Chemistry · Chemical Kinetics · NEET
It means how fast a reaction happens. As a reaction runs, reactants get used up and products get made. Rate measures how much the concentration changes in a fixed amount of time. High rate = fast reaction (like burning). Low rate = slow reaction (like iron rusting or diamond turning to graphite). For NEET, always think: rate = concentration change ÷ time.
Reactant concentration falls as the reaction goes on, so its change Δ[R] is negative. But rate must be a positive number (a reaction cannot go 'negative fast'). So we put a minus sign in front: Rate = −Δ[R]/Δt. The minus cancels the negative change and gives a positive rate. Products get made, so Δ[P] is already positive: Rate = +Δ[P]/Δt. NEET loves testing this sign.
Rate = concentration ÷ time = (mol L⁻¹) ÷ (s), so the unit is mol L⁻¹ s⁻¹ (also written mol/L/s). Do NOT confuse this with the unit of the rate constant k, which changes with the order of the reaction. The rate of reaction always has the same unit: mol L⁻¹ s⁻¹.
No. Rate is how fast the reaction goes RIGHT NOW, and it changes as concentrations change. The rate constant k is a fixed number for a given reaction at a given temperature; it does not depend on concentration. They are linked by the rate law, for example Rate = k[A]. When concentration drops, rate drops, but k stays the same until you change the temperature.
Usually no. At the start there is a lot of reactant, so collisions are frequent and the rate is high. As reactant gets used up, the rate slows down. This is why we later split it into 'average rate' (over a time gap) and 'instantaneous rate' (at one instant). Only a zero-order reaction keeps a constant rate.
For a reaction like aA → bB, one rate expression must describe the whole reaction, so we divide each change by its coefficient: Rate = −(1/a)Δ[A]/Δt = +(1/b)Δ[B]/Δt. This is why a species with a bigger coefficient disappears or appears faster. Getting this ratio right is a common NEET question.
For a reaction 3A → 2B, the average rate of appearance of B is Δ[B]/Δt. The correct relation between the average rate of appearance of B and the average rate of disappearance of A is:
An increase in the concentration of the reactants of a reaction leads to a change in:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
mol L⁻¹ s⁻¹ (moles per litre per second), because rate = concentration change divided by time.
No. Rate is always positive. The minus sign in −Δ[R]/Δt is only there to cancel the negative change of a reactant so the rate comes out positive.
Reactant concentration keeps falling, so there are fewer particles to collide. Fewer collisions per second means a slower rate as the reaction proceeds.
The single 'rate of reaction' is the same value once you divide each species' change by its coefficient. The raw change of each species can differ if their coefficients differ.
Chemical kinetics questions build on this basic idea. Rate leads to average vs instantaneous rate, rate law, order, and the rate constant k — all high-yield NEET topics.
