Chemical KineticsNEET Chemistry · Class 12 · NCERT Chapter 3

You can expect 3 to 5 questions from Chemical Kinetics in NEET every year. The most tested topics are half-life calculations for first-order reactions and the Arrhenius equation. Questions on determining order from experimental data and rate law expressions also appear regularly. This chapter is manageable because the formulas are small in number but frequently applied.

Order of reaction is determined experimentally from the rate law: rate = k[A]^m[B]^n, where order = m + n. It can be zero, a fraction, or a whole number. Molecularity is a theoretical concept that applies only to an elementary step in a reaction mechanism. It is the number of molecules that collide in that single step and is always a small positive integer (1, 2, or 3). For a complex (multi-step) reaction, order is measurable but molecularity is not defined for the overall reaction.

For a first-order reaction, the integrated rate law gives t½ = 0.693/k. The rate constant k depends only on temperature, not on concentration. So t½ comes out the same no matter what [A]₀ is. Physically, this happens because the rate at which the remaining reactant falls to half always takes the same time when the reaction rate is proportional to the current amount. In contrast, for zero-order reactions t½ = [A]₀/2k, which does depend on initial concentration.

When you have rate constants k₁ and k₂ at temperatures T₁ and T₂, use the two-temperature form: ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂), where R = 8.314 J/mol/K and temperatures are in Kelvin. Rearrange to get Ea = R × ln(k₂/k₁) / (1/T₁ - 1/T₂). Make sure to convert temperature to Kelvin (add 273 to Celsius) before plugging in. The result gives Ea in J/mol; divide by 1000 to convert to kJ/mol.

A pseudo-first-order reaction is one that is actually second-order (or higher) but behaves as first-order because one reactant is present in such a large excess that its concentration barely changes. The classic example is the hydrolysis of sucrose: sucrose + H₂O → glucose + fructose. The rate law is rate = k[sucrose][H₂O], which is second-order overall. But water is the solvent and is present in huge excess, so [H₂O] is essentially constant. The reaction then follows rate = k'[sucrose], where k' = k[H₂O]. This is called the pseudo-first-order rate constant.

The most common method in NEET problems is the initial rates method. You compare two experiments where one reactant concentration is doubled while the other is kept constant. If doubling [A] doubles the rate, the order with respect to A is 1. If it quadruples the rate, the order is 2. If the rate does not change, the order is 0. Mathematically: m = log(r₂/r₁) / log([A]₂/[A]₁). You can also use concentration-time graphs: a linear [A] vs t graph means zero-order, a linear ln[A] vs t graph means first-order, and a linear 1/[A] vs t graph means second-order.

Activation energy (Ea) is the minimum energy that colliding molecules must have for a reaction to occur. Even if two molecules collide, the collision is productive only if the combined kinetic energy at the moment of collision is at least equal to Ea. You can picture it as an energy barrier on the reaction profile diagram. Reactants climb this barrier to reach the transition state (activated complex) and then fall down to form products. A higher Ea means fewer molecules in the system have enough energy to react at a given temperature, so the rate is slower.

The rate constant k is a proportionality constant that depends only on temperature (and not on concentration). It is fixed for a given reaction at a given temperature. The rate of reaction r is the actual speed at which the reaction occurs at a specific moment and depends on both k and the current concentrations: r = k[A]^m[B]^n. As the reaction proceeds and concentrations fall, r decreases even though k stays the same. Think of k as a property of the reaction itself, and r as the instantaneous speed that changes as concentrations change.