Rate of a Chemical Reaction

Why kinetics, when thermodynamics already exists? Thermodynamics (Class XI, Unit 7) tells you whether a reaction CAN happen at all under given conditions -- but it is completely silent on how long it takes. Mixing $BaCl_2$ solution with dilute $H_2SO_4$ gives an instant white $BaSO_4$ precipitate, while rusting of iron takes years, yet thermodynamics alone cannot distinguish the two. Chemical kinetics is the branch that fills this gap: the study of the rate and mechanism of a chemical reaction under given conditions of temperature, pressure and concentration. Beyond satisfying curiosity, kinetics is directly useful for optimising real industrial and synthetic processes, deciding how long a reactor needs to run, or how a plant should be heated/cooled/stirred.

Defining rate. Just as velocity is displacement per unit time, the rate of a chemical reaction is the change in concentration of a reacting species per unit time. For the simplest possible reaction, $A\rightarrow B$, measuring $[A]$ at two moments $t_1$ and $t_2$ ($t_2>t_1$) as $[A_1]$ and $[A_2]$ gives

Since the reactant is being consumed, $[A_2]<[A_1]$, so $\Delta[A]$ comes out negative; the minus sign in front is inserted purely by convention so that the rate itself is always reported as a positive number. If you instead track the PRODUCT B, whose concentration only rises, $\text{Rate}=+\dfrac{\Delta[B]}{\Delta t}$ needs no such sign flip. …