Chemistry · Chemical Kinetics · NEET
An elementary reaction is a reaction that finishes in ONE single step. The reacting particles collide once and turn straight into products. There is no hidden middle step. Example: 2HI -> H2 + I2. Because it is one step, you CAN read its rate law and molecularity directly from that step.
A complex reaction does NOT finish in one step. It happens as a series of many elementary steps, one after another. This whole set of steps is called the mechanism (or reaction mechanism). Most real reactions are complex reactions, not one-step reactions.
Simple rule: an elementary reaction = one step. A complex reaction = a sequence of many elementary steps (a mechanism). For an elementary reaction the balanced equation IS the real event. For a complex reaction the balanced equation only shows the start and the end, not the steps in between.
No. NCERT says clearly: a balanced equation never gives a true picture of how a reaction takes place, because a reaction rarely finishes in one step. The equation tells you what goes in and what comes out. It does NOT tell you the middle steps. Only for a true elementary reaction does the equation match the real step.
For many molecules to react together, they must all collide at the same instant. The chance of three molecules hitting at the exact same moment is very small; for four or more it is almost zero. So nature breaks the job into small 2-molecule collisions. That is why big reactions become complex, multi-step reactions.
In a complex reaction, the different steps run at different speeds. The slowest step controls the overall speed of the whole reaction. It is called the rate-determining step. NCERT uses a relay race: the team's speed depends on its slowest runner. So the rate law of a complex reaction comes from its slow step, not from the full balanced equation.
No, and this is the trap NEET loves. For a complex reaction you must NOT use the balanced-equation coefficients as the order. NCERT's example KClO3 + 6FeSO4 + 3H2SO4 -> ... looks like tenth order from the equation, but it is actually second order because it goes in several steps. Order for such reactions is found only by experiment or from the slow step.
Mechanism of a hypothetical reaction X2 + Y2 -> 2XY is given below: (i) X2 <=> X + X (fast); (ii) X + Y2 <=> XY + Y (slow); (iii) X + Y -> XY (fast). The overall order of the reaction will be:
For an elementary chemical reaction, the Arrhenius plot (ln k vs 1/T) is given. If the activation energy is 6.64 kJ/mol and R = 8.3 J/K/mol, the temperature at which the rate constant becomes e^2 per min is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Most real reactions are complex. NCERT states that a reaction rarely finishes in one step, so single-step elementary reactions are the exception, not the rule.
A reaction mechanism is the full sequence of elementary steps that together turn reactants into products. A complex reaction is defined by its mechanism.
No. Molecularity is defined only for an elementary reaction (a single step). For a complex reaction you talk about the molecularity of each individual step, not of the whole reaction.
Because the balanced equation hides the real steps. Only the slow step decides the rate, so the true order must be measured experimentally or worked out from the mechanism, not read off the equation.
NEET gives you a mechanism with fast and slow steps and asks for the order. If you wrongly add the coefficients of the overall equation you get the wrong answer. Knowing elementary vs complex tells you to use the slow step instead.
