REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 147

As the equilibrium concentrations of reactants and products have to be such that

r f = r b ð Eq : 5 : 37 Þ

it is clear that these external factors have to influence the forward and reverse reaction
rates in different ways. These factors are external change of concentrations of one of
the species, temperature, and pressure.
To predict the effect of a change in conditions on a chemical equilibrium, le
Chatelier

s principle can be used. The principle can be summarized as follows: if a
chemical system at equilibrium experiences a change in concentration, temperature,
or (partial) pressure, then the equilibrium shifts to counteract the imposed change and
a new equilibrium is established, i.e., more generally, any change in status quo
prompts an opposing reaction in the system.
Let us analyze the effects of the three factors mentioned.

'

Effect of Change in Concentration :
As an example, consider the reaction

ðÞ !

2NH 3 g ðÞ

N 2 g ðÞ +3H 2 g ð RX : 5 : 21 Þ

which is at equilibrium at a certain temperature.
If we add any quantity of hydrogen, nitrogen, or both, in order to minimize its
effect, these reactants turn into the product ammonia, and a new equilibrium, shifted
to the right side of RX. 5.21, is reached. Similarly, if ammonia is removed, new
ammonia is formed, which results in a new equilibrium shifted to the right side. If,
on the contrary, ammonia is added, to minimize the effect of this alteration, the con-
centrations of the reactants increase until a new equilibrium is reached. Similarly, if
hydrogen and/or nitrogen is removed, this would cause the reaction to fill the ”

and favor the side where the species was/were reduced. This observation is supported
by the collision theory. Increasing the concentration of hydrogen and/or nitrogen also
increases the frequency of successful collisions of these reactants, resulting in an
increase of the rate of the forward reaction and generation of ammonia.
These phenomena do not contradict the law of mass action since both the numer-
ator and the denominator in the equation for K eq change so the value of K eq remains
the same.

“ gap

Effect of Change in Temperature : Almost all reactions are accompanied by either
the release or consumption of energy. So, the effect of changing the temperature on the
equilibrium can be made clear by incorporating heat as either a reactant or a product.
For an exothermic reaction, i.e., a reaction releasing energy and having a negative
enthalpy of reaction (

Δ r H), we include heat as a product; for an endothermic reaction,
which consumes energy ( Δ r H is positive), we include it as a reactant. Obviously, if

the forward reaction is exothermic, the reverse reaction is endothermic and the
other way around. We can determine whether increasing or decreasing the tempera-
ture favors the forward or reverse reaction by applying the same principle as with
concentration changes.