Chemical reactions ordinarily occur as a result of collisions between reacting particles.

Consider the reaction:

CO_(g) + NO_2(g) ----> CO_2(g) + NO_(g)

rate = k(conc CO)(conc NO_2(g)

• doubling the concentration of CO, holding NO₂ constant, the number of collisions in a given time doubles.
  
• doubling the concentration of NO₂, holding CO constant, has the same effect.

In general, the number of collisions per unit time is directly proportional to the concentration of CO or NO₂.

The fact that the rate is directly proportional to these concentrations implies that reaction occurs as a direct result of collisions between CO and NO₂ molecules.

NOT EVERY COLLISION LEADS TO REACTION!!!!!

It is possible to calculate the rate at which molecules collide with each other by using the kinetic theory.

Consider a mixture of CO and NO₂ at 700 K and a concentration of 0.10 mol/L

• every molecule would collide with about a billion other molecules in one second!
• if every collision resulted in a reaction, then the whole mixture would be reacted in a fraction of a second.
  
• the actual reaction takes about 20 seconds.

In order for collisions to be effective, there must be considerable force in the colllisions. The slower moving molecules do not have enough kinetic energy to react when they collide...they bounce off one another and retain their identity.

Only those molecules moving at high speed have enough energy for collisions to result in a reaction.

Every reaction requires a certain minimum energy for the reaction to occur--it is called activation energy, E_a, and is expressed in kJ.

The CO + NO₂ mixture from above has an E_a = 134 kJ. This means that the colliding molecules must have a total KE of 134 kJ/mol if they are to react.

SO, what are the properties of the activation energy, E_a?

• it is a positive quantity, E_a>0
  
• it depends upon the nature of the reaction. "Fast" reactions usually have a small E_a; those with a large E_a usually proceed slowly.
  
• it is independent of temperature and concentrations.

Activated Complex-an intermediate species that is an unstable, high energy species that must be formed before the reaction can occur.

The ... stands for partial bonds. The O-N bond has been partially broken and the C-O bond has started to form.
• forward reaction exothermic (DH<0), E_a is smaller than E_a¹
  
• forward reaction endothermic (DH>0), E_a is larger than E_a¹
  
• if DH = +200 kJ, then E_a = E_a¹ + DH = E_a¹ + 200 kJ