Page 241 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 16

Rates and

Equilibrium

16.1. INTRODUCTION
We have learned (Chap. 8) that some reactions occur under one set of conditions while an opposite reaction
occurs under another set of conditions. For example, we learned that sodium and chlorine combine when treated
with each other, but that molten NaCl decomposes when electrolyzed:
2Na + Cl 2 −→ 2 NaCl
electricity
2 NaCl(molten) −−−→ 2Na + Cl 2
However, some sets of substances can undergo both a forward and a reverse reaction under the same set of
conditions. This circumstances leads to a state called chemical equilibrium. Before we take up equilibrium,
however, we have to learn about the factors that affect the rate of a 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.
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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.
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