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

HBr is a strong inorganic acid (pK_a 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 Br₂ + 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 + SO₃ = 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^– + SO₄^2- = 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⁺ & SO₄^2- ).
• 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⁺ + SO₄^2- = 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^–. SO₃ and 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)

S^VI + 2 e^– → S^IV (reduction)

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.

Conclusion

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.

Banabithi

Hi! This is Banabithi. I did Ph.D. in Chemistry from Saha Institute of Nuclear Physics, Kolkata. After that, I have been involved in teaching and research works at different international institutes and universities in India, Germany and UK. I am passionate about writing articles and love to explore the role of chemistry in our existence. Let us connect through