Introduction to Soil Chemistry

38 soil basics ii
simply states that the change in enthalpy is the difference between the energy
at the beginning H i and at the end of the reaction H e. If a reaction is endothermic
it will be at a higher energy level at the end that at the beginning. The
reverse is true for an exothermic reaction. It is expected that the exothermic
reaction leads to the more stable product and thus is more likely to be formed.
In addition to enthalpy, there is also the consideration of entropy or randomness.
Reactions generally tend to go to a more random state. It would
seem that any reaction involving the formation of a highly ordered crystal
would be going to a state of decreased randomness. Thus there would be an
increase in entropy, which would not favor the reaction. However, if the overall
entropy of the system increases, then the reaction is favored.
The total energy of a system involves both enthalpy and entropy. Thus,
whichever causes the greater change during a reaction will be the one controlling
the reaction and determining whether it is exothermic, endothermic,
spontaneous, or not spontaneous [13].
2.5. REACTION PATHS
Reaction paths involve following the energy path during a reaction. Typically
reactants have a starting energy (R–R, Figure 2.5) which increases through
either some transition state or reaction intermediate or both, leading to an
ending energy. In Figure 2.5 the transition states (indicated by TS) are hypothetical
species or structures that are in transition and cannot be isolated and
identified. As indicated by the slight energy trough, the reactive intermediates
are stable enough to be isolated and studied. Often reactive intermediates are
ions such as carbanions.
PE
P l
TS
TS
RI
TS
RI
TS
R + R
Figure 2.5. A potential energy (PE) diagram for a reaction that can occur in two different ways
producing two different products (P): one kinetically and the other thermodynamically controlled
(R—reactant; TS—transition state; RI—reactive intermediate; P—product).
P r