Question

a) Distinguish between sigma and pi bond.
b) Distinguish between reversible and irreversible reaction.
c) State and explain law of mass action.

Answer 1

Hint: Chemical bonds hold the atoms together. Sigma and pi bonds are types of covalent bonds. Saturated compounds have sigma bonds while unsaturated compounds have sigma and pi bonds.
In thermodynamics there are two types of reactions-reversible and irreversible reactions. Chemical equations are used to represent chemical reactions. In order to represent irreversible reactions, these equations typically have a unidirectional arrow. A bidirectional harpoon may have other chemical equations that represent reversible reactions.
Law of mass action is also called the law of Chemical equilibrium. It gives the relation between the concentration of reactant and product at equilibrium. However it is now understood that the rate terms it offers only refer to elementary reactions.

Complete step by step answer:
Let us differentiate the two type of bonds here;

Sigma bond	Pi bond
It is formed by the overlap of atomic orbitals.	It is formed by the lateral or sidewise overlap of the p- atomic orbitals.
It is denoted by the symbol $\sigma $ .	It is denoted by the symbol $\pi $ .
Atoms with sigma bonds are highly reactive.	Atoms having pi bonds are less reactive than atoms having sigma bonds.
In this bond orbitals can rotate freely.	Here, rotation of orbitals is not allowed.
The overlapping orbitals can be two pure orbitals, two hybrid or one pure and one hybrid orbital.	In pi bond formation the overlapping orbitals must be unhybridized.
These bonds have cylindrical charge symmetry around the bond axis.	There is no symmetry in pi bond.
These help to determine the structure of molecules.	These are not useful for determining the structure of molecules.

Reversible reactions can be reversed at any time by shifting the equilibrium. Maximum amount of work is done in a reversible process. Their differences are-

Reversible process	Irreversible process
This process is being carried out infinitesimally slowly.	It occurs rapidly.
Reversible process can be made to proceed in forward and backward direction.	Irreversible processes can proceed in one direction only.
An equilibrium is maintained between the system and the surroundings.	There is no such equilibrium maintained.
Work done in a reversible process is greater than the work done in an irreversible process.	Work done in an irreversible process is not greater than the work done in a reversible process.
It takes infinite time to complete.	It requires finite time for completion.

The law of mass action states that rate of a reaction is proportional to the product of reactant concentration.
Consider a simple reversible reaction where $A$ and $B$ are the reactant and $C$ and $D$ are the product. The reaction is-
$aA + bB\underset{{}}{\overset{{}}{\longleftrightarrow}}cC + dD$
Where $a,b,c,d$ are the stoichiometric coefficients.
We can see that reactant and product are in equilibrium with each other. There is a relation between the equilibrium concentration of reactant and product and is given by-
${K_c} = {{{{[C]}^c}{{[D]}^d}}}{{{{[A]}^a}{{[B]}^b}}}$
Where ${K_c}$ is the equilibrium constant. $a,b,c,d$ are the stoichiometric coefficients. $[A]$ and $[B]$ are the concentrations of reactant species. $[C]$ and $[D]$ are the concentration of product species.
${K_c}$ gives the relation between the equilibrium concentration of reactant and product raised to their stoichiometric coefficients. It has been observed that the equilibrium constant depends on the stoichiometric coefficients of reactants and products. This law is also known as the law of chemical equilibrium.

Note:
A sigma bond is identical to an atomic orbital "s" and a pi pond has the same orbital symmetry as the p orbital (when viewed along the bond axis in both cases). Sigma bonds are usually stronger than Pi bonds. Both are used extensively in molecular orbital theory to predict molecules' behaviour.
Work done in both the cases is different. Work being a form of energy is expressed in either $joules$ or $calories$ . $1calorie = 4.184joules$ .
 The equilibrium constant for the backward reaction can be written as-
${K_c} = {{{{[A]}^a}{{[B]}^b}}}{{{{[C]}^c}{{[D]}^d}}}$Where $a,b,c,d$ are the stoichiometric coefficients. The unit of ${K_c}$ depends on the stoichiometry of the reactants and products and hence varies accordingly.