Rate of Reaction — Average, instantaneous rate
Chemical Kinetics
8
JEE Qs
8%
Hard
50
min
Master the sign conventions and stoichiometric factor applications for relating rates of different species and the overall reaction rate, as this is fundamental to all subsequent concepts in chemical kinetics.
🧮 Key Formulas
✅ Key Points for JEE
- 1Rate of reaction is always a positive quantity. The negative sign for reactants indicates decrease in concentration over time, and positive sign for products indicates increase.
- 2The 'rate of reaction' must be normalized by the stoichiometric coefficients of the reacting species to ensure it's a unique value, independent of which species' change is monitored.
- 3Average rate is the rate measured over a finite time interval, while instantaneous rate is the rate at a specific moment in time (represented by the slope of the tangent to the concentration-time curve).
- 4Units of reaction rate are typically mol L^-1 s^-1 or M s^-1. Ensure consistency in units for time (seconds, minutes, hours) and concentration (Molarity).
- 5Distinguish between the rate of disappearance/appearance of a specific reactant/product and the overall rate of the chemical reaction.
⚠️ Common Mistakes
- ✕Forgetting to divide by the stoichiometric coefficient when converting the rate of change of a specific species to the overall rate of reaction.
- ✕Incorrectly applying the negative sign for products or positive sign for reactants.
- ✕Confusing average rate with instantaneous rate, especially when solving problems involving concentration-time graphs.
- ✕Not paying attention to the units given in the problem and providing answers in inconsistent units.
📝 Practice Questions
See allQ72.For the thermal decomposition of N2O5( g) at constant volume, the following table can be formed, for the reaction mentioned below. 2 N2O5( g) →2 N2O4( g) + O2( g) x = … × 10−3 atm [nearest integer] Given : Rate constant for the reaction is 4.606 × 10−2 s−1 .
Q58.For a reaction, N2O5( g) →2NO2( g) + 12 O2( g) in a constant volume container, no products were present initially. The final pressure of the system when 50% of reaction gets completed is (1) 5 times of initial pressure (2) 5/2 times of initial pressure (3) 7/2 times of initial pressure (4) 7/4 times of initial pressure
Q72. A → B The molecule A changes into its isomeric form B by following a first order kinetics at a temperature of 1000 K . If the energy barrier with respect to reactant energy for such isomeric transformation is 191.48 kJ mol−1 and the frequency factor is 1020 , the time required for 50% molecules of A to become B is _________ picoseconds (nearest integer). [R = 8.314 J K−1 mol−1]
Q55. Consider the given figure and choose the correct option : (1) Activation energy of both forward and backward (2) Activation energy of forward reaction is E1 + E2 reaction is E1 + E2 and reactant is more stable and product is less stable than reactant. than product. (3) Activation energy of backward reaction is E1 and (4) Activation energy of forward reaction is E1 + E2 product is more stable than reactant. and product is more stable than reactant.
Q54.Which of the following graphs most appropriately represents a zero order reaction ? 2025 (23 Jan Shift 2) JEE Main Previous Year Paper (1) (2) (3) (4)
Q73.Consider a complex reaction taking place in three steps with rate constants k1, k2 and k3 respectively. The overall rate constant k is given by the expression k = . If the activation energies of the three steps are √k1k3k2 60,30 and 10 kJ mol−1 respectively, then the overall energy of activation in kJmol−1 is … … . (Nearest integer)
NCERT Chapters
- Class 12 Chemistry Ch 4: Chemical Kinetics