Lecture 7 – Reversible Reactions

Reversible reactions:

Most reactions like combustion;

are irreversible. Some reactions are however, reversible and can take place in both directions.

Example(s):

Manufacture of Ammonia is an example of a reversible reaction. For such reactions; reactants chemically react together to form products and products breakdown to yield reactants. Another example is manufacture of Sulphur trioxide:

Name of the Process	Detail of the Process	Reversible Reaction	Catalyst used
Harber Process	Industrial manufacture of Ammonia (NH3)		Iron
Contact Process	Industrial manufacture of Sulphuric Acid (H2SO4)		Vanadium(V)Oxide

Dynamic equilibrium:

Dynamic equilibrium is defined as the stage of a reversible reaction where the reactants are constantly reacting to form products and products are constantly breaking to form reactants, such that the yield of the product remains same/constant in spite of the progress of the reaction.

Effects of Factors on Reversible reactions:

Factors like concentration of the reactants, particle size of the reactants and presence of a catalyst affect reversible reactions just as they affect irreversible reactions. However, temperature and pressure have a great effect on reversible reactions.

Table 5 summarizes these effects.

Table 5:

	Exothermic Reactions	Endothermic Reactions
Temperature	General Equation: Heat is a product of Exothermic reactions. Therefore: Indirect relation: i.                    Increase in temperature, decreases the rate of the forward reaction ii.                  Decrease in temperature, increases the rate of the forward reaction	General Equation: Heat is a reactant of Endothermic reactions. Therefore: Direct relation: i.                    Increase in temperature, increases the rate of the forward reaction ii.                  Decrease in temperature, decreases the rate of the forward reaction
Pressure	Pressure affects reversible reactions in which both the reactant(s) and product(s) are gases. General Equation: i.                    If a + b > c + d Direct relation: Increase in pressure, increases the rate of the forward reaction. (Explanation: Increase in pressure means that the reactants and products both will collide more often. Reactants will collide to form products and products will collide to covert to reactants, but reactants being more in concentration will shift the reaction equilibrium to the right to yield products) ii.                  If a + b < c + d Indirect relation: Increase in pressure, decreases the rate of the forward reaction. (Explanation: Increase in pressure means that the reactants and products both will collide more often. Reactants will collide to form products and products will collide to covert to reactants, but products being more in concentration will shift the reaction equilibrium to the left to convert to reactants)