: 190
Collision theory explains reaction rates at the molecular level. Rate = Z * f * e^(-Ea/RT). Z = collision frequency (proportional to concentrations and sqrt(T)). f = steric factor (fraction with correct orientation, typically 10^-1 to 10^-9). e^(-Ea/RT) = fraction with sufficient energy. For a collision to be effective, it must satisfy BOTH conditions: (1) kinetic energy >= Ea and (2) correct molecular orientation. Most collisions are ineffective — even at high concentrations, only a tiny fraction leads to reaction. Temperature effect: the Maxwell-Boltzmann distribution shifts right with increasing T, dramatically increasing the high-energy tail. This explains the exponential temperature dependence. Concentration effect: higher concentration means more molecules per unit volume, increasing collision frequency linearly. Limitations of collision theory: it treats molecules as hard spheres (ignores internal structure), overestimates rates for complex molecules, and cannot predict steric factor from first principles. Transition state theory provides a more sophisticated model using the concept of the activated complex.