Ch 03 - Chemical Kinetics12th Chemistry

Thermodynamics: Predicts the feasibility or spontaneity of a reaction (whether it can happen) based on $\Delta_r G < 0$. It also predicts the extent of the reaction (Equilibrium).

Chemical Kinetics: Predicts the rate or speed of the reaction (how fast it happens) and the mechanism by which it proceeds.

Note: A reaction can be thermodynamically highly feasible but kinetically so slow that it is imperceptible (e.g., diamond converting to graphite).

• Instantaneous/Very Fast Reactions: Occur in fractions of a second. Example: Ionic reactions like the precipitation of $AgCl$ upon mixing aqueous $AgNO_3$ and $NaCl$.
• Very Slow Reactions: Take months or years to show noticeable change. Example: Rusting of iron in the presence of air and moisture.
• Moderate Speed Reactions: Proceed at measurable rates at room temperature. Examples: Inversion of cane sugar, hydrolysis of starch.

Variables & Units:

• $r_{av}$ = Average rate of reaction ($mol \, L^{-1} \, s^{-1}$)
• $\Delta[R]$ = Change in concentration of reactant ($mol \, L^{-1}$)
• $\Delta[P]$ = Change in concentration of product ($mol \, L^{-1}$)
• $\Delta t$ = Time interval ($s$ or $min$)

Conditions/Notes:

• The negative sign ($-$) is strictly placed before $\Delta[R]$ to ensure the overall rate value is positive, because reactant concentration strictly decreases over time.