Chemical Kinetics Mock Test for JEE Main 2025-26 Preparation

The rate of a chemical reaction can be influenced by several factors:
- Concentration of reactants
- Temperature
- Presence of a catalyst
- Surface area of reactants
- Nature of reactants
- Pressure (for gases)
These factors alter the frequency and energy of molecular collisions, thereby impacting how quickly a reaction proceeds.

Rate constant (k) is a proportionality constant in the rate law equation that relates the reaction rate to the concentrations of reactants. Units of k vary depending on the reaction order:
- For a first-order reaction, units are s⁻¹
- For a second-order reaction, units are L mol⁻¹ s⁻¹

Average rate measures the change in concentration of reactants or products over a specified time interval, while instantaneous rate is the rate at a specific moment during the reaction, typically obtained from the slope of a concentration vs. time graph at that point.

The rate law expresses the relationship between the reaction rate and the concentrations of reactants, commonly in the form Rate = k[A]^m[B]ⁿ. To determine it experimentally:
1. Measure the initial rates for varying reactant concentrations.
2. Analyze how rate changes with each reactant.
3. Deduce the order with respect to each reactant.

Order of reaction is the sum of powers of the concentration terms in the rate law, while molecularity refers to the number of reactant molecules involved in an elementary step of the reaction mechanism. Order is experimental; molecularity is theoretical and applies only to elementary reactions.

Half-life (t_1/2) is the time required for the concentration of a reactant to reduce to half its initial value. For a first-order reaction, t_1/2 = 0.693/k, where k is the rate constant.

The Arrhenius equation relates the rate constant (k) to temperature (T): k = A exp(−Ea/RT), where A is the frequency factor, E_a is activation energy, R is the gas constant, and T is temperature. Activation energy is the minimum energy required for reactant molecules to successfully collide and react.

A catalyst increases the rate of a chemical reaction by lowering the activation energy, providing an alternative pathway for the reaction to occur. It does not affect the overall thermodynamics or the position of equilibrium.

Zero-order reactions have a rate independent of reactant concentration (e.g., decomposition of ammonia on platinum). First-order reactions have a rate proportional to the concentration of one reactant (e.g., radioactive decay, hydrolysis of esters).

For elementary reactions, molecularity above three is extremely rare because the probability of four or more molecules colliding simultaneously with proper orientation and energy is negligible, making such collisions statistically unlikely.

Half-life (t_1/2) for a first-order reaction is t_1/2 = 0.693/k. Substituting k = 0.2 min⁻¹:
t_1/2 = 0.693 / 0.2 = 3.465 minutes.
Thus, the half-life of the reaction is approximately 3.47 minutes.