Chemistry · Chemical Kinetics · NEET
Faster collisions are NOT the main reason. When you heat a reaction, the number of collisions per second goes up only a little (roughly a few percent). The big change is in ENERGY. Temperature shifts the energy distribution of molecules (the Maxwell-Boltzmann curve) so that many more molecules now have energy above the activation energy (Ea). These high-energy molecules are the only ones that can react. Even a 10°C rise can roughly double the number of molecules that clear the barrier, which is why the rate roughly doubles. This is important for NEET because they test the idea that energy, not just collision frequency, controls the rate.
It is a simple observation: for many reactions, the rate becomes about 2 times (sometimes 2 to 3 times) faster for every 10°C (10 K) rise in temperature. This factor is called the temperature coefficient. It is only an approximate rule, not an exact law. NEET may ask you to estimate how many times faster a reaction goes if temperature rises by 20°C or 30°C: raise the factor to the right power. For example, a 30°C rise with a coefficient of 2 means about 2 to the power 3 = 8 times faster.
Temperature changes the rate constant k itself. The rate = k[A]^x[B]^y. Concentrations do not change when you heat the flask, but k does. As temperature rises, k increases, so the rate increases. This is a favourite NEET point: k depends on temperature (through the Arrhenius equation), while it does not depend on concentration. So when you heat a reaction, you are really increasing k.
Almost always yes, but there is one special exam case. If the activation energy Ea = 0, then temperature has NO effect on the rate constant. This comes straight from the Arrhenius equation: if Ea = 0, the temperature term drops out and k stays the same at every temperature. NEET (2019 Odisha) tested exactly this. So the safe answer is: for a normal reaction with Ea greater than 0, heating increases the rate; but if Ea = 0, k does not change with temperature.
No. Temperature does NOT change the activation energy Ea. Ea is a fixed height of the barrier for a given reaction path. What temperature changes is how many molecules have enough energy to cross that fixed barrier. To actually LOWER Ea you need a catalyst, which gives a new, lower-energy path. Students often mix these up: temperature = more molecules over the same hill; catalyst = a lower hill.
For a reaction, the activation energy Ea = 0 and the rate constant at 200 K is 1.6 x 10^6 s^-1. The rate constant at 400 K will be (Given R = 8.314 J K^-1 mol^-1):
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.
As a rough rule, the rate becomes about 2 to 3 times faster for every 10°C (10 K) rise. This factor is the temperature coefficient. It is approximate, not exact.
Concentration is set by how much reactant you put in the flask; heating does not change it. The rate constant k depends on temperature through the Arrhenius equation, so heating raises k, and therefore raises the rate.
No. Collision frequency rises only a little. The main reason is that many more molecules now have energy above the activation energy, so far more collisions are successful.
No. Only a catalyst lowers activation energy by giving a new path. Temperature keeps Ea the same and just helps more molecules reach it.
If Ea = 0, the rate constant does not change with temperature. This is a common NEET trap; heating gives no speed-up when there is no energy barrier.
