Lesson 9 of 11 · 6 min
Effect of a catalyst
NCERT §3.4.1
Without iodide, the club's peroxide would sit on the bench for days with barely a bubble. A pinch of KI makes it fizz in minutes, and at the end the iodide is all still there.
The lesson in notes
In short
A catalyst increases the rate of a reaction and is chemically unchanged at the end. MnO₂ speeds up the decomposition of KClO₃: 2KClO₃ → 2KCl + 3O₂.
A substance added to slow a reaction is not called a catalyst; it is an inhibitor.
Intermediate complex theory: the catalyst takes part by forming temporary bonds with a reactant to give an intermediate complex, which then breaks up into products and releases the catalyst.
The catalyst provides a new pathway (mechanism) with a lower activation energy, so a larger fraction of molecules can cross the barrier at the same temperature.
A small amount of catalyst is enough to change the rate greatly.
A catalyst does not change ΔG of the reaction. It can speed up only a reaction that is already spontaneous; it cannot make a non-spontaneous reaction happen.
It lowers the barrier equally for forward and reverse reactions, so equilibrium is reached sooner but the equilibrium constant and the equilibrium composition do not change.