Once enough hydrogen ions have been pumped into the intermembrane space, they tend to move back inside to the matrix. what is this movement of particles form areas of high concentration to low concentration called? *

The question pertains to the electron transport chain that occurs in the mitochondria. First, we must identify the basic parts of the mitochondria namely (from outside to inside): Outer mitochondrial membrane, intermembrane space, inner mitochondrial membrane (cristae, convoluted to increase surface area), and the mitrochondrial matrix. The proteins (or complexes) involved in the electron transport chain is embedded in the inner mitochondrial membrane. For every electron transported from one complex to the other, there is a reduction-oxidation reaction of NADH or FADH to NAD+ or FAD+ having hydrogen ions (H+) be pumped against a concentration gradient (to produce a gradient favorable for the last complex, the ATP synthase, to produce ATP). The movement of particles from high to low concentration is called diffusion (either simple or facilitated). 

To kill bacteria from our body especially from stomach

Idont know but good luck; )