Reaction with Iodine

Iodine is a non-metallic, dark-grey/purple-black, lustrous, solid element. It is the most electropositive halogen and the least reactive of the halogens, and even then, it can still form compounds with many elements. 

The chemical element iodine has the symbol ‘I’ and carries an atomic number of 53. Iodine is the heaviest among the stable halogens. Iodine exists in the form of a semi-lustrous, non-metallic solid at STP, which melts at a high temperature of 114 degrees Celsius to form a deep violet colour liquid. At 184° Celsius, iodine boils to a violet gas. Its name was derived from Ancient Greek, which meant ‘violet-coloured.’

As seen in the above table, iodine occurs in many oxidation states, including iodide (I–), iodate IO3– and various other anions. Iodine is the least abundant among halogens, being at the position of the sixty-first number of most abundant elements. Iodine is also the heaviest essential mineral nutrient found in plants. 

In humans, it is essential in the synthesis of thyroid hormones. The deficiency of iodine severely affects almost two billion people in the world. Iodine deficiency is among the world’s single most important preventable causes of intellectual disabilities and mental retardation. 

However, iodine is very reactive yet less reactive than other halogens.

Reaction of Iodine

Reaction of Iodine with Bases

Iodine, I2, reacts with hot aqueous alkali, forming iodate IO3–.

3 I2(s) + 6 OH–(aq) IO3–(aq) + 5 I–(aq) + 3 H2O(l)

Reaction of Iodine with Acids

Iodination of metals gives a lower oxidation state than that of chlorination or bromination. The iodine molecule, I2, dissolves in CCl4 and aliphatic hydrocarbons, which results in bright violet solutions.

In the following equation, iodine, I2, reacts with concentrated nitric acid, forming iodic acid. 

I2(s) + 10 HNO3(aq) 2 HIO3(s) + 10 NO2(g) + 4 H2O(g)

Reaction of Iodine with Halogens

Iodine, I2, reacts with fluorine, F2, at room temperature, forming iodine(V) fluoride. At the high temperature of 250 °C, the product is iodine(VII) fluoride. At the low temperature of -45 °C, suspension in CFCl3, iodine(III) fluoride is formed.

I2(s) + 5 F2(g) ⟶ 2 IF5(l) [colourless]

I2(g) + 7 F2(g) ⟶ 2 IF7(g) [colourless]

I2(s) + 3 F2(g) ⟶ 2 IF3(s) [yellow]

Iodine, I2, reacts with bromine, Br2, forming the very unstable, low melting solid iodine(I) bromide.

I2(s) + Br2(l) ⟶ 2 IBr(s)

When iodine, I2, reacts with excess chlorine, Cl2, at -80 °C, iodine(III) chloride is formed. In the presence of water, iodic acid is formed at room temperature.

I2(s) + 3 Cl2(l) ⟶ I2Cl6(s) [yellow]

I2(aq) + 6 H2O(l) + 5 Cl2(g) ⟶ 2 HIO3(aq) + 10 HCl(g)

Reaction of Iodine with Air

Iodine, I2, like chlorine and bromine, does not react with oxygen, O2, or nitrogen, N2. 

It does, however, react with ozone, O3, forming the unstable compound, I4O9, which has a yellow colour. 

Reaction of Iodine with Water

Iodine, I2, reacts with water, forming the compound of hypoiodite, IO–.

I2(aq) + H2O(l) ⟶ IO– + 2 H+(aq) + I–(aq)

Reaction of Iodine with Metals/metal ions

Although solid cadmium, Cd, does not react within the aqueous phase. However, in the gas phase, Cd and I2 will react, forming CdI2(g). 

For the manufacturing of steel bombs, at high temperature and pressure, an equivalent of Cd and I2 will react, forming CdI.

Cd(s) + I2(aq) ⟶ Cd2+(aq) + 2 I–(aq)

Cd(g) + I2(g) ⟶ CdI2(g)

2 Cd(g) + I2(g) ⟶ 2 CdI(g)

Under the formation of I2, Manganese with an oxidation state greater than 2 gets reduced to Mn(II) by I– under acidic conditions.

MnO2(s) + 2 I–(aq) + 4 H+(aq) ⟶ Mn2+(aq) + I2(aq) + 2 H2O(l)

Elements like molybdenum(IV) oxide would react with aluminium(III) iodide at 230 °C to give molybdenum(II) iodide. 

Halogen exchange occurs during the reaction of tantalum(V) chloride with excess aluminium(III) iodide at 400 °C, which results in the production of tantalum(V) iodide.

Reaction of Iodine with Hydrogen

The simplest compound of iodine is hydrogen iodide, HI, a colourless gas that reacts with oxygen to give a combination of water and iodine.

Hydrogen reacts with I2 to form hydrogen iodide. At room temperature, the speed of the reaction is slow, whereas when the temperature is increased, the speed of the reaction increases. 

2 I2 + N2H4 H2O ⟶ 4 HI + N2

Iodine Oxides and Oxoacids

The four oxoacids: hypoiodous acid (HIO), iodous acid (HIO3), iodic acid (HIO3), and periodic acid (HIO4 or H5IO6), are very important.

In an aqueous solution, iodine dissolves to form the following reaction. 

I2 + H2O ⇌ HIO + H+ + I– ;Kac = 2.0 × 10-13 mol2 l-2

I2 + 2 OH– ⇌ IO– + H2O + I–;Kalk = 30 mol-1 l

Hypoiodous acid is unstable to disproportionation. The hypoiodite ions thus formed disproportionates immediately to give iodide and iodate:

3 IO– ⇌ 2 I– + IO– 3K = 1020

The even less stable are iodous acid (HIO2) and iodite. They only exist as an intermediate in the oxidation of iodide to iodate. 

Conclusion 

All the compounds formed from the reactions with iodine are of great importance. You must have heard that iodine is found in salt and is also essentially important for humans to survive. Iodine naturally occurs in some rocks, seawater and sediments. It is widely used to purify water as a disinfectant for cleaning and is used in skin soaps. Most of the radioactive iodine in nature is man-made. Iodines are also used to treat diseases and are also used in medical tests.