15 Facts on H2SO3 +KClO3: What, How To Balance & FAQs

Potassium chlorate (KClO3) is a highly reactive chlorate salt, and H2SO3 (sulfurous acid) is an inorganic sulphur oxo acid. Let us examine their reaction in detail.

KClO3 is a white monoclinic crystal with a saline taste. It is a chemical oxygen generator in laboratories. As a powerful oxidizing agent, it is primarily used in fireworks. H2SO3 is a colorless inorganic acid, soluble in water. As a reducing agent, it is used as a bleaching agent.

In this article, let us discuss important features of the reaction H2SO3 + KClO3, such as the product formed, the type of reaction, enthalpy change etc.

What is the product of H2SO3 and KClO3?

Potassium chloride (KCl) and sulphuric acid (H2SO4) are the products formed when sulphurous acid reacts with potassium chlorate.

  3H2SO3 + KClO3 ——-> KCl + 3H2SO4

What type of reaction is H2SO3 + KClO3?

H2SO3 + KClO3 is an oxidation-reduction (redox) type of reaction.

How to balance H2SO3 + KClO3?

H2SO3 + KClO3 is balanced using the following steps

 KClO3 + 3H2SO3  = KCl + 3H2SO4

  • To denote the unknown coefficients, assign an alphabet to each compound of the reaction.
  •  a KClO3 + b H2SO3  = c KCl + d H2SO4
  • Create an equation by equating the coefficients of same type, for each element, involved in the reaction.
  • H → 2b=2d, S → b=c, Cl → a=c, K → a=c, O → 3a+3b=4d
  • Determine the values of each assigned coefficient by using the Gaussian elimination method.
  • The value of coefficients obtained after simplification
  • a=1, b=3, c=1, d=3.
  • Therefore, the balanced equation after substituting the values of all the coefficients is –
  • KClO3 + 3H2SO3  =  KCl + 3H2SO4

H2SO3 + KClO3  titration

A redox titration between H2SO3 and KClO3 is carried out to determine the amount of potassium and chlorine.


Burette, burette holder, pipette, wash bottle, conical flask,  volumetric flask, dropper and beakers.


Phenolphthalein is used as the indicator as the titration is done in an acidic medium.


  • The burette is filled with standardized H2SO3.
  • The KClO3 solution is pipetted out and transferred into the conical flask.
  • Add a few drops of phenolphthalein indicator.
  • H2SO3 is added drop by drop into the conical flask, and the flask is swirled constantly.
  • The appearance of pale pink color marks the equivalence point of the titration.
  • Note the burette reading and repeat the procedure for 3 concordant readings.
  • The quantity of potassium and chlorine is estimated by using the formula, V1S= V2S2.

H2SO3 + KClO3  net ionic equation

The net ionic equation between H2SO3 + KClO3 is:

3H2SO3 (aq)+ ClO3 (aq) = Cl (aq) + 2H+ (aq) + SO42- (aq)

To elucidate the net ionic equation, the following steps are followed –

  • The balanced equation along with its physical state is written first.
  • 3H2SO3 (aq) + KClO3 (aq) =  KCl(aq)  + H2SO4 (aq)
  • The electrolytes, which are strong, are split into their respective ions. H2SO3 is a weak electrolyte, while KClO3, KCl, and H2SO4 are strong electrolytes in an aqueous solution. The equation now becomes,
  • 3H2SO3(aq) + K+(aq) + ClO3 (aq) =  K+(aq) + Cl(aq) + 2H+(aq) + SO42-(aq)
  • The spectator ions (K+) are cancelled on both sides of the equation to get the net ionic equation.
  • 3H2SO3 (aq)+ ClO3 (aq) = Cl (aq) + 2H+ (aq) + SO42- (aq)

H2SO3 + KClO3  conjugate pairs

  • Conjugate base of H2SO3 as HSO3
  • KClO3 is a neutral salt, so it has no conjugate pair.

H2SO3 + KClO3  intermolecular forces

H2SO3 + KClO3  reaction enthalpy

H2SO3 + KClO3 reaction enthalpy is -205.3 KJ/mol.

  • Enthalpy values of reactants and products involved in the reaction are listed below –
MoleculeEnthalpy in KJ/mol
Enthalpy values of reactants and products
  • The enthalpy of the reaction is obtained by using the formula: enthalpy of products – enthalpy of reactants.
  • ∆Hf°(reaction) = ∆Hf°(products) – ∆Hf°(reactants)
  • = -1252 – (-1046.7) KJ/mol
  • = -205.3 KJ/mol

Is H2SO3 + KClO3 a buffer solution?

H2SO3 + KClO3 does not form a buffer solution, as KClO3 is a neutral salt formed by the neutralization reaction of a strong acid and a strong base.

Is H2SO3 + KClO3 a complete reaction?

H2SO3 + KClO3  is a complete reaction since the products are completely formed at equilibrium.

Is H2SO3 + KClO3 an exothermic reaction?

H2SO3 + KClO3 is an exothermic reaction following thermodynamics first law. Moreover, the reaction enthalpy is found to be negative.

Is H2SO3 + KClO3 a redox reaction?

H2SO3 + KClO3 is a redox reaction where,

  • Sulphur is oxidized from +4 to +6 oxidation state.
  • Chlorine is reduced from +5 to -1 oxidation state.

Is H2SO3 + KClO3 a precipitation reaction?

H2SO3 + KClO3 is not a precipitation reaction as the products formed are completely soluble in the reaction mixture.

Is H2SO3 + KClO3 an irreversible reaction?

H2SO3KClO3 is an irreversible reaction as the products formed react with each other to form chlorine gas, so reverse reaction is not favoured.

Is H2SO3 + KClO3 a displacement reaction?

H2SO3 + KClO3  is not a displacement reaction, as there is no swapping or displacement of ions during product formation.


The reaction of KClO3 with H2SO3 is a redox and exothermic reaction. KClO3 should be handled carefully as it is combustible in nature. H2SO3 and KClO3 are widely used as reducing and oxidizing agents, respectively.