Is HI Polar or Nonpolar?

HI is a polar molecule because of the electronegativity difference of Iodine (2.66) and hydrogen (2.2) atom which causes the formation of partial positive and negative charges on both ends of the molecule, as a result, the molecule has a net dipole moment.

Detailed Explanation: Why is HI a Polar Molecule?

Hydrogen Iodide is a heteronuclear diatomic molecule and a member of hydrogen halide. An aqueous solution of hydrogen iodide is commonly known as hydroiodic acid or hydriodic acid which is considered a strong acid. It is found in both aqueous and gas states under standard conditions however they are interconvertible. HI is used as a strong reducing agent in a redox reaction.

Before entering into its polarity, let’s have some ideas about what polar and nonpolar molecules are,

Polar Molecules

Those molecules having electronegativity difference between bonded atoms so that the formation of dipole charges on the molecules are known as polar molecules. The net dipole moment shouldn’t be zero in case of polar molecule because of the induction of partial positive and negative charges on either end of the molecule.

Polar molecules show little ionic characteristics as they can conduct heat and electricity, can be soluble in water, and have a strong electrostatic force of attraction.

Examples of Polar molecules: Water (H2O), Nitrogen dioxide (NO2), etc.

Nonpolar Molecules

Those molecules having the same electronegativity value for bonded atoms so that there are not any form of partial positive and negative charges within the molecule resulting in the net dipole moment become zero are known as nonpolar molecules. In many cases, there may exist polar bonds within the molecules but whole molecules are considered as nonpolar due to the symmetrical geometry of molecules which caused dipole charges of molecules to get canceled.

Nonpolar molecules are pure covalent bonds as they only show covalent nature i.e. bad conductor of heat and electricity, insoluble in polar solvents, etc.

Examples of Nonpolar molecules: Carbon Dioxide (CO2), Carbon Tetrachloride (CCl4), Methane (CH4) etc.

Hydrochloric acid (HI) Polar or Nonpolar (Based on characteristics)

HI is a polar molecule and the Iodine atom closest to the negative side as iodine has a higher electronegativity value. The shared pair of electrons stay closer to the I atom, as a result, induced partial positive charge on hydrogen atom and negative charge on iodine atom.

These are some of the characteristics of HI which clearly elaborate why HI is a polar molecule:

Electronegativity Difference

Electronegativity is the tendency of an atom to attract shared pair of electrons from its binding partner within a molecule. Higher the electronegativity value, more strength to pull electrons. If the electronegativity difference between two atoms is equal, shared pair of electrons are centralized but when the difference occurs, atom having higher E.N, attracts electrons closer to its nucleus.

In HI molecules,

Electronegativity of H atom= 2.2

Electronegativity of I atom= 2.66

Electronegativity difference= 2.66-2.2= 0.46

Here electronegativity difference between hydrogen and Iodine atom is 0.46 and according to the Pauli scale if the electronegativity difference between two atoms is between 0.4 to 2.0, the bond is considered as a polar bond so that the H-I bond is also polar.

Lewis Structure & Molecular Geometry

In the HI molecule, a total of 8 valence electrons take part in molecule formation in which hydrogen has only one valence electron and iodine has seven valence electrons. By mutual sharing of one electron from each atom, the Lewis dot structure completed.

Hydrogen Iodide has a linear geometry (also called terminus) because it is made up of two different atoms with mutual sharing of electrons. In heteronuclear diatomic molecule, induced charges do not oppose each other so their resultant dipole charge is also not canceled.

Dipole Moment

Mathematically, the dipole moment is the product of charge and the separation between the atom within the molecule. It is denoted by and given be,

Dipole moment (µ)= Charge (Q) * distance of separation (r)

It is measured in Debye units denoted by ‘D’. 1 D = 3.33564 × 10^-30 C.m, where C is Coulomb and m denotes a meter.

The dipole moment is a vector quantity and depends on the resultant of the dipole charges i.e. if a molecule has symmetrical geometry, the net dipole moment will be zero. But HI is a heteronuclear diatomic molecule so that induced charges do not cancel each other as a result net dipole moment of the molecule is 0.38 D which makes the molecule polar.

Electron Affinity

Electron affinity is the possibility of finding an electron inside an atom or a molecule. It is impossible to determine the exact position of an electron at a particular time but we can estimate the probable position of an electron according to its behavior and orbit.

Here in the case of HI, Iodine has a higher electronegativity value so it pulls a shared pair of electrons from the hydrogen atom and that’s why the chances of finding an electron is higher closer to the iodine atom.

Solubility Principle

As we know the solubility principle “like dissolves like”, HI is mostly soluble in only polar solvents and very rarely soluble in nonpolar solvents which directly reflects its polarity nature. Further, it is extremely soluble in water (H2O) which is also a polar solvent.

Clarify your doubts

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Sources & Preparation of HI

HI is prepared by reacting Iodine gas (I2) and Hydrazine (N2H4) for industrial purposes on a large scale which yields hydrogen iodide and nitrogen gas.

I2 + N2H4 à HI + N2

In general, it can be prepared by the normal reaction of H2 and I2.

 I2 + H2 à 2HI

Properties of Hydrogen Iodide (HI)

1. It is a colorless gas, has a molar mass of 127.904 g/mol with melting and boiling point -50.80 °C and -35.36 °C respectively.

2. It is a strong reducing agent and highly soluble in water, gives a fume of hydroiodic acid when it comes in contact with moist air.

3. It is a toxic and corrosive gas that causes severe hazards to death when it comes to contact with skin or inhalation. Mainly the vapor of HI is severely irritating to the mucous membrane of the nose and eyes.

Uses of Hydroiodic acid (HI)

• It is used as a strong reducing agent mostly used for the preparation of acetic acid which is further used at the time of production of vinegar.
• It is highly acidic in nature so it can easily kill many harmful germs and bacteria. This is the main reason it is used as a disinfectant and sanitizer for medical tools.
• It is used in the field of agriculture, forestry, fishing and hunting, mostly as non-pesticidal.