15 Facts on HBr + Mg3P2: What, How To Balance & FAQs

Magnesium phosphide (Mg3P2) is a salt composed of three Mg metals and two P non-metals. Hydrogen bromide (HBr) is a strong acid. Let us examine the reaction of HBr and Mg3P2.

Magnesium phosphide is a white crystalline cubic solid. It ignites on contact with air and reacts intensely with water. It is a strong reducing agent and is primarily used as a fumigant in agriculture. HBr is a pungent acid and a strong oxidizing agent used in producing organobromides.

In this article, we discuss various facts and FAQs on HBr + Mg3P2 chemical reaction, the type of reaction, the product formed, reaction enthalpy, etc.

What is the product of HBr and Mg3P2?

HBr reacts with magnesium phosphide to form magnesium bromide (MgBr2) along with the evolution of phosphine (PH3) gas.

6HBr + Mg3P2 —–> 3MgBr2 + 2PH3

What type of reaction is HBr + Mg3P2?    

The reaction of HBr + Mg3P2 is an example of a double displacement reaction, also known as a salt metathesis reaction.

How to balance HBr + Mg3P2?

The steps to balance HBr + Mg3P2 are as follows –

  • Assign an alphabet to each compound in the reactant and product in order to represent the unknown coefficients.
  • a HBr + b Mg3P2 —–> c MgBr2 + d PH3
  • Generate an equation by summing all the coefficients of each element involved in the reaction.
  • H → a=3d, Br → a=2c, Mg → 3b=c, P → 2b=d
  • Apply the Gaussian elimination approach to determine the values of each assigned coefficient.
  • The value of coefficients obtained after simplification is a= 6, b=1, c=3, d=2.
  • The overall balanced equation is –
  • 6HBr + Mg3P2 —–> 3MgBr2 + 2PH3

HBr + Mg3P2 titration

HBr + Mg3P2 titration is generally avoided as their reaction is vigorous in nature with the liberation of toxic phosphine gas.

HBr + Mg3P2 net ionic equation

The net ionic equation between HBr+ Mg3P2 is:

6H+ (aq) + Mg3P2 (s) = 3Mg2+ (aq) + PH3 (g)

The net ionic equation is derived by the following steps:

  • Write the balanced equation indicating the physical states of each molecule.
  • 6HBr (aq) + Mg3P2 (s) = 3MgBr2 (aq) + 2PH3 (g)
  • Write the ionic form of soluble electrolytes that are present in the reaction. Hence, the complete ionic equation is –
  • 6H+ (aq) + 6Br (aq) + Mg3P2(s)  =  6Mg2+ (aq) + 6Br (aq) + PH3 (g)
  • Finally, to get the net ionic equation, cancel the spectator ions (6Br) on either side of the reaction.
  • 6H+ (aq) + Mg3P2 (s) = 3Mg2+ (aq) + PH3 (g)

HBr + Mg3P2 conjugate pairs

  • HBr is a strong acid with Bras its conjugate base.
  • There is no conjugate pair Mg3P2 as it is moisture-sensitive.

HBr + Mg3P2 intermolecular forces

HBr + Mg3P2 reaction enthalpy

HBr + Mg3P2 reaction enthalpy is -399.34 KJ/mol. The enthalpy of formation values is given below –

Reactants and ProductsEnthalpy in KJ/mol
Enthalpy values
  • ∆Hf°(reaction) = ∆Hf°(products) – ∆Hf°(reactants)

= -515 – (-115.66)

= -399.34 KJ/mol

Is HBr + Mg3P2 a buffer solution?

HBr + Mg3P2 is not a buffer solution, as HBr is a strong acid, and Mg3P2 is an inorganic salt that can neither form a buffer solution nor maintain the pH level of other solutions.

Is HBr + Mg3P2 a complete reaction?

HBr + Mg3P2 is a complete reaction, as water-soluble magnesium bromide salt is formed along with phosphine gas.

Is HBr + Mg3P2 an exothermic reaction?

HBr + Mg3P2 is an exothermic reaction, as tremendous heat is expelled along with the evolution of phosphine gas.

Is HBr + Mg3P2 a redox reaction?

HBr + Mg3P2  is not a redox reaction, as no transfer of electrons occurs within the reaction. Moreover, there is no change in the oxidation state of any element in HBr + Mg3P2 reaction; thus, it is not a redox reaction.

Is HBr + Mg3P2 a precipitation reaction?

HBr + Mg3P2 is not a precipitation reaction as the product (MgBr2) formed during the chemical reaction is an electrically neutral salt, soluble in water.

Is HBr + Mg3P2 an irreversible reaction?

HBr + Mg3P2 is an irreversible reaction since the reaction can never attain equilibrium due to the evolution of phosphine gas.

Is HBr + Mg3P2 a displacement reaction?

HBr + Mg3P2 is a double displacement reaction because the more reactive magnesium displaces hydrogen from HBr, while H combines with the non-metal P to form PH3.


Screenshot 2022 12 05 174815
Double Displacement Reaction


Magnesium phosphide is a binary alkali metal salt, mildly soluble in acids. MgBr2 formed in this reaction, is mainly used as a tranquilizer and as a catalyst in organic compound synthesis. PH3 is used in the fumigation of food grains.