Subtopics - Chemical Kinetics (NEET)
Seven topic blocks: rate of reaction with factors affecting rate, molecularity vs order of reaction, first order kinetics with integrated rate equation and half-life, higher order reactions (zero, second, third), Arrhenius equation with collision theory and catalysis, experimental methods for order determination, and photochemical reactions.
1) Rate of Reaction
Defines rate as change in concentration per unit time. For aA + bB to cC + dD: rate = -(1/a)(d[A]/dt) = (1/c)(d[C]/dt). Distinguishes average rate from instantaneous rate. Lists six factors affecting rate: nature of reactant, concentration, surface area, catalyst, temperature, and light.
2) Molecularity and Order of Reaction
Molecularity is the number of reacting species in an elementary step (always a positive integer, rarely above 3). Order is the sum of the powers in the experimentally determined rate law (can be zero, fractional, or negative). For complex reactions, molecularity of the rate-determining step governs the kinetics. Includes pseudo-unimolecular reactions.
3) First Order Kinetics, Rate Equation, and Half-Life
The integrated first order rate equation: k = (2.303/t) log([A]0/[A]t). A plot of log[A]t vs t gives a straight line with slope -k/2.303. Half-life t1/2 = 0.693/k is independent of initial concentration. Time for 75% completion = 2 t1/2; for 99.9% = 10 t1/2. Gaseous first order reactions use partial pressure: k = (2.303/t) log(P1/P2). Multiple cases for total pressure calculations.
4) Zero, Second, and Third Order Reactions
Zero order: rate = k, [A]t = [A]0 - kt, t1/2 = [A]0/(2k). Second order: 1/[A]t = kt + 1/[A]0, t1/2 = 1/(k[A]0). Third order: more complex expression, t1/2 proportional to [A]0^(-2). General nth order: unit of k is (mol/L)^(1-n) time^(-1), and t1/2 proportional to [A]0^(1-n).
5) Arrhenius Equation, Collision Theory, and Effect of Catalyst
The Arrhenius equation k = Ae^(-Ea/RT) quantifies the temperature dependence of rate constants. Log k vs 1/T gives a straight line with slope -Ea/(2.303R). Two-temperature form: log(k2/k1) = (Ea/2.303R)(1/T1 - 1/T2). Collision theory explains why only a fraction of collisions are effective (energy barrier + orientation barrier). A catalyst lowers Ea equally for forward and reverse reactions without shifting equilibrium.
6) Experimental Determination of Order
Four methods to determine order experimentally: integrated equation or hit-and-trial (substitute data into rate equations for different orders and check which gives constant k), fractional change method (compare half-lives at different initial concentrations), graphical method (plot rate vs concentration), and Ostwald isolation method (take all reactants except one in excess).
7) Photochemical Reactions
Reactions initiated by absorption of light. Einstein's law: each molecule absorbs one photon (E = hc/lambda per molecule, E = Nhc/lambda per mole). Rate depends on intensity of absorbed radiation, not temperature. Quantum yield phi = molecules reacted / photons absorbed. Photosensitisation: foreign substance absorbs light and transfers energy to reactant.
Chemical Kinetics Download Notes & Weightage Plan
For each topic in the Chemical Kinetics chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Rate definition, stoichiometric rate expression, average vs instantaneous rate, six factors affecting rate.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Writing the correct rate expression with stoichiometric coefficients. For 2A + B to 3C: rate = -(1/2)(d[A]/dt) = -d[B]/dt = (1/3)(d[C]/dt).
- High-risk Area: Forgetting to divide by the stoichiometric coefficient. For 2N2O5 to 4NO2 + O2: the rate of disappearance of N2O5 is -(1/2)(d[N2O5]/dt), not -(d[N2O5]/dt). Missing the 1/2 introduces a factor-of-two error.
- Best Practice Style: Always write the balanced equation first. Each d[species]/dt term is divided by its coefficient.
Molecularity and Order of Reaction
Distinguishing molecularity (theoretical, from mechanism) from order (experimental). Pseudo-unimolecular reactions.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Molecularity can NEVER be zero or fractional. Order CAN be zero, fractional, or negative. Molecularity applies only to elementary steps. For complex reactions, only the rate-determining step's molecularity matters.
- High-risk Area: Assuming order = molecularity for all reactions. This is true only for elementary reactions. For complex reactions, the overall order can differ significantly from the sum of stoichiometric coefficients. NEET tests this distinction directly.
- Best Practice Style: If the question says 'elementary reaction', then order = molecularity. Otherwise, order must be determined experimentally.
First Order Kinetics, Rate Equation, and Half-Life
The most important topic for NEET. Integrated rate equation, half-life formula, pressure-based cases for gaseous reactions.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Three high-yield results: (1) k = (2.303/t) log([A]0/[A]t), (2) t1/2 = 0.693/k (independent of [A]0 for first order), (3) for gaseous A to B+C: PA at time t = 2P(initial) - P(total at time t). Master these three and you can solve most first order problems.
- High-risk Area: In gaseous Case II (A to B+C, given total pressure), students confuse P(total) with P(A). At time t, P(total) = P1 + x (where x = extent of reaction), not P1 - x. The pressure of A at time t is 2P1 - P(total), which is less than P1. Getting this algebra wrong inverts the log term.
- Best Practice Style: For gaseous reactions: (1) write the ICE table in terms of partial pressures, (2) express P(A) at time t in terms of given total pressures, (3) substitute into k = (2.303/t) log(P(A,0)/P(A,t)).
Zero, Second, and Third Order Reactions
Zero order: rate is constant. Second order: 1/[A]t = kt + 1/[A]0. General nth order: unit of k and t1/2 proportional to [A]0^(1-n).
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Two quick identification tools: (1) Unit of k: if k has units of time^-1, it is first order. If mol^-1 L time^-1, it is second order. (2) Half-life independence of [A]0 means first order. Half-life proportional to [A]0 means zero order. Half-life inversely proportional to [A]0 means second order.
- High-risk Area: For zero order, [A]t = [A]0 - kt can give negative concentrations if t > [A]0/k. The reaction stops when [A] = 0, not when the formula gives a negative number. Students who extrapolate beyond completion time get physically meaningless results.
- Best Practice Style: Use the half-life dependence on initial concentration as the fastest diagnostic: t1/2 proportional to [A]0^(1-n). n=0: proportional to [A]0. n=1: independent. n=2: inversely proportional.
Arrhenius Equation, Collision Theory, and Effect of Catalyst
k = Ae^(-Ea/RT). Two-temperature form for calculating Ea. Collision theory: energy barrier and orientation barrier. Catalyst lowers Ea without changing K(eq).
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: The two-temperature Arrhenius formula is the most tested: log(k2/k1) = (Ea/2.303R)(1/T1 - 1/T2). Common NEET trick: providing temperatures in Celsius (must convert to Kelvin). Catalyst effect: lowers Ea, does not change delta-H or K(eq).
- High-risk Area: Forgetting to convert Celsius to Kelvin in the Arrhenius equation. Using T = 25 instead of T = 298 K makes the 1/T term 12 times too large, giving an Ea that is absurdly small. NEET typically gives temperatures in Celsius to catch this error.
- Best Practice Style: Step 1: convert all temperatures to Kelvin. Step 2: substitute into the two-temperature form. Step 3: use R = 8.314 J/mol K if Ea is in J/mol, or R = 2 cal/mol K if Ea is in cal/mol.
Experimental Determination of Order
Four methods: integrated equation (hit and trial), fractional change, graphical, and Ostwald isolation.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Fastest identification: if log[A] vs t is linear, the reaction is first order. If 1/[A] vs t is linear, it is second order. If [A] vs t is linear, it is zero order.
- High-risk Area: Confusing the y-axis variables: [A] vs log[A] vs 1/[A]. Each corresponds to a different order. Plotting the wrong variable gives a curve instead of a straight line.
- Best Practice Style: For graphical identification, try the first-order plot first (log[A] vs t) since first order is the most commonly tested. If linear, done.
Reactions initiated by light absorption. Einstein's law, quantum yield, characteristics distinguishing photochemical from thermal reactions.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Key distinction: photochemical rate depends on light intensity (not temperature). Quantum yield phi > 1 is possible for chain reactions (HCl synthesis: phi = 10^6).
- High-risk Area: Assuming photochemical reactions always have phi = 1. Chain reactions like HCl synthesis have enormously high quantum yields because one absorbed photon triggers thousands of reaction cycles.
- Best Practice Style: For photochemical MCQs: rate depends on I(abs), not T. delta-G can be positive. Each molecule absorbs exactly one photon.
Chemical Kinetics Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Chemical Kinetics chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Using natural log formula but reading common log value: k = (1/t) ln([A]0/[A]t) uses natural log (ln), while k = (2.303/t) log([A]0/[A]t) uses common log (log10). Mixing the two gives k that is off by a factor of 2.303.
- Thinking t(75%) = 1.5 times t(1/2): After one half-life, 50% remains. After two half-lives, 25% remains (75% complete). So t(75%) = 2 t(1/2), not 1.5 t(1/2). Students who confuse fraction remaining with fraction completed make this error.
A first order reaction has k = 0.01 min^-1. t1/2 = 0.693/0.01 = 69.3 min. t(75%) = 2 x 69.3 = 138.6 min. If one mistakenly uses t(75%) = 1.5 x 69.3 = 104 min, the answer is wrong because 75% completion means only 25% remains, which takes exactly two half-lives.
How NEET Frames The Trap
NEET gives k and asks for time to complete 75% or 87.5% of the reaction.
Q. The half-life of a first order reaction is 20 minutes. The time required for 75% completion is
A. 40 min B. 30 min C. 60 min D. 20 min
Trick: 75% completion means 25% remains, which is (1/2)^2 of the original. So it takes 2 half-lives = 40 min (Option A). Option B uses 1.5 x t1/2. Option C uses 3 x t1/2 (which would be 87.5%).
Mistake Snapshot (What Students Do Wrong)
- Using total pressure instead of partial pressure of reactant: For A to B+C, the total pressure at time t is P(total) = P(A) + P(B) + P(C) = P1 + x. The partial pressure of A is P(A) = P1 - x = 2P1 - P(total). Substituting P(total) directly as P(A) gives wrong k.
- Confusing P1 + x with P1 - x: Total pressure increases (P1 + x) because 1 mole of A produces 2 moles of products. But the partial pressure of A decreases (P1 - x). Students who assume total pressure decreases get the algebra inverted.
A(g) to B(g) + C(g). Initial pressure = 100 mmHg. Total pressure at time t = 130 mmHg. x = 130 - 100 = 30. P(A) = 100 - 30 = 70 mmHg. k = (2.303/t) log(100/70). If student uses P(total) as P(A): k = (2.303/t) log(100/130), which gives a negative log (impossible for k).
How NEET Frames The Trap
NEET gives initial pressure and total pressure at time t for a gaseous decomposition and asks for the rate constant.
Q. For the decomposition A(g) to 2B(g), initial pressure of A = 200 mmHg. Total pressure after 10 min = 300 mmHg. Rate constant is
A. 0.0693 min^-1 B. 0.0231 min^-1 C. 0.0462 min^-1 D. 0.1386 min^-1
Trick: A to 2B: at time t, P(A) = P1 - x, P(B) = 2x. P(total) = P1 - x + 2x = P1 + x. So x = 300 - 200 = 100. P(A) = 200 - 100 = 100. k = (2.303/10) log(200/100) = 0.2303 x 0.301 = 0.0693 min^-1 (Option A). Option B uses P(total) as P(A).
Mistake Snapshot (What Students Do Wrong)
- Using temperature in Celsius instead of Kelvin: The Arrhenius equation requires absolute temperature (Kelvin). Using Celsius (e.g., 25 instead of 298) makes 1/T about 12 times too large, giving Ea that is 12 times too small.
- Swapping T1 and T2 in the two-temperature formula: The formula is log(k2/k1) = (Ea/2.303R)(1/T1 - 1/T2) where T2 > T1 and k2 > k1. Swapping gives a negative Ea, which is physically impossible for most reactions.
k = 2 x 10^-5 s^-1 at 300 K and 4 x 10^-5 s^-1 at 310 K. log(4/2) = (Ea/2.303 x 8.314)(1/300 - 1/310). 0.301 = (Ea/19.15)(10/93000) = Ea x 5.61 x 10^-6. Ea = 0.301/5.61 x 10^-6 = 53,655 J/mol = 53.7 kJ/mol. Using T = 27 and 37 instead of 300 and 310: 1/27 - 1/37 = 0.010, which gives Ea = 0.301/(0.010 x Ea/19.15), a completely wrong value.
How NEET Frames The Trap
NEET gives temperatures in Celsius and rate constants at those temperatures. Students must convert to Kelvin first.
Q. A reaction has Ea = 60 kJ/mol. If k at 27 degrees C is 1.5 x 10^-3 s^-1, what is k at 37 degrees C? (R = 8.314 J/mol K)
A. 3.0 x 10^-3 B. 1.5 x 10^-3 C. 6.0 x 10^-3 D. 0.75 x 10^-3
Trick: Convert: T1 = 300 K, T2 = 310 K. log(k2/1.5 x 10^-3) = (60000/2.303 x 8.314)(1/300 - 1/310) = (3134)(1.075 x 10^-4) = 0.337. k2/k1 = 10^0.337 = 2.17. k2 = 2.17 x 1.5 x 10^-3 = 3.3 x 10^-3, closest to Option A (3.0 x 10^-3). Option B assumes no temperature effect. Option D swaps T1 and T2.
Mistake Snapshot (What Students Do Wrong)
- Stating molecularity can be zero or fractional: Molecularity is always a positive integer (1, 2, or 3). It is the number of species in the elementary step. Order CAN be zero, fractional, or negative.
- Equating order with stoichiometric coefficients for complex reactions: For the overall reaction 2A + B to products, the order is NOT necessarily 3 (2+1). The order is determined experimentally and depends on the rate-determining step mechanism.
Overall reaction: 2N2O5 to 4NO2 + O2. This is a complex reaction with a multi-step mechanism. The experimental rate law is rate = k[N2O5], so the order is 1, not 2. Molecularity of the rate-determining step (unimolecular decomposition of N2O5) is 1.
How NEET Frames The Trap
NEET gives an overall reaction equation and asks for the order. The distractor is the sum of stoichiometric coefficients.
Q. Which of the following is true about molecularity?
A. It is always a whole number B. It can be zero C. It can be fractional D. It can be negative
Trick: Option A: Molecularity is always a positive whole number (1, 2, or 3). Options B, C, D describe properties of ORDER, not molecularity. Order can be zero (option B), fractional (option C), or negative (option D).
Mistake Snapshot (What Students Do Wrong)
- Thinking catalyst changes equilibrium position: A catalyst lowers Ea for both forward and reverse reactions equally. It speeds up both directions equally. The equilibrium constant K and equilibrium position remain unchanged.
- Thinking catalyst changes delta-H of reaction: The catalyst changes the pathway but not the initial and final states. Since H is a state function, delta-H remains the same with or without catalyst.
For N2 + 3H2 to 2NH3, adding Fe catalyst: forward and reverse rates both increase equally. K(eq) remains the same. delta-H remains -92 kJ. Only Ea is lowered. Without catalyst, Ea might be 200 kJ; with Fe, it might be 120 kJ. But delta-H is always -92 kJ.
How NEET Frames The Trap
NEET asks what a catalyst changes: Ea, K, delta-H, or equilibrium position.
Q. Which of the following statements about a catalyst is correct?
A. It lowers the activation energy of the reaction B. It changes the enthalpy of the reaction C. It changes the equilibrium constant D. It shifts the equilibrium position to the right
Trick: Option A is correct. A catalyst provides an alternative pathway with lower Ea. It does NOT change delta-H (state function), K(eq) (depends only on delta-G), or equilibrium position. It only speeds up the approach to equilibrium.