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Sulfurous acid

Sulfurous acid

Last updated: March 19, 2023

Sulfurous acid is an inorganic compound with formula H₂SO₃.

Table of contents

Names

List of substance names

Nomenclature

Name

Typical name: Sulfurous acid

Compositional nomenclature: Hydrogen sulfite

Additive nomenclature: Dihydroxidooxidosulfur

Hydrogen nomenclature: Dihydrogen(trioxidosulfate)

IUPAC acceptable common names: Sulfurous acid

Formulae and structures

List of formulae

Formula name

Formula

Typical formula: H₂SO₃

Molecular formula: H₂SO₃

Compositional formula: H₂SO₃

Structural formula

Structural formula with no lone pair

Other structural formulas

Lewis structure

Colored Lewis structure

Properties

List of substance properties

Item

Value

Name: Sulfurous acid

Formula: H₂SO₃

Appearance: Colorless aqueous solution

Odor: Struck match smell

Molar mass: 82.07 g/mol

Density: –

Melting point: –

Boiling point: –

Constituents

Constituent atoms

Atom	Name	Oxidation state	Number
H	Hydrogen	+1	2
S	Sulfur	+4	1
O	Oxygen	−2	3

Ratio of atoms

Atom	Atomic weight	Number	Atomic ratio	Weight ratio
H	1.008	2	33.33%	2.46%
S	32.06	1	16.67%	39.06%
O	15.999	3	50.00%	58.48%

Thermodynamic properties

Phase transition properties

Item

Value

Enthalpy of fusion: –

Enthalpy of vaporization: –

Enthalpy of vaporization at 25°C: –

Enthalpy of other transition: –

Standard thermodynamic properties

State	Standard enthalpy of formation Δ_fH° kJ · mol⁻¹	Standard Gibbs energy of formation Δ_fG° kJ · mol⁻¹	Standard molar entropy S° J · K⁻¹ · mol⁻¹	Standard molar heat capacity at constant pressure C_p° J · K⁻¹ · mol⁻¹
Un−ionized aqueous solution	−608.81^[1]	−537.81^[1]	232.2^[1]	–

Solubility

Qualitative solubility

Reactive

Miscible

Very soluble

Soluble: H₂OWater

Slightly soluble

Very slightly soluble

Insoluble

Preparations

Electrolysis

Electrolysis of liquid can yield sulfurous acid.

Electrolysis of sulfuric acid ◆ Δ_rG 413.44 kJ/mol K 0.37 × 10⁻⁷² pK 72.43: 2H₂SO₄Sulfuric acid
⚡
⟶
2H₂SO₃Sulfurous acid + O₂↑Oxygen

Electrolysis of sulfuric acid ◆ Δ_rG 783.4 kJ/mol K 0.57 × 10⁻¹³⁷ pK 137.25: 3H₂SO₄Sulfuric acid
⚡
⟶
3H₂SO₃Sulfurous acid + O₃↑Ozone

Comproportionation

The reaction of hydrogen sulfide and copper(II) sulfate can yield sulfurous acid.

Reaction of hydrogen sulfide and copper(II) sulfate ◆ Δ_rG −124.2 kJ/mol K 5.74 × 10²¹ pK −21.76: 4H₂SHydrogen sulfide + 3CuSO₄Copper(II) sulfate
⟶
4H₂SO₃Sulfurous acid + 3CuSCopper(II) sulfide

Reaction of hydrogen sulfide and silver(I) sulfate ◆ Δ_rG −391.03 kJ/mol K 3.20 × 10⁶⁸ pK −68.51: 4H₂SHydrogen sulfide + 3Ag₂SO₄Silver(I) sulfate
⟶
4H₂SO₃Sulfurous acid + 3Ag₂SSilver(I) sulfide

Reaction of hydrogen sulfide and mercury(II) sulfate ◆ Δ_rG −427.1 kJ/mol K 6.68 × 10⁷⁴ pK −74.82: 4H₂SHydrogen sulfide + 3HgSO₄Mercury(II) sulfate
⟶
4H₂SO₃Sulfurous acid + 3HgSMercury(II) sulfide

Chemical reactions

Electrolytic dissociation

Electrolytic dissociation of sulfurous acid yields hydrogen ion and anion.

Electrolytic dissociation of sulfurous acid ◆ Δ_rG 10.08 kJ/mol K 0.17 × 10⁻¹ pK 1.77: H₂SO₃Sulfurous acid
⟶
H⁺Hydrogen ion + HSO₃⁻Hydrogensulfurous ion

Electrolytic dissociation of sulfurous acid ◆ Δ_rG 51.3 kJ/mol K 0.10 × 10⁻⁸ pK 8.99: H₂SO₃Sulfurous acid
⟶
2H⁺Hydrogen ion + SO₃²⁻Sulfurous ion

Reaction with base

The reaction of sulfurous acid and base yields salt.

Reaction of sulfurous acid and ammonia ◆ Δ_rG −54.2 kJ/mol K 3.13 × 10⁹ pK −9.50: H₂SO₃Sulfurous acid + 2NH₃Ammonia
⟶
(NH₄)₂SO₃Ammonium sulfite

Reaction of sulfurous acid and lithium hydroxide: H₂SO₃Sulfurous acid + 2LiOHLithium hydroxide
⟶
Li₂SO₃Lithium sulfite + 2H₂OWater

Reaction of sulfurous acid and sodium hydroxide ◆ Δ_rG −108.54 kJ/mol K 1.04 × 10¹⁹ pK −19.02: H₂SO₃Sulfurous acid + 2NaOHSodium hydroxide
⟶
Na₂SO₃Sodium sulfite + 2H₂OWater

Reaction of sulfurous acid and magnesium hydroxide ◆ Δ_rG −26.7 kJ/mol K 4.76 × 10⁴ pK −4.68: H₂SO₃Sulfurous acid + Mg(OH)₂Magnesium hydroxide
⟶
MgSO₃Magnesium sulfite + 2H₂OWater

Reaction of sulfurous acid and potassium hydroxide ◆ Δ_rG −108.5 kJ/mol K 1.02 × 10¹⁹ pK −19.01: H₂SO₃Sulfurous acid + 2KOHPotassium hydroxide
⟶
K₂SO₃Potassium sulfite + 2H₂OWater

Reaction with basic oxide

The reaction of basic oxide and sulfurous acid yields salt and water.

Reaction of lithium oxide and sulfurous acid: Li₂OLithium oxide + H₂SO₃Sulfurous acid
⟶
Li₂SO₃Lithium sulfite + H₂OWater

Reaction of sodium oxide and sulfurous acid ◆ Δ_rG −334.25 kJ/mol K 3.61 × 10⁵⁸ pK −58.56: Na₂OSodium oxide + H₂SO₃Sulfurous acid
⟶
Na₂SO₃Sodium sulfite + H₂OWater

Reaction of magnesium oxide and sulfurous acid ◆ Δ_rG −57.2 kJ/mol K 1.05 × 10¹⁰ pK −10.02: MgOMagnesium oxide + H₂SO₃Sulfurous acid
⟶
MgSO₃Magnesium sulfite + H₂OWater

Reaction of potassium oxide and sulfurous acid ◆ Δ_rG −430.3 kJ/mol K 2.43 × 10⁷⁵ pK −75.39: K₂OPotassium oxide + H₂SO₃Sulfurous acid
⟶
K₂SO₃Potassium sulfite + H₂OWater

Reaction of calcium oxide and sulfurous acid: CaOCalcium oxide + H₂SO₃Sulfurous acid
⟶
CaSO₃Calcium sulfite + H₂OWater

Reaction with dehydrating acidic oxide

The reaction of sulfurous acid and dehydrating acidic oxide yields sulfur dioxide and oxoacid.

Reaction of sulfurous acid and tetraphosphorus decaoxide ◆ Δ_rG −446.8 kJ/mol K 1.89 × 10⁷⁸ pK −78.28: 6H₂SO₃Sulfurous acid + P₄O₁₀Tetraphosphorus decaoxide
🔥
⟶
6SO₂↑Sulfur dioxide + 4H₃PO₄Phosphoric acid

Reaction of sulfurous acid and selenium trioxide: H₂SO₃Sulfurous acid + SeO₃Selenium trioxide
🔥
⟶
SO₂↑Sulfur dioxide + H₂SeO₄Selenic acid

Reaction with anion

The reaction of sulfurous acid and sulfurous ion yields hydrogensulfurous ion.

Reaction of sulfurous acid and sulfurous ion ◆ Δ_rG −31.2 kJ/mol K 2.92 × 10⁵ pK −5.47: H₂SO₃Sulfurous acid + SO₃²⁻Sulfurous ion
⟶
2HSO₃⁻Hydrogensulfurous ion

Reaction with active metal

The reaction of active metal and sulfurous acid yields salt and hydrogen.

Reaction of lithium and sulfurous acid: 2LiLithium + H₂SO₃Sulfurous acid
⟶
Li₂SO₃Lithium sulfite + H₂↑Hydrogen

Reaction of sodium and sulfurous acid ◆ Δ_rG −472.58 kJ/mol K 6.20 × 10⁸² pK −82.79: 2NaSodium + H₂SO₃Sulfurous acid
⟶
Na₂SO₃Sodium sulfite + H₂↑Hydrogen

Reaction of magnesium and sulfurous acid ◆ Δ_rG −386.0 kJ/mol K 4.21 × 10⁶⁷ pK −67.62: MgMagnesium + H₂SO₃Sulfurous acid
⟶
MgSO₃Magnesium sulfite + H₂↑Hydrogen

Reaction of potassium and sulfurous acid ◆ Δ_rG −515.3 kJ/mol K 1.89 × 10⁹⁰ pK −90.28: 2KPotassium + H₂SO₃Sulfurous acid
⟶
K₂SO₃Potassium sulfite + H₂↑Hydrogen

Reaction of calcium and sulfurous acid: CaCalcium + H₂SO₃Sulfurous acid
⟶
CaSO₃Calcium sulfite + H₂↑Hydrogen

Reaction with self-redoxing species

The reaction of self-redoxing species and sulfurous acid yields a variety of products.

Reaction of carbon and sulfurous acid ◆ Δ_rG 105.92 kJ/mol K 0.28 × 10⁻¹⁸ pK 18.56: 2CCarbon + 2H₂SO₃Sulfurous acid
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⟶
2SO₂↑Sulfur dioxide + CH₃COOHAcetic acid

Reaction of sulfur and sulfurous acid ◆ Δ_rG 107.92 kJ/mol K 0.12 × 10⁻¹⁸ pK 18.91: 3SSulfur + 2H₂SO₃Sulfurous acid
🔥
⟶
3SO₂↑Sulfur dioxide + 2H₂S↑Hydrogen sulfide

Reaction of nitrogen dioxide and sulfurous acid ◆ Δ_rG −26.9 kJ/mol K 5.16 × 10⁴ pK −4.71: 2NO₂Nitrogen dioxide + H₂SO₃Sulfurous acid
⟶
HNO₃Nitric acid + HNO₂Nitrous acid + SO₂Sulfur dioxide

Reaction of carbon and sulfurous acid ◆ Δ_rG 30.15 kJ/mol K 0.52 × 10⁻⁵ pK 5.28: 2CCarbon + 2H₂SO₃Sulfurous acid
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⟶
2SO₂↑Sulfur dioxide + CO₂↑Carbon dioxide + CH₄↑Methane

Reaction of nitrogen dioxide and sulfurous acid ◆ Δ_rG −52.26 kJ/mol K 1.43 × 10⁹ pK −9.16: 3NO₂Nitrogen dioxide + H₂SO₃Sulfurous acid
⟶
2HNO₃Nitric acid + NONitrogen monoxide + SO₂Sulfur dioxide

Reaction with oxidizing species

The reaction of sulfurous acid and oxidizing species yields a variety of products.

Reaction of sulfurous acid and oxygen ◆ Δ_rG −429.8 kJ/mol K 1.98 × 10⁷⁵ pK −75.30: 2H₂SO₃Sulfurous acid + O₂Oxygen
⟶
2H₂SO₄Sulfuric acid

Reaction of sulfurous acid and oxygen ◆ Δ_rG −163.5 kJ/mol K 4.41 × 10²⁸ pK −28.64: 2H₂SO₃Sulfurous acid + O₂Oxygen
⟶
2SO₃Sulfur trioxide + 2H₂OWater

Reaction of sulfurous acid and chlorine ◆ Δ_rG −33.94 kJ/mol K 8.83 × 10⁵ pK −5.95: H₂SO₃Sulfurous acid + Cl₂Chlorine
⟶
SO₃Sulfur trioxide + 2HClHydrogen chloride

Reaction of sulfurous acid and nitric acid ◆ Δ_rG −146.1 kJ/mol K 3.94 × 10²⁵ pK −25.60: H₂SO₃Sulfurous acid + HNO₃Nitric acid
⟶
H₂SO₄Sulfuric acid + HNO₂Nitrous acid

Reaction of sulfurous acid and nitric acid ◆ Δ_rG −62.12 kJ/mol K 7.64 × 10¹⁰ pK −10.88: 3H₂SO₃Sulfurous acid + 2HNO₃Nitric acid
⟶
3SO₃Sulfur trioxide + 2NO↑Nitrogen monoxide + 4H₂OWater

Reaction with reducing species

The reaction of reducing species and sulfurous acid yields a variety of products.

Reaction of hydrogen and sulfurous acid ◆ Δ_rG −208.8 kJ/mol K 3.80 × 10³⁶ pK −36.58: 2H₂Hydrogen + H₂SO₃Sulfurous acid
⟶
SSulfur + 3H₂OWater

Reaction of hydrogen sulfide and sulfurous acid ◆ Δ_rG −117.92 kJ/mol K 4.56 × 10²⁰ pK −20.66: 2H₂SHydrogen sulfide + H₂SO₃Sulfurous acid
⟶
3SSulfur + 3H₂OWater

Reaction of hydrogen and sulfurous acid ◆ Δ_rG −259.9 kJ/mol K 3.41 × 10⁴⁵ pK −45.53: 3H₂Hydrogen + H₂SO₃Sulfurous acid
⟶
H₂SHydrogen sulfide + 3H₂OWater

Reaction of manganese and sulfurous acid ◆ Δ_rG −386.92 kJ/mol K 6.10 × 10⁶⁷ pK −67.79: 3MnManganese + 2H₂SO₃Sulfurous acid
⟶
3MnO₂Manganese(IV) oxide + 2H₂S↑Hydrogen sulfide

Reaction of manganese and sulfurous acid ◆ Δ_rG −319.80 kJ/mol K 1.06 × 10⁵⁶ pK −56.03: 3MnManganese + 2H₂SO₃Sulfurous acid
⟶
3MnO₂Manganese(IV) oxide + 2SSulfur + 2H₂↑Hydrogen

Precipitation reaction

When a certain chemical species is present in aqueous solution, it reacts with sulfurous acid to form a precipitate.

Reaction of magnesium hydrogencarbonate and sulfurous acid: Mg(HCO₃)₂Magnesium hydrogencarbonate + H₂SO₃Sulfurous acid
💧
⟶
MgSO₃↓Magnesium sulfite + 2H₂CO₃Carbonic acid

Reaction of magnesium nitrite and sulfurous acid: Mg(NO₂)₂Magnesium nitrite + H₂SO₃Sulfurous acid
💧
⟶
MgSO₃↓Magnesium sulfite + 2HNO₂Nitrous acid

Reaction of calcium hydrogencarbonate and sulfurous acid: Ca(HCO₃)₂Calcium hydrogencarbonate + H₂SO₃Sulfurous acid
💧
⟶
CaSO₃↓Calcium sulfite + 2H₂CO₃Carbonic acid

Reaction of calcium cyanide and sulfurous acid: Ca(CN)₂Calcium cyanide + H₂SO₃Sulfurous acid
💧
⟶
CaSO₃↓Calcium sulfite + 2HCNHydrogen cyanide

Reaction of calcium nitrite and sulfurous acid: Ca(NO₂)₂Calcium nitrite + H₂SO₃Sulfurous acid
💧
⟶
CaSO₃↓Calcium sulfite + 2HNO₂Nitrous acid

Decomposition

Decomposition of sulfurous acid yields sulfur dioxide and water.

Decomposition of sulfurous acid ◆ Δ_rG 0.49 kJ/mol K 0.82 × 10⁰ pK 0.09: H₂SO₃Sulfurous acid
🔥|🕒
⟶
SO₂↑Sulfur dioxide + H₂OWater

Decomposition of sulfurous acid ◆ Δ_rG 507.37 kJ/mol K 0.13 × 10⁻⁸⁸ pK 88.89: 2H₂SO₃Sulfurous acid
🔥
⟶
SSulfur + 2H₂O₂Hydrogen peroxide + SO₂↑Sulfur dioxide

Decomposition of sulfurous acid ◆ Δ_rG 815.01 kJ/mol K 0.16 × 10⁻¹⁴² pK 142.78: 4H₂SO₃Sulfurous acid
🔥
⟶
H₂S↑Hydrogen sulfide + 3H₂O₂Hydrogen peroxide + 3SO₂↑Sulfur dioxide

Decomposition of sulfurous acid ◆ Δ_rG −115.91 kJ/mol K 2.03 × 10²⁰ pK −20.31: 4H₂SO₃Sulfurous acid
🔥|🕒
⟶
3H₂SO₄Sulfuric acid + H₂S↑Hydrogen sulfide

Decomposition of sulfurous acid ◆ Δ_rG 30.90 kJ/mol K 0.39 × 10⁻⁵ pK 5.41: 2H₂SO₃Sulfurous acid
🔥|🕒
⟶
H₂SO₄Sulfuric acid + SO₂↑Sulfur dioxide + H₂↑Hydrogen

Electrolysis of aqueous solution

Electrolysis of aqueous sulfurous acid yields a variety of products.

Electrolysis of aqueous sulfurous acid without water as reactant ◆ Δ_rG 163.60 kJ/mol K 0.22 × 10⁻²⁸ pK 28.66: H₂SO₃Sulfurous acid
💧⚡
⟶
SO₃Sulfur trioxide + H₂↑Hydrogen

Electrolysis of aqueous sulfurous acid with water as non-redox agent ◆ Δ_rG 30.41 kJ/mol K 0.47 × 10⁻⁵ pK 5.33: H₂SO₃Sulfurous acid + H₂OWater
💧⚡
⟶
H₂SO₄Sulfuric acid + H₂↑Hydrogen

Electrolysis of aqueous sulfurous acid without water as reactant ◆ Δ_rG 30.90 kJ/mol K 0.39 × 10⁻⁵ pK 5.41: 2H₂SO₃Sulfurous acid
💧⚡
⟶
H₂SO₄Sulfuric acid + SO₂↑Sulfur dioxide + H₂↑Hydrogen

Electrolysis of water ◆ Δ_rG 474.258 kJ/mol K 0.82 × 10⁻⁸³ pK 83.09: 2H₂OWater
⚡
⟶
2H₂↑Hydrogen + O₂↑Oxygen

Electrolysis of aqueous sulfurous acid with water as non-redox agent: 2H₂SO₃Sulfurous acid + H₂OWater
💧⚡
⟶
H₂S₂O₃Thiosulfuric acid + 2H₂O₂Hydrogen peroxide

References

List of references

1
Janiel J. Reed (1989)
The NBS Tables of Chemical Thermodynamic Properties: Selected Values for Inorganic and C1 and C2 Organic Substances in SI Units
https://doi.org/10.18434/M32124
National Institute of Standards and Technology (NIST)
Citation list

Δ_rG	0.00 kJ/mol
K	1.00 × 10⁰
pK	0.00

Δ_rG	−68.7 kJ/mol
K	1.09 × 10¹²
pK	−12.04

Δ_rG	−68.8 kJ/mol
K	1.13 × 10¹²
pK	−12.05

Δ_rG	−69.0 kJ/mol
K	1.23 × 10¹²
pK	−12.09

Sulfurous acid

Names

List of substance names

Formulae and structures

List of formulae

Properties

List of substance properties

Constituents

Constituent atoms

Ratio of atoms

Thermodynamic properties

Phase transition properties

Standard thermodynamic properties

Solubility

Qualitative solubility

Preparations

Reaction of acidic oxide and water

Reaction of salt of weak acid and strong acid

Reaction of salt of volatile acid and nonvolatile acid

Decomposition

Hydrolysis

Reaction of nonmetal and water

Electrolysis

Comproportionation

Chemical reactions

Electrolytic dissociation

Reaction with base

Reaction with basic oxide

Reaction with dehydrating acidic oxide

Reaction with anion

Reaction with active metal

Reaction with self-redoxing species

Reaction with oxidizing species

Reaction with reducing species

Precipitation reaction

Decomposition

Electrolysis of aqueous solution

References

List of references

Related substances

Sulfur oxoacid

Acidic oxide

Sulfite

Related categories