RATES OF CHEMICAL REACTION

16.2. RATES OF CHEMICAL REACTION

Some chemical reactions proceed very slowly, others with explosive speed, and still others somewhere in between. The “dissolving” of underground limestone deposits by water containing carbon dioxide to form caverns is an example of a slow reaction; it can take centuries. The explosion of trinitrotoluene (TNT) is an example of

a very rapid reaction. The rate of a reaction is defined as the change in concentration of any of its reactants or products per unit time. There are six factors that affect the rate of a reaction:

1. The nature of the reactants. Carbon tetrachloride (CCl 4 ) does not burn in oxygen, but methane (CH 4 ) burns very well indeed. In fact, CCl 4 used to be used in fire extinguishers, while CH 4 is the major component of natural gas. This factor is least controllable by the chemist, and so is of least interest here.

2. Temperature. In general, the higher the temperature of a system, the faster the chemical reaction will proceed.

A rough rule of thumb is that a 10 ◦

C rise in temperature will approximately double the rate of a reaction.

3. The presence of a catalyst. A catalyst is a substance that can accelerate (or slow down) a chemical reaction without undergoing a permanent change in its own composition. For example, the decomposition of KClO 3 by heat is accelerated by the presence of a small quantity of MnO 2 . After the reaction, the KClO 3 has been

changed to KCl and O 2 , but the MnO 2 is still MnO 2 .

4. The concentration of the reactants. In general, the higher the concentration of the reactants, the faster the reaction.

CHAP. 16]

RATES AND EQUILIBRIUM

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 ball

E a Products

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

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.

RATES AND EQUILIBRIUM

[ CHAP. 16