Chemistry · Chemical Kinetics · NEET
Average rate is measured over a period of time (a gap between two moments), so it uses a large change: rate = -Δ[R]/Δt. Instantaneous rate is measured at one single instant, so the time gap becomes very tiny (Δt approaches 0), and it is written as rate = -d[R]/dt. In short: average rate uses Δ (a finite gap), instantaneous rate uses d (an infinitely small gap).
Draw a tangent line that just touches the curve at the exact time you want. The slope (steepness) of that tangent gives the instantaneous rate at that moment. For a reactant, the concentration falls, so the slope is negative; you take the magnitude. This matters for NEET because many graph questions ask you to read the rate at a specific instant, not over an interval.
As reactants get used up, their concentration drops, so collisions happen less often and the reaction slows down. This is why the average rate over the whole reaction is different from the instantaneous rate at the start (which is fastest) and at the end (which is slowest). The instantaneous rate at the very beginning is called the initial rate.
For a reaction R to P, average rate = -Δ[R]/Δt = +Δ[P]/Δt. The minus sign is used for the reactant because its concentration decreases (Δ[R] is negative), and rate must always be a positive number. The product uses a plus sign because its concentration increases.
Initial rate is a special instantaneous rate taken at time t = 0, right when the reaction starts. So initial rate is one particular instantaneous rate. It is very useful in NEET because at t = 0 the concentrations are exactly what you started with, which makes rate-law calculations easier.
Use average rate when you are given two concentration readings and the time between them (an interval). Use instantaneous rate when the question asks for the rate at one specific time or shows a tangent on a graph. For rate law and order-of-reaction problems, instantaneous (or initial) rate is what the rate equation actually refers to.
For a reaction 3A -> 2B, the average rate of appearance of B is given by Δ[B]/Δt. The correct relation between the average rate of appearance of B and the average rate of disappearance of A is:
For a certain reaction R -> Product, the plot of concentration [R] versus time is a straight line with a constant negative slope. The order of the reaction is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. For a zero-order reaction the concentration-time graph is a straight line, so the slope is the same everywhere. Then the average rate over any interval equals the instantaneous rate at every point. For curved graphs (first, second order) they are generally different.
A reactant is used up, so its concentration change Δ[R] is negative. The minus sign flips it to positive, because reaction rate is always reported as a positive quantity.
Rate is change in concentration per unit time, so its units are mol L⁻¹ s⁻¹ (or mol L⁻¹ min⁻¹). This is true for both average and instantaneous rate.
The rate law (rate = k[A]^x...) always refers to the instantaneous rate, because k describes the rate at the actual concentrations present at that instant. Initial rate (instantaneous at t = 0) is commonly used to find the order.
