Lesson 9 of 11 · 9 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.
Simulations in this lesson
Each on its own page