Question

In your previous studies last semester, you encountered reactions went to completion and were written A + B → C + D. You are now learning about reactions that achieve equilibrium and are written A + B ↔ C + D. Give the definitions of both types of reactions and justify the statement using mathematical relations that "reactions that go to completion are a subclass of the more general equilibrium reaction."

Answer 1

In a chemical reaction, reactants react with each other by breaking of bonds of the reactant molecules and forming new bonds with the other reactants to form new product molecules.

In the process of breaking of bonds of the reactants, energy is needed and making of new bonds releases energy. If the amount of energy released to form products is appreciably high than that needed to break bonds in the reactant molecules, then the reaction goes to completion and entire reactant molecules react to form products. This means the molecules wants to achieve low energy stable state by forming product.

However, if energy released in product formation is comparable to the energy required for bond breaking then the products,then the reaction becomes reversible and chemical equilibrium state is reached.

Chemical equilibrium is the state of a reaction in which there is no net change in concentrations of both reactants and products and then the forward reaction proceeds at the same rate as the reverse reaction.

Now, the reaction rate only could be changed if any changes are made in the concentrations of reactant or product, or the reaction condition like temperature and pressure.

Suppose for reaction ,A+B↔C+D

Rate of forward reaction=r=kf[A][B] where the reaction rate depends on initial concentration of reactants(rate law),k is rate const

Similarly

Rate of reverse reaction=r=kb[C][D]

Keq=equilibrium const=kb[C][D]/ kf[A][B]

If A][B]>>> [C][D] (high reactant concentration) then keq tends to zero or vanish and the reaction goes to completion

This verifies that "reactions that go to completion are a subclass of the more general equilibrium reaction."