Chemistry · Chemical Kinetics · NEET
A reaction mechanism is the actual set of small steps a reaction goes through to change reactants into products. Most real reactions do not happen in one jump. They happen in a series of simple steps called elementary steps. Adding up all these steps gives the balanced (overall) equation. NCERT gives the example of hydrogen peroxide breaking down: 2H2O2 to 2H2O + O2. This looks simple, but it actually happens in more than one step.
The rate-determining step is the slowest step in the mechanism. NCERT compares it to a relay race: the whole team is only as fast as its slowest runner. In the same way, the overall reaction cannot go faster than its slowest step. So the rate of the whole reaction is equal to the rate of this slow step. This is why it is also called the rate-controlling step.
The balanced equation only tells you how much reactant and product there is, not how the reaction really happens. The rate law depends on how the reaction actually proceeds, which is decided by the slow step. So you take the reactants of the slow (rate-determining) step and put their concentrations into the rate law. This is a very common NEET trap: the order is NOT always the same as the coefficients in the balanced equation.
An intermediate is a species that is made in one step and used up in a later step. It does not appear in the final balanced equation. If the slow step contains an intermediate, you cannot leave it in the rate law, because we cannot measure an intermediate easily. You replace it using the fast step before it (called a fast pre-equilibrium). For example, if a fast step gives X2 in equilibrium with 2X, then [X] = sqrt(Keq[X2]). You substitute this into the slow-step rate to get the final rate law in terms of real reactants.
Step 1: write the rate law using only the reactants of the slow step. Step 2: if any of these are intermediates, replace them using the fast equilibrium step before it. Step 3: add up all the powers in the final rate law. That sum is the overall order. This is exactly what the NEET 2017 question tested, and the answer came out to be 1.5, not a whole number.
An elementary reaction happens in a single step. For an elementary step only, the order is equal to its molecularity, so you can read the rate law directly from that step. A complex reaction happens in two or more steps (a mechanism). For a complex reaction you must find the slow step first. So order equals molecularity only for elementary steps, never automatically for the overall complex reaction.
Mechanism of a hypothetical reaction X2 + Y2 to 2XY is given below: (i) X2 in equilibrium with X + X (fast); (ii) X + Y2 in equilibrium with XY + Y (slow); (iii) X + Y to XY (fast). The overall order of the reaction will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The rate-determining step is defined as the slowest step in the mechanism. The whole reaction cannot go faster than this step, so it controls the overall rate.
Yes. When the mechanism has a fast pre-equilibrium that gives a square-root term, the order can be a fraction like 1/2 or 1.5. NEET 2017 had exactly this, giving order 1.5.
It can. If the slow step contains an intermediate, you must replace that intermediate using the fast equilibrium step before it, so that the final rate law is written only in terms of measurable reactants.
Because the rate law and order of a reaction are decided by the mechanism, not the balanced equation. NEET regularly gives a multi-step mechanism and asks you to find the order from the slow step. Getting this idea clear stops you from losing easy marks.
Next you should learn about the activated complex and the energy barrier, which explains why the slow step is slow. That is the concept of threshold energy and the top of the energy hill that reactants must cross.
