An example of a reversible but nonspontaneous equilibrium reaction is the formation of carbonic acid (H2CO3) from carbon dioxide (CO2) and water (H2O):
CO2 + H2O ⇌ H2CO3
This reaction can occur in both directions, indicating reversibility, but it is nonspontaneous under normal conditions. It means that the reaction does not proceed in the forward direction without external influence or the addition of energy.
In the case of carbonic acid formation, the reaction is nonspontaneous because it requires an input of energy to overcome the thermodynamic stability of carbon dioxide and water. However, once carbonic acid is formed, it can dissociate back into carbon dioxide and water spontaneously, as indicated by the reversible arrow in the equation.
To drive the reaction forward and favor the formation of carbonic acid, one would need to supply energy or adjust the conditions such as temperature, pressure, or concentration of reactants. For instance, increasing the pressure of carbon dioxide or decreasing the temperature can shift the equilibrium towards the formation of carbonic acid.