Acid-base Behavior of the Oxides – Chemistry LibreTexts

Sulfur Oxides

Two oxides are considered: sulfur dioxide, SO2, and sulfur trioxide, SO3.

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Sulfur dioxide: Sulfur dioxide is fairly soluble in water, reacting to give a solution of sulfurous acid (also known as sulfuric(IV) acid), H2SO3, as shown in the reaction below. This species only exists in solution, and any attempt to isolate it gives off sulfur dioxide.

[ SO_2 + H_2O rightarrow H_2SO_3]

The protonated acid has the following structure:

Sulfurous acid is also a relatively weak acid, with a pKa of around 1.8, but slightly stronger than the two phosphorus-containing acids above. A reasonably concentrated solution of sulfurous acid has a pH of about 1.

Sulfur dioxide also reacts directly with bases such as sodium hydroxide solution. Bubbling sulfur dioxide through sodium hydroxide solution first forms sodium sulfite solution, followed by sodium hydrogen sulfite solution if the sulfur dioxide is in excess.

[ SO_2 + 2NaOH rightarrow Na_2SO_3 + H_2O]

[Na_2SO_3 + H_2O rightarrow 2NaHSO_3]

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Another important reaction of sulfur dioxide is with the base calcium oxide to form calcium sulfite (also known as calcium sulfate(IV)). This is of the important methods of removing sulfur dioxide from flue gases in power stations.

[CaO + SO_2 rightarrow CaSO_3]

Sulfur trioxide: Sulfur trioxide reacts violently with water to produce a fog of concentrated sulfuric acid droplets.

[ SO_3 + H_2O rightarrow H_2SO_4]

Pure, fully-protonated sulfuric acid has the structure:

Sulfuric acid is a strong acid, and solutions will typically have a pH around 0. The acid reacts with water to give a hydronium ion (a hydrogen ion in solution) and a hydrogen sulfate ion. This reaction runs essentially to completion:

[ H_2SO_4 (aq) + H_2O (l) rightarrow H_3P^+ + HSO_4^- (aq)]

The second proton is more difficult to remove. In fact, the hydrogen sulfate ion is a relatively weak acid, similar in strength to the acids discussed above. This reaction is more appropriately described as an equilibrium:

[ HSO_4^- (aq) + H_2O rightleftharpoons H_3O^+ (aq) + SO_4^{2-} (aq)]

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It is useful if you understand the reason that sulfuric acid is a stronger acid than sulfurous acid. You can apply the same reasoning to other acids that you find on this page as well.

Sulfuric acid is stronger than sulfurous acid because when a hydrogen ion is lost from one of the -OH groups on sulfuric acid, the negative charge left on the oxygen is spread out (delocalized) over the ion by interacting with the doubly-bonded oxygen atoms. It follows that more double bonded oxygen atoms in the ion make more delocalization possible; more delocalization leads to greater stability, making the ion less likely to recombine with a hydrogen ion and revert to the non-ionized acid.

Sulfurous acid only has one double bonded oxygen, whereas sulfuric acid has two; the extra double bond provides much more effective delocalization, a much more stable ion, and a stronger acid. Sulfuric acid displays all the reactions characteristic of a strong acid. For example, a reaction with sodium hydroxide forms sodium sulfate; in this reaction, both of the acidic protons react with hydroxide ions as shown:

[2NaOH +H_2SO_4 rightarrow Na_2SO_4 + 2H_2O]

In principle, sodium hydrogen sulfate can be formed by using half as much sodium hydroxide; in this case, only one of the acidic hydrogen atoms is removed.

Sulfur trioxide itself also reacts directly with bases such as calcium oxide, forming calcium sulfate:

[ CaO + SO_3 rightarrow CaSO_4]

This reaction is similar to the reaction with sulfur dioxide discussed above.