How can we make a reversible reaction irreversible ? Give an example.

All reactions are reversible in some respect, it is the rate constant of the reaction that determines whether it can be considered irreversible.

if A+B —→ C+D is reaction for which forward rate constant is k1 and backward reaction rate constant is k2,

then if k1/k2> 10^5, we can say reaction is practically irreversible, and that C and D would be the only components present in system when reaction is completed.

Values of k1 and k2 depend upon the activation energy of reaction.

(1) would happen only if molecules collide with an energy >= E1 (activation energy for forward reaction). Similarly, (2) happens only if collision energy >= E2. By Boltzmann distribution (or, intuitively) molecules with higher energies are lesser in no. So, in this case, more atoms would convert via (1) than via (2). So, reaction proceeds in forward reaction.

The greater the value of dE=E2-E1, the more is the irreversibility of reaction.

So, only if E2-E1 is close to zero the reaction is reversible.