What is the rate of reaction? — The change in concentration of a reactant or product per unit time. Rate = Δ[concentration] / Δtime (mol/L/s or mol dm⁻³ s⁻¹). Rate decreases over time as reactants are consumed.

What four factors affect the rate of reaction? — 1. Concentration (more particles → more collisions). 2. Temperature (more energy → more successful collisions). 3. Surface area (more contact → more collisions). 4. Catalyst (lowers activation energy → more successful collisions).

What is collision theory? — Reactions occur only when reactant particles collide with sufficient energy (≥ activation energy) and with the correct orientation. Increasing any factor that raises collision frequency or energy increases reaction rate.

What is activation energy (Ea)? — The minimum energy required for a collision to result in a reaction. Only particles with energy ≥ Ea upon collision will react. A lower Ea means more particles can react → faster rate.

What is the Maxwell-Boltzmann distribution? — A graph showing the distribution of kinetic energies among particles in a gas/liquid at a given temperature. The area under the curve beyond Ea represents the fraction of particles that can react. Increasing T shifts the curve right — more particles exceed Ea.

How does increasing temperature affect reaction rate? — Rate approximately doubles for every 10°C rise in temperature (rule of thumb). More particles have kinetic energy ≥ Ea, and collision frequency increases. The Maxwell-Boltzmann curve shifts to higher energies.

What is the rate law (rate equation)? — Rate = k[A]ᵐ[B]ⁿ where k = rate constant, [A] and [B] = concentrations of reactants, m and n = orders of reaction with respect to each reactant. The rate law must be determined experimentally — it cannot be deduced from the balanced equation.

What is the order of reaction? — The power to which a reactant's concentration is raised in the rate law. Zero order: Rate independent of [A] — rate = k. First order: Rate ∝ [A] — rate = k[A]. Second order: Rate ∝ [A]² — rate = k[A]². Overall order = sum of all individual orders.

How do you determine order of reaction experimentally? — Using the initial rates method: Run experiments with varying concentrations. Compare how rate changes when one reactant's concentration is doubled (or halved). If rate doubles → 1st order. Rate quadruples → 2nd order. No change → zero order.

What is the rate constant k and what affects it? — A proportionality constant in the rate law. k is specific to a reaction at a given temperature. It increases with increasing temperature. A large k = fast reaction. Units depend on the overall order of reaction.

What are the units of k for different reaction orders? — Zero order: mol L⁻¹ s⁻¹. First order: s⁻¹. Second order: L mol⁻¹ s⁻¹. Third order: L² mol⁻² s⁻¹. Units are derived from: Rate (mol L⁻¹ s⁻¹) = k × [concentration]ⁿ.

What is the half-life (t½) of a reaction? — The time taken for the concentration of a reactant to fall to half its initial value. First-order reaction: t½ = ln2/k = 0.693/k — constant, independent of initial concentration. Used in radioactive decay calculations.

What is the Arrhenius equation? — k = Ae^(−Ea/RT) where k = rate constant, A = frequency factor (collision frequency with correct orientation), Ea = activation energy, R = 8.314 J/mol·K, T = temperature (K). Shows the exponential relationship between k and temperature.

How is the Arrhenius equation used graphically? — Plot ln k vs 1/T — gives a straight line with slope = −Ea/R. Therefore: Ea = −slope × R. y-intercept = ln A. Used to calculate Ea from experimental rate constant data at different temperatures.

What is a catalyst and how does it work? — A substance that increases reaction rate without being consumed. It provides an alternative reaction pathway with lower activation energy. Does not change the thermodynamics (ΔG, ΔH) or the equilibrium position.

What is a homogeneous catalyst? — A catalyst in the same phase as the reactants. Example: H⁺(aq) catalyzing ester hydrolysis in aqueous solution. Often forms an intermediate with the reactant, which then breaks down to give products and regenerate the catalyst.

What is a heterogeneous catalyst? — A catalyst in a different phase from the reactants. Usually a solid catalyst with gaseous or liquid reactants. Reaction occurs at the catalyst surface (adsorption). Examples: Fe in the Haber Process, V₂O₅ in the Contact Process, Pt in catalytic converters.

What is the rate-determining step? — The slowest step in a multi-step reaction mechanism. It limits the overall rate of the reaction. The rate law reflects the concentrations of species involved in (or before) the rate-determining step.

What is a reaction mechanism? — A step-by-step sequence of elementary reactions showing exactly how reactants are converted to products. Each step is called an elementary step. The overall balanced equation is the sum of all steps.

What is a reaction intermediate? — A species produced in one step of a reaction mechanism and consumed in a subsequent step. It does not appear in the overall balanced equation. Different from a transition state (which cannot be isolated).

What is a transition state (activated complex)? — An unstable, high-energy arrangement of atoms at the top of the energy barrier between reactants and products. It is transient — it cannot be isolated. The energy difference between reactants and the transition state = activation energy Ea.

How does surface area affect reaction rate? — A larger surface area exposes more particles to collisions. Dividing a solid into smaller pieces increases surface area dramatically. Example: powdered calcium carbonate reacts faster with HCl than large lumps. Relevant in heterogeneous catalysis.

What is the difference between rate and rate constant? — Rate: Changes during a reaction as concentrations change (usually decreases over time). Rate constant (k): Fixed for a given reaction at a given temperature — independent of concentration. Changing temperature changes k but not the rate law expression.

How does a catalyst appear on an energy profile diagram? — The catalysed pathway shows a lower activation energy (lower energy hump). If the mechanism involves multiple steps, there may be multiple humps, all lower than the uncatalysed pathway. The start and end energy levels (reactants and products) remain the same.

What is enzyme kinetics (Michaelis-Menten)? — Enzymes are biological catalysts. At low substrate concentration: rate ∝ [substrate] (first order). At high substrate concentration: rate plateaus at Vmax (zero order — enzyme saturated). Km = substrate concentration at ½Vmax — measures enzyme-substrate affinity.