Page 242 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 16] RATES AND EQUILIBRIUM 231

5. The pressure of gaseous reactants. In general, the higher the pressure of gaseous reactants, the faster the reac-
tion. This factor is merely a corollary of factor 4, since the higher pressure is in effect a higher concentration
(Chap. 12).
6. State of subdivision. The smaller the pieces of a solid reactant—the smaller the state of subdivision—the
faster the reaction. Wood shavings burn faster than solid wood, for example, because they have greater
surface area in contact with the oxygen with which they are combining (for a given mass of wood). In a
sense, this is also a corollary of factor 4.

The collision theory is presented to explain the factors that affect reaction rates. The theory considers the
molecules undergoing reaction to explain the observed phenomena. The theory postulates that in order for a
reaction to occur, molecules must collide with one another with sufficient energy to break chemical bonds in
the reactants. A very energetic and highly unstable species is formed, called an activated complex. Not every
collision between reacting molecules, even those with sufficient energy, produces products. The molecules might
be oriented in the wrong directions to produce products, or the activated complex may break up to re-form the
reactants instead of forming the products. But the huge majority of collisions do not have enough energy to cause
bond breakage in the first place.
The minimum energy that may cause a reaction to occur is called the activation energy, designated E a .
An everyday analogy to activation energy is a golfer whose ball has landed in a deep bunker near the green
(Fig. 16-1). It does not matter if the green is above the bunker or below, the golfer must give the ball sufficient
energy to get over the hill that separates the ball and the green. If the ball is hit with too little energy, it will
merely return to its original level in the bunker. Similarly, if molecular collisions are not energetic enough, the
molecules will merely return to their original states even if temporarily they have been somewhat deformed.

Activated complex
Top of hill
Golf
E Products
ball a
Green
Bunker Reactants
Fig. 16-1. Activation energy

EXAMPLE 16.1. If the activation energy of a certain reaction is 15 kJ/mol and the overall reaction process produces
25 kJ/mol of energy in going from reactants to products, how much energy is given off when the activated complex is
converted to products?
Ans. That process produces 40 kJ/mol:

Activated complex
Reactants 15 kJ
25 kJ
Products

The collision theory allows us to explain the factors that affect the reaction rate. There is a wide range of
energies among molecules in any sample, and generally only the most energetic molecules can undergo reaction.
An increase in temperature increases the number of molecules that have sufficient energy to react (the activation
energy); a rise in temperature of 10 C about doubles the number of molecules with that energy. An increase
◦
in concentration or pressure causes the molecules to collide more often; with more collisions, more effective
collisions are expected. The state of subdivision of a solid affects the reaction rate because the more surface area
there is, the more collisions there are between the fluid molecules and the solid surface. A catalyst works by
reducing the activation energy, making an easier path for the reactants to get to products. Since more reactant
molecules have this (lower) activation energy, the reaction goes faster.