What is Equilibrium? 

Equilibrium does not always imply an equal presence of reactants and products. This signifies that the reaction reaches a point when reactant and product quantities remain constant as the rate of forward and backward reaction is the same. Molecular structures of various compounds can help in predicting equilibrium. 

Chemical Equilibrium 

If the quantities of reactants and products remain constant, a chemical reaction is said to be in equilibrium that is their ratio does not change along with time. Equilibrium is attained when forward and backward events occur at similar rates. 

"A graph showing chemical equilibrium.”

Factors Affecting Chemical Equilibrium 

Le Chatelier’s Principle 

The concept of the Le Chatelier asserts that an alteration in the equilibrium system will lead to a predicted modification that counters the modification. The principle of the Le Chatelier is to observe chemical process equilibrium. It indicates that variations in climate pressure, volume or concentration of a system are likely to lead to predictable and counter-system adjustments to attain a new degree of balance. 

Concentration change 

The introduction of an extra reacting substance to the system shifts the balance to the right side towards the product according to the Le Chatelier principle. The same principle also changes the balance to the right, when the concentration of any product lowered. 

When a system adds extra products then the balance shifts to the left to generate more reactants. Or equilibrium will also change to the left if we remove reactants from the system. 

Pressure change 

A variation in pressure or volume is intended to re-establish equilibrium by producing more or fewer gaseous particles. For instance, as the pressure in a medium rises or the volume drops, the balance moves to the side with fewer gas molecules. Likewise, the creation of extra gas moles would be preferred if the volume of the system is increased or the pressure decreased. 

Temperature change 

The effect of temperature alteration has on equilibrium is directly correlated with the heat evolved during a reaction. For an endothermic reaction, the value of ΔH is positive as heat is being absorbed in the reaction while for an exothermic reaction the value for ΔH is negative as heat is being released. It is just like adding additional reactant to the system while raising the temperature of an endothermic reaction, and thus the balance is going to shift the right one according to Le Chatelier's principle. In contrast, a reduction in a thermal reaction temperature changes the balance to the left, as the decrease in temperature is equal to the removal of a reactant in this situation. Heat is a product of an exothermic process. As a result, the temperature increase will push the balance to the left, whereas the temperature drop will push the balance to the right. 

Effect of catalyst 

The inclusion of a catalyst, also including reversible reactions with a final equilibrium can accelerate reactions. In the existence of, a catalyst the front and back reaction frequency will accelerate identically and enable the system to balance more quickly. The inclusion of a catalyst does not have any influence on the ultimate balance of the process. It's only getting faster then. 

Valence-Shell Electron-Pair Repulsion (VSEPR) Model  

The VSEPR model can anticipate the structure of almost every ion wherein the central atom has a non-metal, and even the shape of several polyatomic particles with a center metal atom. The VSEPR theory assumes that the electron pairs situated in bonds and lonely pairs oppose each other, thereby adopting the geometry which distinguishes electron pairs well beyond feasible. This concept is quite straightforward and takes care not just of the complexities of orbital interaction which affect molecule forms, but of three-dimensional structures of enormous quantities of compounds which cannot be counted reliably by the basic Lewis electron-pair method. 

"An image showing VSEPR model of atoms.”
Atom effects 

The capacity of an atom to draw electrons to it as connections are formed is called electronegativity. Once a negative charge was added, atoms with enhanced electronegativity are much more stabilized. The relation of a hydrogen atom linked to an oxygen atom to a hydrogen atom linked to a carbon atom demonstrates an explanation of the influence of the electronegativity on acidification. Oxygen is the most electronegative molecule and therefore the charge is more stable than a carbon with a charge. This equilibrium leads to low pKa than those of carbon or oxygen, hence demonstrates that oxygen is a stronger acid. 

Resonance delocalization 

A negative charge can be delocalized for compounds with resonance properties. Each of several or more Lewis models with many equivalent interpretations is a resonance structure. This relocation is a stabilizing factor, and when the number of atoms increases the more the charge is scattered, the more the anion is stable. If the resonance framework of the base is not stable, it would be a poor base and would be produced from strong acid. 

For instance, the resonance is the reason for equilibrium of carboxylate anion while it is not for ethoxide anion. Resonance is the process used by organic chemists to manage organic compounds containing two and sometimes more Lewis’s arrangements with many alternatives. 

Phenol has forms with resonance however cyclohexanol does not show any. Phenol is also a stronger acid since the resonance arrangement it produces strengthens the conjugate base. Resonance frequency frameworks help strengthen a conjugate base and hence lower the core of the conjugate base that ensures that a strong acid produces the conjugate base. If a compound has over a Lewis structure, the associated resonance forms can be described. Not all configurations of resonance are completely stable. Indeed, the resonance mixture has to be the best stable resonance form because the electron concentration is delocated over a larger number of atoms. There are several factors that you need to consider to evaluate which resonance pattern is much more stabilized. 

  • Atoms usually "do not like" charges, and hence it's best to have no charge. Sometimes charges cannot be avoided hence the octet rule has to be addressed when both resonating patterns are charged.  
  • If there are charges in the resonating patterns and if the octet does not also determine, then the equilibrium should depend on the general patterns in stabilizing negative and positive charges should be investigated. 
"An image showing resonance stabilization on molecules.”

Common Mistakes 

The student should know the structure of the compound to predict its nature so it’s important to know the structures well. 

Context and Application 

This topic is useful for courses in Bachelors and Masters in Chemistry. 

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