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Bromine is a volatile reddish-brown liquid at room temperature, and it can readily form a reddish-brown vapour. Its name was derived from ancient Greek, referring to its sharp and pungent smell. The properties of bromine lie somewhere in between that of chlorine and iodine, making it the middle child.
Reaction with Bromine
With an electronic configuration of [Ar] 3d10 4s2 4p5, bromine falls in the p block with 2, 8, 18, 7 in its electron shell. Bond energies to bromine tend to be lower than those to chlorine but higher than those to iodine. Bromine is a weaker oxidising agent than chlorine but a stronger one than iodine.
This can be represented as:
Bond Energy:
Br < Cl
Br > I
Oxidising Agent:
Br < Cl
Br > I
Similar to chlorine, bromine does not occur freely in nature. Still, just as chlorine occurs in table salt, bromine also exists in the form of colourless soluble crystalline mineral halide salts. Although bromine is rare in Earth’s crust, bromide ion, with its high solubility, has caused accumulation in the oceans.
Fun Fact: After mercury, bromine is the only other liquid element under STP.
Several organobromine compounds such as the pesticide methyl bromide are no longer used, resulting in ozone depletion in high temperatures.
However, one can find the use of bromine compounds in well drilling fluids, photographic film, and fire retardants.
Bromine forms a reaction with water, hydrogen, carbon monoxide, phosphorus, sulphur, halides, alkanes, alkenes, and alkynes.
Oxidation states: −1, +1, +3, +4, +5, +7 (a strongly acidic oxide)
The Reaction of Bromine with Air
Bromine, Br, does not react with oxygen, O2, or nitrogen N2 at all. So, if you’re ever presented with a reaction, say
Br2 + O2 → NO REACTION
However, bromine does react with ozone, O3, at -78 °C, very low temperature, forming bromine(IV) oxide, BrO2. It can also be said that bromine reacts with ozone under special conditions.
Br2(l) + 2 O3(g) → 2 BrO2(s) + O2(g)
Bromine, Br2 also forms a reaction with poisonous carbon monoxide, CO, forming COBr2.
Br2(l) + CO(g) → COBr2(l)
The Reaction of Bromine with Bases
Bromine, Br2, reacts with hot aqueous alkali, forming bromate, BrO3–. Out of all, only one-sixth of the total bromine is converted in the following reaction.
3 Br2(g) + 6 OH–(aq) → BrO3–(aq) + 5 Br–(aq) + 3 H2O(l)
The Reaction of Bromine with Halogens
Bromine, Br2, reacts with fluorine, F2, in the gas phase, forming a compound, BrF . This is a difficult product to obtain as BrF reacts with itself, forming the compounds, Br2, BrF2 and BrF5.
Br2(g) + F2(g) → 2 BrF(g)
3 BrF(g) → Br2(l) + BrF3(l)
5 BrF(g) + 2Br2(l) → BrF5(l)
Using excess fluorine at a high temperature of 150 °C, bromine reacts with fluorine forming BrF5.
Br2(l) + 5 F2(g) → 2 BrF5(l)
Chlorine, Cl2 , reacts with bromine, Br2, in the gas phase, forming the unstable bromine(I) chloride, ClBr.
Cl2(g) +Br2(g) → 2 ClBr(g)
Bromine, Br2, reacts with iodine, I2, at room temperature, forming bromine(I) iodide, BrI.
Br2(l) + I2(s) → 2 IBr(s)
The Reaction of Bromine with Hydrogen
Hydrogen reacts with Br2 forming hydrogen bromide. The reaction is slow at room temperature, and increases in speed with increasing temperatures.
H2(g) + Br2(g) → 2 HBr(g)
Another way to produce HBr is with red phosphorous:
2 P + 6 H2O + 3 Br2 → 6 HBr + 2 H3PO3
H3PO3 + H2O + Br2 → 2 HBr + H3PO4
The simplest compound of bromine is HBr and is a colourless gas.
Aqueous hydrogen bromide is known as hydrobromic acid, which is a strong acid (pKa = −9)
The Reaction of Bromine with Metals/Metal Ions
Nearly all the elements in the periodic table form binary bromide.
Cd reacts directly with Br2 in aqueous solution and Br2(g) at 450 °C.
Cd(s) + Br2(aq) → Cd2+(aq) + 2 Br–(aq)
Cd(s) + Br2(g) → CdBr2(s) [pale yellow]
Mn(II)-ions are readily oxidised to MnO2 by brome under alkaline conditions
Mn2+(aq) + Br2(aq) + 2 OH–(aq) → MnO2(s) [brown-black] + 2 HBr(aq)
Manganese with oxidation steps >2 will be reduced to Mn(II) by Br under acidic conditions under the formation of Br2, e.g.
MnO2(s) + 2 Br–(aq) + 4 H+(aq) → Mn2+(aq) + Br2(aq) + 2 H2O(l)
Nickel(II) can be oxidised to nickel(III) using Br2 under alkaline conditions
2 Ni(OH)2(s) + Br2(aq) + 2 OH–(aq) → 2 Ni(OH)3(s) + 2 Br–(aq)
Reaction of Bromine with Phosphorus
Phosphorus reacts with excess Br2 forming phosphorus(V)chloride.
2 P(s) + 5 Br2(l) → 2 PBr5(s)
with excess phosphorus, the reaction is
2 P(s) + 3 Br2(l) → 2 PBr3(s)
PBr3(s) + Br2(l) → PBr5(s)
Reaction of Bromine with Sulphur
Sulphur reacts with excess bromine forming sulphur(I)bromide or sulphur(II)bromide:
2 S(s) + Br2(g) → S2Br2(s)
S(s) + Br2(g) → SBr2(s)
Bromine reacts with H2S forming HBr:
S(s) + Br2(g) → SBr2(s)
Reaction of Bromine with Water
Bromine, Br2, reacts with water, forming hypobromite, BrO–.
Br2(l) + H2O(l) → BrO–(aq) + 2 H+(aq) + Br–(aq)
The reaction with bromine gives an alkyl bromide.
Alkanes undergo a substitution reaction with halogens in the presence of light.
For example, under ultraviolet light, methane reacts with halogen molecules such as bromine and chlorine.
For example:
methane + bromine → methyl bromide + hydrogen bromide
CH4(g) + Br2(l) –h–> CH3Br(g) + HBr(g)
Unsaturated hydrocarbons such as alkenes and alkynes are much more reactive than the parent alkanes. They react rapidly with bromine, for example, to add a Br2 molecule across the C=C double bond.
This reaction provides a way to test for alkenes or alkynes. Solutions of bromine in CCl4 have an intense red-orange colour. When Br2 in CCl4 is mixed with a sample of an alkane, no change is initially observed. When it is mixed with an alkene or alkyne, the colour of Br2rapidly disappears.
Conclusion
Bromine is a very reactive halogen that forms a compound with most of the elements in the periodic table.
Bromine is an essential element of our atmosphere and living world. The role of biological organobromine compounds in sea life such as algae has been known for much longer. As a pharmaceutical, the simple bromide ion (Br−) has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative.
Bromine and its compound reactions can be found everywhere around us. In this article we learnt about the reaction with bromine.
