Advanced Topic 446 of 1102 Kinetics & Equilibrium
Pre-exponential Factor
The constant A representing collision frequency and orientation probability.
Practise Kinetics & Equilibrium All Kinetics & Equilibrium topics
In-depth explanation of Pre-exponential Factor
The pre-exponential factor A is the constant in the Arrhenius equation that represents the frequency of collisions with the correct orientation. It combines the collision frequency with the probability that colliding molecules have the required geometry to react. For bimolecular gas phase reactions A is typically of the order of 10^10 to 10^13 L mol^-1 s^-1, while for unimolecular reactions it has units of s^-1. A small value of A suggests that only a narrow range of orientations leads to reaction, which often happens with large or complex molecules. A large value suggests that most collisions are favourably oriented.
Going deeper: Pre-exponential Factor
Key ideas: Interpret A as roughly P x Z. Collision theory gives Z about 10^11 L per mol per s for simple gas reactions, and measured A values for larger molecules can be 10^6 to 10^8, giving P well below one. In the Eyring picture A is related to kT/h times exp(delta S double dagger/R), connecting A to entropy of activation.
Common mistakes and connections: Students think A is a temperature-independent constant in all cases, but it has a weak temperature dependence. Another error is to ignore its units: A has the same units as k. A tiny A does not mean a high barrier, it indicates an unfavourable orientation or a very ordered transition state. This links to the steric factor and Eyring equation.
Visualization of Pre-exponential Factor
Both reactions have Ea = 50 kJ/mol, but the one with the smaller A (stricter orientation needed) is slower at every temperature, by a constant factor of 100. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Reactions between large, flexible molecules often have small steric factors, which lower A well below the simple collision value.
Frequently asked questions about Pre-exponential Factor
What is Pre-exponential Factor?
The constant A representing collision frequency and orientation probability.
Is Pre-exponential Factor a beginner, intermediate or advanced topic?
Pre-exponential Factor is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Pre-exponential Factor?
Study these earlier topics in Kinetics & Equilibrium first: Activated Complex, Arrhenius Equation, Arrhenius Plot.
What comes after Pre-exponential Factor?
The next topics in Kinetics & Equilibrium are Temperature Effect on Rate and Effect of Concentration on Rate.
Can I practise Pre-exponential Factor with quizzes?
Yes. The interactive lesson includes a self-check question on Pre-exponential Factor, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Pre-exponential Factor fits in Kinetics & Equilibrium
Pre-exponential Factor is topic 27 of 91 in Kinetics & Equilibrium.