Chemistry · Chemical Kinetics · NEET
Because different species are used up or made at different speeds. In N2 + 3H2 to 2NH3, three H2 molecules disappear for every one N2. So d[H2]/dt is 3 times faster than d[N2]/dt. If we did not divide by the coefficient, we would get three different 'rates' for one reaction. Dividing each term by its coefficient forces them to all equal ONE common number, called the rate of reaction. That is why NEET answers always show 1/3, 1/2 etc. in front.
Reactants are being used up, so their concentration falls with time. That makes d[reactant]/dt a negative value. Rate is a speed, so it must be positive. We add a minus sign to flip the negative into a positive. Products are being made, so their concentration rises and d[product]/dt is already positive, so it keeps a plus sign. Rule: minus for reactants, plus for products.
Rate of disappearance of a reactant is just how fast that one species vanishes, like -d[A]/dt. Rate of reaction is the single value that describes the whole reaction. They are equal ONLY when the coefficient is 1. When the coefficient is bigger, rate of reaction = (1/coefficient) x rate of disappearance. So they are not always the same number. NEET often traps students here.
Divide each species by its coefficient and add the sign. Rate = -d[N2]/dt = -(1/3)d[H2]/dt = +(1/2)d[NH3]/dt. From this you can pull any pair. For example -(1/3)d[H2]/dt = (1/2)d[NH3]/dt. This exact reaction came in NEET 2019, so learn to write the full chain first, then compare the two terms the question asks about.
Set the two rate-of-reaction terms equal, then rearrange. For 3A to 2B: -(1/3)d[A]/dt = +(1/2)d[B]/dt. To get d[B]/dt alone, multiply both sides by 2: d[B]/dt = -(2/3)d[A]/dt. The ratio of the two rates equals the ratio of their coefficients (product coeff over reactant coeff). This is the NEET 2023 answer.
For the chemical reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the correct option is:
For a reaction 3A -> 2B, the average rate of appearance of B is Δ[B]/Δt. The correct relation between the rate of appearance of B and the rate of disappearance of A is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. The whole point of dividing by the stoichiometric coefficient is that you get the SAME single number for every species. That common value is the rate of the reaction.
Only when that species has a stoichiometric coefficient of 1. For example in A -> B, rate = -d[A]/dt = +d[B]/dt. If the coefficient is 2 or 3, you must divide, so they are no longer equal.
Concentration divided by time, so mol L^-1 s^-1 (or mol L^-1 min^-1). The units are the same whichever species you track, because dividing by a pure-number coefficient does not change units.
No. This stoichiometry expression tells you how the rates of different species relate to each other. The rate law (Rate = k[A]^x[B]^y) tells you how rate depends on concentration, and its powers come from experiment, not from the coefficients.
It is a repeat favourite. The N2 + 3H2 reaction (2019) and the 3A -> 2B relation (2023) are both direct one-step questions. Getting the signs and the fraction right is an almost guaranteed mark.
