Chemistry · Chemical Kinetics · NEET
They are not the same. Threshold energy is the TOTAL minimum energy that colliding molecules must have to react. Activation energy (Ea) is only the EXTRA energy reactants must gain on top of the energy they already have. In one line: Ea = Threshold energy − Average energy of reactant molecules. So threshold energy is measured from zero, but Ea is measured from where the reactants start. NEET loves this exact distinction.
It is a short-lived, unstable arrangement of atoms that forms at the very top of the energy barrier, when old bonds are half-broken and new bonds are half-formed. It is also called the transition state. It exists for a tiny fraction of a second and then either falls forward to give products or falls back to reactants. It is NOT a normal stable molecule you can bottle.
No, and NEET can trap you here. The activated complex sits at the PEAK of the energy curve (highest energy, cannot be isolated). A reaction intermediate sits in a small DIP (a valley) between two peaks in a multi-step reaction, so it is comparatively more stable and can sometimes be detected. Peak = activated complex; valley = intermediate.
Yes. If the reactant molecules already possess energy equal to the threshold value, then no extra energy is needed, so Ea = 0. This is exactly why NEET 2023 marked the assertion 'a reaction can have zero activation energy' as TRUE. A zero-Ea reaction is extremely fast because no barrier has to be climbed.
The energy barrier is the hump on the potential-energy vs reaction-coordinate graph that separates reactants from products. Molecules must have at least the threshold energy to cross it. A HIGH barrier (large Ea) means fewer molecules can cross, so the reaction is slow. A LOW barrier means more molecules cross, so the reaction is fast. This is why Ea controls reaction speed.
A catalyst gives the reaction a new, alternative pathway with a LOWER activation energy. It does this by helping form a more stable (lower-energy) activated complex. So the barrier becomes shorter and more molecules can cross it, making the reaction faster. A catalyst does NOT change enthalpy (ΔH), internal energy, or entropy of the reaction, only Ea.
Assertion A: A reaction can have zero activation energy. Reason R: The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to the threshold value is called activation energy. Choose the correct option.
The addition of a catalyst during a chemical reaction alters which of the following quantities?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Threshold energy = activation energy + average energy of the reactant molecules. Since the average energy of reactants is not negative, threshold energy is always greater than or equal to Ea. They become equal only in the special case when the average energy of reactants is zero.
It sits exactly at the highest point (the peak) of the potential-energy vs reaction-coordinate curve, at the top of the energy barrier between reactants and products.
Yes. A higher Ea means a taller barrier, so fewer molecules have enough energy to cross it at a given temperature, making the reaction slower. This is why raising temperature or adding a catalyst (which lowers Ea) speeds reactions up.
Because at the peak the old bonds are partly broken and the new bonds are only partly formed, giving it maximum energy. High energy means low stability, so it exists only for a very short time before turning into products or reactants.
No. Whether the reaction releases or absorbs energy depends on the energy of products versus reactants (the ΔH). The activated complex only sets the height of the barrier, which controls the SPEED, not whether the reaction is exo- or endothermic.
