The weak temperature dependence of the preexponential factor is so small compared to the exponential factor that it cannot be measured experimentally, that is, "it is not feasible to establish, on the basis of temperature studies of the rate constant, whether the predicted T½ dependence of the preexponential factor is observed experimentally". Steric factor If the values of the predicted rate constants are compared with the values of known rate constants, it is noticed that collision theory fails to estimate the constants correctly, and the more complex the molecules are, the more it fails. The reason for this is that particles have been supposed to be spherical and able to react in all directions, which is not true, as the orientation of the collisions is not always proper for the reaction. For example, in the hydrogenation reaction of ethylene the H2 molecule must approach the bonding zone between the atoms, and only a few of all the possible collisions fulfill this requirement. To alleviate this problem, a new concept must be introduced: the steric factor ρ. It is defined as the ratio between the experimental value and the predicted one (or the ratio between the frequency factor and the collision frequency): and it is most often less than unity. Usually, the more complex the reactant molecules, the lower the steric factor. Nevertheless, some reactions exhibit steric factors greater than unity: the harpoon reactions, which involve atoms that exchange electrons, producing ions. The deviation from unity can have different causes: the molecules are not spherical, so different geometries are possible; not all the kinetic energy is delivered into the right spot; the presence of a solvent (when applied to solutions), etc. {| class="wikitable" |+ Experimental rate constants compared to the ones predicted by collision theory for gas phase reactions |- ! Reaction ! A, s−1M−1 ! Z, s−1M−1 ! Steric factor |- | 2ClNO → 2Cl + 2NO || 9.4 || 5.9 || 0.16 |- | 2ClO → Cl2 + O2 || 6.3 || 2.5 || 2.3 |- | H2 + C2H4 → C2H6 || 1.24 || 7.3 || 1.7 |- | Br2 + K → KBr + Br || 1.0 || 2.1 || 4.3 |- |} Collision theory can be applied to reactions in solution; in that case, the solvent cage has an effect on the reactant molecules, and several collisions can take place in a single encounter, which leads