Quote by answerseeker
but have another query: for reaction kinetics, when monitoring the change in a closed system, would you only look at the reactant side for the change?

Changes in the reactant side are related to changes in the product side through the stoichiometry of the reaction. For instance :
[tex]2HBr \leftrightarrow H_2 + Br_2 [/tex]
Consuming 2 moles of HBr produces a mole each of H
_{2} and Br
_{2}. So, removing any x moles of HBr yields an increase by x/2 moles each of H
_{2} and Br
_{2}.
[tex]\Delta [H_2] = \Delta [Br_2] = \frac {1}{2} \Delta [HBr] [/tex]
[tex] \implies \frac {d}{dt} [H_2] = \frac {d}{dt} [Br_2] = \frac {1}{2} \frac {d}{dt}
[HBr] [/tex]