Global reaction is a simplification of a system of chemical reactions. Real chemical reactions may contain 100s of reactions that happen at molecular level. These individual reactions area called *[[Detailed chemistry]]*. You might not want to track all those reactions. They maybe simplified into a set of few reactions. These are called *reduced mechanisms*. Good rule of thumb is to breakdown a global reaction based on its *rate determining step*. Ter-molecular reactions are only important in high pressures (3 molecules need to combine for the reactions to happen)