Chemistry · Chemical Kinetics · NEET
It changes ONLY the activation energy (Ea). A catalyst lowers Ea by giving a new low-energy path (a different transition state). Enthalpy change ΔH is fixed by the energy of reactants and products, and the catalyst does not touch those. So Ea goes down, but ΔH stays exactly the same. This is the single most tested idea in NEET (2016 asked this directly).
Because those are state functions or fixed properties. ΔH, internal energy (ΔU), and entropy (ΔS) depend only on the START (reactants) and END (products) states. A catalyst only changes the PATH in between, not the two ends, so these values cannot change. K also stays fixed because K depends on ΔG (which comes from ΔH and ΔS). The catalyst just helps you reach equilibrium faster, at the same K.
NCERT (6.8.5) says: 'A catalyst increases the rate by making available a new low energy pathway.' The catalyst forms a temporary bond with reactants (or an intermediate/activated complex) that needs less energy. With a smaller energy barrier, a larger fraction of molecules already have enough energy to cross it. More molecules crossing per second = faster rate.
BOTH, and by the SAME amount. NCERT says the catalyst lowers activation energy for the forward AND reverse reactions equally. That is why it speeds up both directions equally and reaches equilibrium faster, but K (the ratio) does not change. If a question says a catalyst changes K, that statement is wrong.
No. A catalyst cannot make an impossible reaction happen. It only speeds up a reaction that is already feasible (already has a chance to go). If the equilibrium constant K is extremely small, a catalyst is of little help (NCERT note). Remember: 'Catalyst does not initiate; it only accelerates.' NEET 2017 tested this exact line.
A catalyst lowers activation energy (the barrier). Collision frequency is changed by concentration or temperature, NOT by a catalyst. Do not mix these up: increasing concentration → more collisions per second (collision frequency); catalyst → lower Ea (easier barrier). NEET 2020 tested that concentration changes collision frequency.
The addition of a catalyst during a chemical reaction alters which of the following quantities?
Which one of the following statements is NOT correct?
An increase in the concentration of the reactants of a reaction leads to change in:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It does not appear in the balanced overall equation and is not in the equilibrium constant expression (NCERT). A catalyst is regenerated at the end. It CAN appear in the rate law if it takes part in the slow (rate-determining) step, but for NEET at this level treat it as not consumed and not in the overall equation.
No. A catalyst is not consumed overall. It may bond temporarily with reactants to form an intermediate, but it is released again at the end, so the same catalyst keeps working.
A positive catalyst lowers activation energy and speeds up the reaction (the normal case). A negative catalyst (inhibitor) raises the effective barrier or blocks the path and slows the reaction down.
Yes. Enzymes are biological catalysts. They lower the activation energy of biochemical reactions in the body. Coenzymes increase the catalytic activity of enzymes. NEET has tested these enzyme facts along with catalyst theory.
Yes. NEET 2023 confirmed a reaction can have Ea = 0 when the reactant molecules already have energy equal to the threshold value. In that case, from the Arrhenius equation the rate constant does not change with temperature.
