Neutralization reactions

Neutralization mechanisms

In the classical view, the neutralization of a strong acid by a strong base is described as a simple interaction between a proton and a hydroxide ion. However, taking into account the existence of the hydronium ion, the equation takes a more precise form: $H_3O^+ + OH^- = 2H_2O$.

If a weak acid and a strong base react, the process acquires different features:

• The interaction is explicitly reversible in nature.
• The reacting entities are not ions, but directly molecules of the weak acid, since it is practically undissociated.
• Example: when potassium hydroxide is added to hypochlorous acid, a mobile equilibrium is established: $HClO + OH^- \rightleftarrows ClO^- + H_2O$.

During the neutralization of a weak base (e.g., ammonia) by a strong acid, the process resembles a combination reaction. At the ionic level, this means a proton transfer from the hydronium ion to the ammonia molecule: $NH_3 + H_3O^+ \rightleftarrows NH_4^+ + H_2O$.

Interaction of acids with salts (Protolytic theory)

Acids are capable of reacting not only with hydroxides, but also with salts. According to the protolytic theory, the salt anion acts as a base, as it is capable of accepting a proton from the acid.

The mechanism of such reactions boils down to displacing a weak acid from its salt. It is important to remember the main rule: to displace an acid, the reagent does not necessarily have to be a strong acid. It is sufficient that the displacing acid is simply stronger than the one contained in the salt.

Typical example: when sulfuric acid is added to sodium acetate, a characteristic vinegar odor instantly appears. The ionic equation clearly demonstrates proton transfer to the base anion: $CH_3COO^- + H^+ \rightleftarrows CH_3COOH$.

Acid strength series and conjugate bases

To accurately predict the feasibility of a reaction, a special series is used where acids are arranged in order of decreasing proton-donating ability.

1. Strong acids (the first eight representatives, including perchloric, hydrochloric, sulfuric, and nitric acids). They are almost completely ionized (degree of dissociation $\alpha \approx 1$), so mutual displacement in aqueous solutions is uncharacteristic for them.
2. Weak acids (from chloric acid and below). Their degree of dissociation is less than one, and their anions actively compete with each other for proton capture.

Particles exhibiting acidic properties can be molecular, charged (residues of polybasic acids), or cationic (ammonium).

There is a strict inverse relationship between the strength of an acid and its conjugate base (the anion remaining after proton abstraction). The weaker the acid in the activity series, the stronger its anion is as a base. A classical example: water is an extremely weak acid, but its conjugate base (the hydroxide ion $OH^-$) is one of the strongest bases.

Practical application of the activity series

Analyzing the position of substances in the acid strength series allows for the flawless prediction of chemical reaction outcomes. The key principle is that the system always strives to form a weaker electrolyte.

Stepwise displacement example: Will a reaction occur between benzoic acid ($C_6H_5COOH$) and sodium sulfite ($Na_2SO_3$)?

• We evaluate the strength using the series: $H_2SO_3 > C_6H_5COOH > HSO_3^-$.
• Benzoic acid is stronger than the hydrogensulfite ion, but weaker than sulfurous acid itself.
• Result: the reaction will proceed only at the first stage. Benzoic acid will donate one proton, converting the sulfite into hydrogensulfite, but will not be able to completely displace sulfurous acid: $Na_2SO_3 + C_6H_5COOH = NaHSO_3 + C_6H_5COONa$.

Equilibrium shift example: In the reaction between hypochlorous acid and sodium bicarbonate ($HClO + NaHCO_3 \rightleftarrows NaClO + H_2CO_3$), two weak acids are involved — $HClO$ and $H_2CO_3$. In the activity series, they are located very close to each other. Because their strengths are practically equal, the equilibrium will not shift either to the left or to the right, and the reaction will remain highly reversible.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Competition of anions for proton addition
• Stepwise displacement in polybasic acids
• The relationship between acid strength and salt hydrolysis
• Evaluating chemical equilibrium shifts using the activity series

Other topics in "Chapter 7. Classification of chemical reactions by electronic processes"

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