Chemistry · Chemical Kinetics · NEET
Ea is the minimum EXTRA energy that reactant molecules must absorb so their energy becomes equal to the threshold energy. Only molecules with energy above this barrier can cross over and become products. In the formula k = A·e^(-Ea/RT), Ea is measured in J/mol (or kJ/mol). A big Ea means only a few molecules can cross the barrier, so k is small and the reaction is slow. A small Ea means many molecules make it, so the reaction is fast.
A is called the Arrhenius factor, frequency factor, or pre-exponential factor. It stands for the total number of collisions per second between reactant molecules (with the right orientation). It has the same units as k. A does not change much with temperature. Think of A as 'how many attempts' the molecules make, while e^(-Ea/RT) is 'the fraction of attempts that succeed'.
When T goes up, the value of -Ea/RT gets closer to zero, so e^(-Ea/RT) becomes larger. The term e^(-Ea/RT) is the fraction of molecules that have enough energy to cross the barrier. More heat means more molecules cross the barrier, so k increases and the reaction runs faster. A common NEET fact: near room temperature, a rise of just 10 K roughly doubles the rate for many reactions.
Take natural log of both sides: ln k = ln A − Ea/RT. In base-10 log it becomes log k = log A − Ea/(2.303 RT). We use this form because it is a straight line: if you plot ln k on the y-axis against 1/T on the x-axis, you get a line with slope = −Ea/R. This makes it easy to FIND Ea from experiment — you just measure the slope.
Use the two-temperature form: log(k2/k1) = (Ea/2.303R) × (1/T1 − 1/T2), or equivalently ln(k2/k1) = (Ea/R)(T2−T1)/(T1·T2). Plug in the two rate constants k1, k2 and their temperatures T1, T2, then solve for Ea. NEET fact: to calculate Ea you MUST know the rate constants at two different temperatures — one value is not enough.
Yes, Ea CAN be zero. If Ea = 0, then e^(-Ea/RT) = e^0 = 1, so k = A. This means k does not depend on temperature at all — the rate constant is the same at every temperature. This is exactly what NEET 2019 (Odisha) and NEET 2023 tested. A zero Ea reaction means every collision is already energetic enough to react.
Threshold energy is the TOTAL minimum energy the molecules must HAVE to react. Activation energy Ea is the EXTRA energy they must absorb, measured from the average energy of the reactants. So Ea = Threshold energy − Average energy of reactants. Ea is the height of the barrier above the reactant level; threshold energy is the actual top of the barrier.
Activation energy of any chemical reaction can be calculated if one knows the value of:
For a reaction, the activation energy Ea = 0 and the rate constant at 200 K is 1.6×10⁶ s⁻¹. The rate constant at 400 K will be (R = 8.314 J K⁻¹ mol⁻¹):
For a first-order reaction, ln k = 14.34 − (1.25×10⁴)/T, with k in s⁻¹ and R = 1.987 cal mol⁻¹ K⁻¹. The energy of activation in kcal mol⁻¹ is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
k = A·e^(-Ea/RT), where k is the rate constant, A is the frequency factor, Ea is activation energy, R is the gas constant (8.314 J K⁻¹ mol⁻¹), and T is temperature in Kelvin. The log form is log k = log A − Ea/(2.303RT).
Ea is measured in J mol⁻¹ or kJ mol⁻¹ (sometimes cal mol⁻¹ or kcal mol⁻¹). In NEET numericals, always match Ea units with the R value you use: R = 8.314 J K⁻¹ mol⁻¹ gives Ea in joules, while R = 1.987 cal K⁻¹ mol⁻¹ gives Ea in calories.
Yes. A catalyst lowers the activation energy Ea by giving an alternative path with a smaller energy barrier. It does NOT change enthalpy, entropy, internal energy, or the equilibrium constant. This was directly asked in NEET 2016 — the catalyst alters only Ea.
R = 8.314 J K⁻¹ mol⁻¹ when Ea is in joules. If a NEET question gives Ea or the answer in calories, use R = 1.987 cal K⁻¹ mol⁻¹ (about 2 cal). Always check the units the question wants.
Because the equation is based on the energy distribution of molecules, which depends on absolute temperature. Kelvin starts at absolute zero, so it correctly represents molecular energy. Using Celsius would give wrong results, so always convert to Kelvin (T[K] = t[°C] + 273).
