What Happens When an Acid Reacts With a Base?

Initiation of the Reaction

The reaction is considered initiated when the acid and base are in the same solution but have not reacted yet. That is, their concentrations are still initial concentrations at this point. The acid-base reaction begins when one of the acidic molecules encounters a basic molecule in solution.

Proton Transfer

Consider the general acid-base reaction: HA + B- ' HB + A', where "A" represents the anion of the acid and conjugate base and "B" represents the anion of the base and conjugate acid.

The reaction between the HA and B- is motivated by the base's willingness to accept a proton to cancel its negative charge. The rationale behind this is known as the mechanism of acid-base interaction and is detailed as follows:

When a molecule of HA encounters a molecule of B, B acts as a nucleophile (positive charge-loving molecule). The base attacks the protonated acid due to its affinity for positive charge. The electrons in the H-A single bond are then cleaved heterolytically and are pushed onto the A molecule to produce "A-."

Meanwhile, the H+ ion cleaved from HA becomes attracted to the negatively charged base and gives one molecule of HB.

Establishment of Equilibrium

Depending on the relative strength of the initial acid and base, equilibrium may favor either the products (conjugate acid and conjugate base), or they may favor the reactants, in which case the reaction does not proceed to a large extent. This is measured by a property called "K," which describes the ratio of products equilibrium concentrations to those of the reactants.

It is important to understand that even though acid-base reactions seem to follow a general set of steps, the reverse reaction is constantly occurring as well. The same mechanism can then be applied to the reverse of the reaction given earlier to produce HB + A' ' HA + B-.

Not all acid-base reactions involve strong acids and bases; for reactions involving weak acids and weak bases (such as those without a negative charge), it is important to notice that the reverse reaction may be favored at equilibrium. For more help understanding weak acids and bases, see the link provided in the "Additional Resources" section.

References

• Acid-Base Fundamentals

Resources

• Understanding Weak Acid Dissociation Constants