A neutralization reaction is one that occurs between an acidic and a basic species in a quantitative way. In general, in this type of reaction in aqueous medium, water and a salt are produced (ionic species composed of a cation different from H + and an anion different from OH – or O 2- ) according to the following equation: acid + base → salt + water.
In a neutralization reaction, electrolytes have an impact, which are those substances that, when dissolved in water, generate a solution that allows electrical conductivity. Acids, bases, and salts are considered electrolytes.
In this way, strong electrolytes are those species that completely dissociate into their constituent ions when they are in solution, while weak electrolytes only partially ionize (they have less capacity to conduct an electric current; that is, they are not good conductors such as strong electrolytes).
In the first place, it must be emphasized that if a neutralization reaction is started with equal amounts of the acid and the base (in moles), when said reaction ends, only one salt is obtained; that is, there are no residual amounts of acid or base.
Also, a very important property of acid-base reactions is pH, which indicates how acidic or basic a solution is. This is determined by the amount of H + ions found in the measured solutions.
On the other hand, there are several concepts of acidity and basicity depending on the parameters that are taken into consideration. One concept that stands out is that of Brønsted and Lowry, who consider an acid as a species capable of donating protons (H + ) and a base as the species capable of accepting them.
To study properly and quantitatively a neutralization reaction between an acid and a base, a technique called acid-base titration (or titration) is applied.
Acid-base titrations consist of determining the concentration of acid or base necessary to neutralize a certain amount of base or acid of known concentration.
In practice, a standard solution (whose concentration is known exactly) must be gradually added to the solution whose concentration is unknown until the equivalence point is reached, where one of the species has completely neutralized the other.
The equivalence point is detected by the violent change in color of the indicator that has been added to the solution of unknown concentration when the chemical reaction between both solutions has been completed.
For example, in the case of the neutralization of phosphoric acid (H 3 PO 4 ) there will be an equivalence point for each proton that is released from the acid; that is, there will be three equivalence points and three color changes will be observed.
Products of a neutralization reaction
In the reactions of a strong acid with a strong base the complete neutralization of the species takes place, as in the reaction between hydrochloric acid and barium hydroxide:
2HCl (aq) + Ba (OH) 2 (aq) → BaCl 2 (aq) + 2H 2 O (l)
So no excess H + or OH – ions are generated , which means that the pH of strong electrolyte solutions that have been neutralized is intrinsically related to the acid character of their reactants.
On the contrary, in the case of neutralization between a weak and a strong electrolyte (strong acid + weak base or weak acid + strong base) the partial dissociation of the weak electrolyte is obtained and the acid dissociation constant (K a ) appears or of the weak base (K b ), to determine the acidic or basic character of the net reaction by calculating the pH.
For example, we have the reaction between hydrocyanic acid and sodium hydroxide:
HCN (aq) + NaOH (aq) → NaCN (aq) + H 2 O (l)
In this reaction, the weak electrolyte does not ionize noticeably in the solution, so the net ionic equation is represented as follows:
HCN (aq) + OH – (aq) → CN – (aq) + H 2 O (l)
This is obtained after writing the reaction with strong electrolytes in their dissociated form (Na + (ac) + OH – (ac) on the reactant side, and Na + (ac) + CN – (ac) on the side of the products), where only the sodium ion is a bystander.
Finally, in the case of the reaction between a weak acid and a weak base, said neutralization does not occur. This is because both electrolytes partially dissociate, without resulting in the expected water and salt.
Strong acid + strong base
The given reaction between sulfuric acid and potassium hydroxide in an aqueous medium is taken as an example, according to the following equation:
H 2 SO 4 (aq) + 2KOH (aq) → K 2 SO 4 (aq) + 2H 2 O (l)
It can be seen that both acid and hydroxide are strong electrolytes; therefore, they completely ionize in solution. The pH of this solution will depend on the strong electrolyte that is in the highest proportion.
Strong acid + weak base
Neutralization of nitric acid with ammonia results in the compound ammonium nitrate, as shown below:
HNO 3 (aq) + NH 3 (aq) → NH 4 NO 3 (aq)
In this case, the water produced with the salt is not observed, because it would have to be represented as:
HNO 3 (aq) + NH 4 + (aq) + OH – (aq) → NH 4 NO 3 (aq) + H 2 O (l)
So water can be seen as a reaction product. In this case, the solution will have an essentially acidic pH.
Weak acid + strong base
The reaction that occurs between acetic acid and sodium hydroxide is shown below:
CH 3 COOH (aq) + NaOH (aq) → CH 3 COONa (aq) + H 2 O (l)
As acetic acid is a weak electrolyte, it partially dissociates, resulting in sodium acetate and water, the solution of which will have a basic pH.
Weak acid + weak base
Lastly, and as previously mentioned, a weak base cannot neutralize a weak acid; neither is the opposite. Both species hydrolyze in aqueous solution and the pH of the solution will depend on the “strength” of the acid and the base.
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