What chemical reaction is used as energy is used as an energy source to make electricity?

Answer 1

It is not that simple

Electricity made in the conventional steam cycle uses a fuel to boil water to make high pressure steam; the high pressure steam is forced through turbine blades spinning them and generating electricity.

If a fossil fuel is used as the fuel, the carbon reacts with oxygen making carbon dioxide; the hydrogen reacts with oxygen making water.

If a nuclear fuel is used, it is not a chemical but a nucear reaction where the attoms are split and due to slight mass differences this creates much heat used to boil the water.

This is a simplification.

Answer 2

All you would have to ask yourself is what are all of the chemical reactions that occur in your own body that generate the energy for you to live from day to day.