15 Facts on HCl + SO2: What, How To Balance & FAQs

Hydrochloric acid and sulphur(IV)oxide are carcinogenic chemicals which react with each other. Let us learn about some interesting phenomena of the mixture of HCl and SO2

SO2 on burning gives a rotten egg-like smell. It is used for the preparation of sulphuric acid. Aqueous HCl is also known as muriatic acid.

In this article, we will learn how to balance redox reaction between HCl and SO2, the products formed in this reaction, net ionic equation, conjugate pairs and many other related facts.

What is the product of HCl and SO2?

Solid sulphur (S), chlorine gas (Cl2), and water (H2O) are produced when SO2 reacts with HCl.

SO2(l)+ HCl(aq) = S(s) + Cl2(g) + H2O(l)

What type of reaction is HCl + SO2?

The chemical reaction of HCl and SO2 follows a redox reaction.

How to balance HCl + SO2?

The unbalanced equation for the reaction HCl + SO2 is

SO2(l)  + HCl(aq)  = S(s) + Cl2(g) + H2O(l)

To balance the reaction follow the procedures mentioned below.

  • First, give each reactant and product a symbol and a description of their state.
  • SO2(l)  + HCl(aq)  =  S(s) + Cl2(g) + H2O(l)
  • Fill in the Table by recording the number of various atom kinds in the reactant and product.
Atom typeReactantProduct
Table shows the number of atoms present in reactant and product
  • Put two in front of HCl since there are not identical amounts of hydrogen and chlorine atoms. Check the number of atoms once more.
  • SO2(l) + 2HCl(aq)  = S(s)  + Cl2(g) + H2O(l)
  • Now the oxygen atoms are not identical in both the reactant and product. Therefore put 2 in front of H2O. 
  • SO2(l) + 2HCl(aq)  =  S(s) + Cl2(g)  + 2H2O(l)
  • Since the number of Hydrogen atom are not same, therefore change the stoichiometry of HCl with 4 and balance hydrogen and chlorine atoms.
  • SO2(l) + 4HCl(aq) = S(s) + 2Cl2(g) + 2H2O(l)
  • Thus the balanced equation is
  • SO2(l) + 4HCl(aq) = S(s) + 2Cl2(g) + 2H2O(l)

HCl + SO2 titration

Redox titration is utilised for the HCl and SO2 combination.


Funnel, Conical flask, Burette stand, Burette, Pipette, Stirrer, White glazed tile, Potassium permanganate, Hydrochloric acid, Sulphur dioxide


  • Potassium permanganate (acts as self-indicator)
  • Phenolphthalein


  • The HCl and SO2 should first be normalized using KMnO4, which serves as a self-indicator.
  • Using M/10 KMnO4 solution, rinse the burette, and record the initial reading.
  • Transfer 10 ml of HCl solution into a conical flask using a pipette.
  • Drop by drop, add KMnO4 solution until a bright pink colour is achieved.
  • Make a note of the final reading and calculate the HCl concentration.
  • Next, wash, rinse, and titrate the burette with SO2 and HCl.
  • A few drops of the indicator phenolphthalein should be added.
  • Drop by drop, add HCl solution to the SO2 solution until the mixture turns pink.
  • Record the burette’s final reading and calculate the SO2 concentration.
  • The concentration of SO2 can be calculated using the normality equation (NSO2VSO2=NHClVHCl) where N represent the normality and V is the volume of respective chemical.

HCl + SO2 net ionic equation

The net ionic equation for HCl + SO2 is-

SO2(l) + 4H+(aq) + 4Cl(aq) = S(s) + 2Cl2(g) + 2H2O(l)

The actions listed below must be taken to obtain the net ionic equation. 

  • Write the balanced chemical equation initially regarding each substance’s physical state.
  • SO2(l)  + 4HCl(aq) =  S(s) + 2 Cl2(g) + 2H2O(l)
  • Pure substances like SO2, S, Cl2, and H2O do not have molecules that can split into ions. However, the powerful acid HCl can split into a proton (H+) and chloride (Cl) ions. 
  • Thus the net ionic equation is-    
  • SO2(l) + 4H+(aq) + 4Cl(aq) = S(s) + 2Cl2(g) + 2H2O(l)

HCl + SO2 conjugate pairs

The conjugate pairs do not form when HCl and SO2 are combined. The ionic equation shows that, neither HCl nor SO2 can give or take a proton (H+). Therefore, the mixture mentioned above cannot be regarded as conjugate pairs.

HCl and SO2 intermolecular forces

The intermolecular forces present between HCl + SO2 are:

  • The dipole-dipole intermolecular force of attraction is demonstrated with HCl and SO2 where the net dipole moment of SO2 is not zero due to the bent shape.
  • HCl molecule shows permanent dipole moments due to the electronegativity mismatch between H and Cl .

HCl + SO2 reaction enthalpy

The standard enthalpy of the reaction for HCl + SO2 is 194.1 kJmol-1 which is calculated using the listed values as follows.

S.No.Chemical CompoundStandard enthalpy of formation (Hform) (kJmol-1)
Table shows the standard enthalpy of formation
  • Hrxn = HS + 2HCl2 + 2HH2O – HSO2 – 4HHCl
  • = (0.0-2*285.8-o.0+4*92.5+395.7) kJmol-1
  • = 194.1 kJmol-1

Is HCl + SO2 a buffer solution?

The mixture of HCl and SO2 will not create a buffer solution because SO2 and HCl easily react with each other to produce chlorine gas.

Is HCl + SO2 a complete reaction?

HCl + SO2 is a complete reaction because SO2 works as an oxidizing agent and oxidizes chloride to chlorine gas.

Is HCl + SO2 an exothermic or endothermic reaction?

HCl + SO2 is an endothermic reaction because the calculated standard enthalpy of the reaction comes out to be positive (194.1 kJmol-1). 

Is HCl + SO2 a redox reaction?

HCl + SO2 is a redox reaction because both oxidation and reduction take place simultaneously in this process, where the oxidation state of the chlorine atom grow from -1 to zero and the sulphur atom decrease from +4 to 0.

SO2(l) + 4H+(aq) + 4Cl(aq) = S(s) + 2Cl2(g) + 2H2O(l)

Is HCl + SO2 a precipitation reaction?

HCl + SO2 is a precipitation reaction because the product solid sulphur precipitates out from the reaction mixture after the completion of the reaction.

Is HCl + SO2 reversible or irreversible reaction?

HCl + SO2 is an irreversible reaction because Cl2 gas produced will escape out of the reaction mixture. Therefore, it was impossible to reverse the process.

Is HCl + SO2 displacement reaction?

The chemical reaction HCl + SO2 is not a displacement reaction because the reduction potential of hydrochloric acid is more negative than sulphur dioxide.


The reaction of HCl with SO2 is an endothermic reaction in which both oxidation and reduction take place simultaneously. The sulphur produced in the reaction can be used for the vulcanization of rubber which increases the resistant power of natural rubber.