15 Facts on HBr + SO3: What, How To Balance & FAQs

HBr is a strong inorganic acid (pKa of -9). SO3, the anhydride of sulfuric acid, is a strong Lewis acid. Let us explore the reaction between these two acidic chemical reagents.

SO3, a precursor to H2SO4, is the most beneficial sulfur oxide in industry. It is highly hygroscopic, able to form H2SO4 mist even in relatively dry atmosphere. Colorless HBr, mostly available in gaseous state, is a reducing agent. Aqueous solution of HBr, that forms HBr acid, is normally used for chemical reactions.

In this following article, we will understand different aspects of HBr+ SO3 reaction in detail.

What is the product of HBr and SO3

  • In aqueous medium, SO3 and HBr reacts with each other and produces dark reddish-brown dibromine (Br2), acidic sulfur dioxide (SO2) and neutral water (H2O).
  • SO3(l)+2HBr(aq)→Br2(aq)+SO2(aq)+H2O(l)
  • In non-aqueous medium, reaction between SO3 and HBr produces bromosulfuric acid (HSO3Br) at very low temperature.
  • SO3 + HBr → HSO3Br
  • HSO3Br is the congener of super acid fluorosulfuric acid (HSO3F)

What type of reaction is HBr + SO3?

HBr + SO3 reaction is an example of oxidation – reduction (redox) reaction.

How to balance HBr + SO3

The balanced chemical equation for HBr + SO3 reaction is,

2HBr + SO3 = Br2 + SO2 + H2O

We will follow the following steps to balance the equation.

  • Write the unbalanced chemical equation for the HBr + SO3 reaction,
  • HBr + SO3 = Br2 + SO2 + H2O
  • Assign coefficients to both reactants and products to balance the number of atoms on both sides of the equation like this, a HBr + b SO3 = c Br+ d SO2 + e H2O
  • Make a linear equation with the above reaction, H= a= 2e, Br =a =2c, S =b = d, O = 3b=e+2d
  • Solve the above equation using gauss elimination method and multiply a coefficient of a, b, c, d, and e with HBr, SO3, Br2, SO2 and H2O respectively in order to get a balanced equation. In this case, a=2, b= 1, c= 1, d= 1, e =1

Thus, the balanced equation of HBr + SO3 is : 2HBr + SO= Br2 + SO2 + H2O

HBr + SO3 titration

HBr + SO3 does not undergo titration. The reaction is highly exothermic, therefore any titration experiment associated with HBr + SO3 reaction is difficult to perform.

HBr + SO3 net ionic equation

The net ionic equation is: 3H+ + 2Br + SO42- = Br2 + H2O + HSO3

  • In aqueous medium, starting material SO3 form H2SO4 which is strong acid. Therefore, in aqueous medium it remains in ionic form s (H+ & SO42- ).
  • Other starting material HBr is also a strong acid and remains in H+ & Br ionic forms.
  • Product Br2 is neutral
  • Other product SO2 in aqueous medium remains as HSO3 & H+ .

So, the equation in ionic form is:

2H+ + 2Br + 2H+ + SO42- = Br2 + H2O + HSO3 + H+

HBr + SO3 conjugate pairs

HBr + SO3 reaction does not form conjugate pairs. HBr after reaction forms Br2 instead of its conjugate base Br. SOand SO2 both possess Lewis acid characteristics, therefore don’t have conjugate base.

HBr and SO3 intermolecular forces

The intermolecular forces that play role in HBr + SO3 reaction are:

  • In highly polar HBr, dipole-dipole and London dispersion forces, arising due to high electronegativity difference between H and Br, are present.
  • In SO3 and SO2 molecule covalent, lone pair repulsion and van der Waals forces exist.

HBr + SO3 reaction enthalpy

The reaction enthalpy of HBr + SO3 is -39.543 kJ (Negative).

SO3 + HBr = Br2 + SO2 + H2O;

ΣΔH°f(reactants) > ΣΔH°f(products),

ΔH°rxn = -39.543 kJ

Is HBr + SO3 a buffer solution

HBr + SO3 is not a buffer solution as HBr a strong acid and SO3 is an also a strong Lewis acid.

Is HBr + SO3 a complete reaction

HBr + SO3 is a complete reaction as all the reactant molecules get converted into corresponding stable products.

Is HBr + SO3 an exothermic reaction

HBr + SO3 reaction is an exothermic reaction. Heat is generated in this reaction as the enthalpy of this reaction (ΔH°rxn  = -39.543 kJ) is found negative.

Is HBr + SO3 a redox reaction

HBr + SO3 is a redox reaction.

  • SO3 is an oxidizing agent, Oxidation state of S in SO3 is +6
  • HBr is a reducing agent, Oxidation sate of Br in HBr is -1
  • After the reaction, SO3 reduced to SO2, (Oxidation No. of S is +4)
  • After the reaction Br in HBr oxidised to Br2 (Oxidation No. is 0)

2 Br-I  2 e  2 Br0 (oxidation)

SVI + 2 e → SIV (reduction)

HBr SO3 image webp
Oxidation-reduction in HBr+SO3 reaction

Is HBr + SO3 a precipitation reaction

HBr + SO3 is not a precipitation reaction as the products formed during the chemical reaction are water soluble.

Is HBr + SO3 reversible reaction

HBr + SO3 is not a reversible reaction. SO3 is the anhydride of H2SO4. Therefore, in aqueous medium SO3 forms H2SO4. The reaction among Br2, SO2 and water can be reversed to H2SO4 and HBr products, but not to SO3. Volatile Br2 makes the reaction forward. The oxidizing and reducing property of Br2 and SO2 play a role.

Is HBr + SO3 displacement reaction

HBr + SO3 is a partially displacement reaction. SO3 displace bromide ion from HBr and form Br2.


In this redox reaction between HBr and SO3, acidic SO2, Br2 and H2O are formed. SO2 is a precursor of sulfuric acid and can also act as a reducing agent. Volatile red-brown Br2, is the only non-metal element that is liquid at room temperature.