Chemical reactions occur to form unique substances that are different from the reactants. Any mechanism in and around the world creates products by combining the reaction’s reactants. From food preparation and digestion to fuel formation, chemical reactions play a crucial role. We use the Equilibrium constant for predicting the direction of a chemical reaction. When the reaction involves gases, the formula is from vapour density measurements.

Chemical Reaction

• A chemical reaction is a sequential process in which two or more chemical substances (reactants) interact to form one or more products.
• In general, a chemical reaction comprises two sides.
• The left side contains the reactants. The right side has the products formed through the reaction between the reactants.
• A chemical reaction in symbolic form is expressed as A + B → C + D.

The direction of a Reaction

The direction of a chemical reaction is how a reaction is taking place. Chemical reactions are classified into two: Irreversible and reversible reactions.

Irreversible Reaction

• In an irreversible reaction, the reactants form the product.
• Such reactions are not reversible like the products giving the reactants.
• The general expression of an irreversible chemical reaction is A + B → C + D

For example,

C + O2 → CO2 [Carbon + Oxygen → Carbon dioxide]

Reversible Reaction

• In a reversible reaction, the reactants can form the products, and the products can react back to yield the reactants.
• It is bidirectional, happening in both directions.
• The general expression of a reversible chemical reaction is A + B ⇄ C + D.

For example,

N2 + O2 ⇄ 2NO [Nitrogen + Oxygen ⇄ 2 Nitrous Oxide]

• The elements involved in the reaction are gases.
• Thus, the direction of the reaction can be predicted from Vapour density measurements.

Equilibrium Constant for predicting the direction of a reaction

Equilibrium of chemical reaction is a state where the reactants and products undergo no further interaction. The reaction remains. This stage in a reversible reaction is where the concentration of the reactants is equal to that of the products, according to the law of mass action.

How to calculate the Equilibrium constant

In a chemical reaction, aA + bB ⇄ cC + dD where,

• A and B are the reactants; a and b are their respective stoichiometric numbers.
• C and D are the products; c and d are the respective stoichiometric numbers.

The equilibrium constant Kc is calculated as

K_c = [C]^c [D]^d /[A]^a [B]^b

An equilibrium constant is the ratio of the concentration of the products to that of the reactants. Here, the chemical reaction is at equilibrium.

How to calculate the Reaction Quotient

In a chemical reaction, aA + bB ⇄ cC + dD where,

• A and B are the reactants; a and b are their respective stoichiometric numbers.
• C and D are the products; c and d are the respective stoichiometric numbers.

The reaction quotient Qc is calculated as

Q_c = [C]^c [D]^d /[A]^a [B]^b

A reaction quotient is the same as the equilibrium constant, except the reaction is not at equilibrium.

Predicting the direction of a reaction

By knowing the values of the equilibrium constant and the reaction quotient of a chemical reaction, the direction of the reaction can be predicted. When the reactants and products are gases, the direction is from vapour density measurements of the gases. The three different conditions of predicting the direction of a reaction are as follows.

Reverse Reaction

Qc > Kc

When the value of Qc is more than the value of Kc, then,

• The reaction is proceeding in the reverse direction.
• It means that the products are reacting back to form the reactants.
• The direction of the reaction is from the right side to the left side.
• The reaction is occurring backwards.

Forward Reaction

Qc < Kc

When the value of Qc is lesser than the value of Kc, then,

• The reaction is advancing in the forward direction.
• It means that the reactants are forming the products.
• The direction of the reaction is from the left side to the right side.

Equilibrium Reaction

Qc = Kc

When the value of Qc is the same as the value of Kc, then,

• The reaction halts.
• It means that the products are at the same concentrations as the reactants.
• There is no net reaction occurring.

Conclusion

Chemical reactions can be irreversible or reversible. In a reversible reaction, certain parameters are considered to determine the direction of the reaction. Apart from using the equilibrium constant for predicting the direction of the reaction, the value of the reaction quotient also plays a crucial role. The reaction is in the backward direction when the chemical reaction quotient is greater than the equilibrium constant. When the equilibrium constant is greater, the chemical reaction is forward. When both the values are equal, the reaction halts.