It is for this reason that E'_a in (32.8) was distinguished from E_a of (32.10). Since (1/2) RT is much smaller than E'_a, the difference between E_a and E'_a is generally not of great significance.

Let us compare some experimental results with the results of collision theory. These are shown in Table 32.1

Reactions	Arrhenius Parameters from experiments A / (M ^{– 1} s ^{– 1}) E_a / ( k J / mol)		Collision theory A / (M ^{– 1} s ^{– 1})	A _Arrhenius / A_{Collision Theory}

2NOCl 2 NO + Cl	9.4 * 10 ⁹	102.0	5.9 * 10 ¹⁰	0.16
2ClO Cl₂ +O₂	6.3 * 10 ⁷	0.0	2.5 * 10 ¹⁰	2.5 * 10 ^-3
H₂ + C₂ H₄ C₂ H₆	1.24 * 10 ⁶	180.0	7.3 * 10 ¹¹	1.7 * 10 ^{- 6}
K₂ + Br₂ KBr + Br	1.0 * 10 ¹²	0	2.1 * 10 ¹¹	4.8

Table 32.1 Arrhenius parameters (from Eq. 32.10) and collision theory results for a few gas phase reactions. 

The comparisons are not encouraging. The above collision theory can not predict E_a. In the first three reactions, the experimental collision frequency is much lower than the collision theory results while in the last reaction, it is higher. Rather than discarding the theory, we seek to improve it and learn a lot in the process (in a manner similar to getting improved equations of state, starting with the ideal gas equation of sate). An explanation of the above difference in the values of A is that the actual "reactive" cross section

is different from the collision cross section

. Consider the collisions between H₂ and C₂ H₄ shown in Fig 32.2.

Figure 32.2 Various collisions of H₂ with C₂ H₄

It is for this reason that E'a in (32.8) was distinguished from Ea of (32.10). Since (1/2) RT is much smaller than E'a, the difference between Ea and E'a is generally not of great significance.

It is for this reason that E'_a in (32.8) was distinguished from E_a of (32.10). Since (1/2) RT is much smaller than E'_a, the difference between E_a and E'_a is generally not of great significance.