechanism now hundreds of years old.

Chemical energy is turned into mechanical energy in the internal combustion engine. The heat of the chemical reaction (fuel and air) expands the gas and this pushes down on a piston. If you remove the piston and just ignite the gasses you have the beginnings of a rocket motor. Put some blades at each end of the cylinder and you have the beginnings of a jet engine.

So they would be the most obvious examples: power stations, steam trains, cars, jets etc and lets not forget that the muscles of the body ultimately draw their energy from chemical relations in the body.