Define Instantaneous Rate of a reaction mathematically for a reactant R.

Why does the rate of any reaction generally decrease during the course of the reaction?

For the reaction $2HI(g) \rightarrow H_2(g) + I_2(g)$ , express the rate of reaction in terms of the disappearance of HI.

What are the units of rate of a reaction for solutions and gases?.

How does the rate of disappearance of a reactant relate to the rate of appearance of a product in $5 Br^{-} + BrO_3^{-} + 6 H^{+} \rightarrow 3 Br_2 + 3 H_2O$ ?.