$(a)$ Explain the following terms and provide the difference between them:
$(i)$ Reagent species
$(ii)$ Substrate species
$(b)$ $(i)$ What is the mechanism of a reaction?
$(ii)$ What are the uses of a reaction mechanism?
$(c)$ Depict the general reaction that occurs between a substrate and a reagent in an organic reaction.

(N/A) $(i)$ Reagent: $A$ reagent is a substance that attacks the substrate to initiate a chemical reaction. It is usually the smaller molecule.
$(ii)$ Substrate: The substrate is the organic molecule upon which the reagent acts to form the product.
Difference: The substrate provides the carbon skeleton,while the reagent provides the attacking group.
$(b)$ $(i)$ Reaction mechanism: It is the step-by-step description of the sequence of elementary reactions by which the overall chemical change occurs.
$(ii)$ Uses: It helps in understanding the path of the reaction,predicting the product,and controlling the reaction conditions.
$(c)$ General reaction: $\text{Substrate} + \text{Reagent}$ $\rightarrow \text{Intermediate}$ $\rightarrow \text{Product} + \text{By-product}$