Is Sulfur dioxide (SO₂) an Acidic Gas?

Sulfur dioxide is an acidic gas. When sulfur dioxide gas dissolves in water, it forms sulfurous acid (H₂SO₃) which is a dibasic weak acid. Sulfur dioxide has the capability to form acid rain. In areas that have high sulfur dioxide emissions, there is a higher possibility of occurrence of acid rains.

Content

• Reaction of sulfur dioxide gas and water
• Partial dissociation of sulfurous acid
• Why is sulfur dioxide acidic?
  • I know nitrogen dioxide is also an acidic gas and causes acid rain. Now, I want to know which gas will be more dangerous to release to the environment considering the acidic strength of acid rain.
• Is sulfur trioxide more acidic than sulfur dioxide?
• What are the other ways of prooving SO₂ is an acidic gas?
  • Using litmus papers
  • Using Phenolphthalein
• Questions asked by Students

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Reaction of sulfur dioxide gas and water

SO_2(g) + H₂O_(l) → H₂SO_3(aq)

Chemical balanced equation is given here. Sulfurous acid is soluble in water and partially dissociate to give H⁺ ions to water.

Partial dissociation of sulfurous acid

Sulfurous acid partially dissociates in two steps to release H₃O⁺ ions. Due to formation of ₃O⁺ ions, aqueous solution become acidic.

First dissociation

H₂SO_3(aq) + H₂O_(l) ⇌ HSO₃^-_(aq) + H₃O⁺_(aq)

In the first step, bisulfite ion (HSO₃^-) and H₃O⁺ ions are formed.

Second dissociation

HSO₃^-_(aq) + H₂O_(l) ⇌ SO₃^2-_(aq) + H₃O⁺_(aq)

In the second step, sulfite ion (SO₃^2-) and H₃O⁺ ions are formed.

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Why is sulfur dioxide acidic?

We know when an element forms several oxides and the oxidation number of that element is high in the oxide, it increases the acidic properties. In sulfur dioxide, sulfur exists at +4 oxidation and maximum oxidation number of sulfur is +6. Because the oxidation number of sulfur in sulfur dioxide is close to +6, sulfur dioxide should have acidic properties.

I know nitrogen dioxide is also an acidic gas and causes acid rain. Now, I want to know which gas will be more dangerous to release to the environment considering the acidic strength of acid rain.

Actually, we cannot give an exact answer for this question. Due to the presence of nitrogen dioxide gas, nitric acid, very strong acid is formed when gas contacts with water.

As well, Sulfur dioxide will give sulfurous acid. However, sulfurous acid can be oxidized to sulfuric acid by oxygen gas or oxidizing agent. Sulfuric acid is also a strong acid that can harm the environment as nitric acid.

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Is sulfur trioxide more acidic than sulfur dioxide?

According to the theories, sulfur trioxide (SO₃) should be more acidic than sulfur dioxide becaause sulfur trioxide reacts violently with water and form sulfuric acid which is a stronger acid than sulfurus acid. Therefore, SO₃ is more acidic than SO₂.

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What are the other ways of prooving SO₂ is an acidic gas?

Using litmus papers

Wet blue litmus will turn into red when SO₂ gas is contacted.

Using Phenolphthalein

In acidic conditions, colourless solution will be given by phenolphthalein. Therefore, when SO gas is sent to a aqueous solution with phenolphthalein, the colourless state is not changed.

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Questions asked by Students

Ask your chemistry questions and find the answers

SO₂ strong or weak acid?

Because Sulfur dioxide gas give a weak acidic aqueous solution, Sulfur dioxide is considered as a weak acidic gas.

How do I prove SO₂ is an acid gas with a aqueous sodium hydroxide solution which has pH of 10.5?

Send some amount of SO₂ gas to the aqueous sodium hydroxide solution and check pH value of the solution. You may see pH value is decreased to value below 10.5

It means, OH^- concentration has been reduced in the solution. To happen that, H₃O⁺ ion should be supplied and SO₂ should have the ability to do that.

Therefore, SO₂ can give H₃O⁺ ions in the water and can prove that SO₂ is an acidic gas.

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