Definition of Reaction Rate and Chemical Equilibrium

All chemical reaction has a certain spontaneity towards him Balance, and to investigate it we do it through the sign of ∆G, Energy Gibbs free, which implies that, through the value of this magnitude, we can predict whether a reaction will occur in a certain sense or not.

The variation of the Gibbs Free Energy is expressed, in general, under standard conditions as the difference between the energies of products and reactants also in standard state:

Whereas, if the reaction occurs under non-standard conditions, the relationship between ∆Gº and ∆G is determined by the following expression:

Where Q is the reaction quotient.

To understand the implication of the reaction speed and the chemical equilibrium we must study the sign of ∆G:

If ∆G is negative, it implies that the reaction is spontaneous (occurs) in the direct sense.

If ∆G is positive, it implies that the reaction is not spontaneous (does not occur) in the direct sense.

Whereas, if ∆G = 0, there will be no change, since the system is in equilibrium, and as already mentioned, the speed Direct reaction rate equals indirect reaction rate. This implies that the reaction quotient Q is equal to the equilibrium constant K, so there is no tendency to favor a specific direction of the reaction.

Since Q is defined as:

For a generic reaction:

While K takes the same form, but with the concentrations in equilibrium.

If we return to the case where ∆G is negative, this implies that the reaction quotient Q is less than K (constant of equilibrium), implies that the product concentrations are lower than they should be if the reaction were in Balance. Therefore, in terms of spontaneity, it becomes spontaneous in the direct sense.

Whereas, if ∆G is positive, there will be a preponderance of products above those that should exist if the system were in equilibrium, with Q greater than K. Therefore, the reaction is spontaneous in the reverse direction.

It should be noted that the strict definition of Q and K is given in terms of the activities of the products and reactants, defining the activity in terms of concentration or pressures as:

O well:

From there it follows that both Q and K are dimensionless and can be raised both in concentrations and partial pressures.

When the concentrations or partial pressures of products and reactants are kept constant over time, the situation occurs chemical equilibrium, insofar as a situation of dynamic equilibrium is reached because the rate of direct and inverse reaction with identical. It is important to highlight the dynamicity of equilibrium, the speed with which they are formed and consume products and reagents is the same, that is why the concentrations or partial pressures do not it varies.

If the condition moves away from the equilibrium situation, certain species will prevail over another and from there arises the expression that relates the direct and inverse reaction speed, Kc:

Suppose the reaction seen above:

Where Kd and Ki are the reaction rate constants in the forward or reverse direction respectively.

Again, if Kc> 1, it implies that Ki is less than Kd, therefore, there is a high degree of conversion of products into reactants. In this case, the equilibrium is shifted towards products.

The reverse occurs if Kc <1, implying that the direct reaction rate is less than the indirect reaction rate and there is little consumption of reactants, the equilibrium is shifted towards reactants. Whereas, if Kc = 1, the speeds are equal and the system is in equilibrium. It is important to define two issues: first, the value of this constant depends exclusively on the temperature and, in turn, varies according to the magnitude used to express the concentrations or pressures of products and reactants. Finally, the law Chemical equilibrium adjusts to dilute solutions or gases under low pressure.

Topics in Reaction Rate and Chemical Equilibrium