Halogen notes by Bimochan Dhungel

1. Halogens
Presented by Hikmat Khanal
L.R.I school, Kalanki, KTM

2. Syllabus according to NEB
9.5.1 General characteristics of halogens
9.5.2 Comparative study on preparation (no diagram and description is
required),
9.5.2.1 Chemical properties [with water, alkali, ammonia, oxidizing
character, bleaching action] and uses of halogens (Cl2, Br2 and I2)
9.5.3 Test for Cl2, Br2 and I2
9.5.4 Comparative study on preparation (no diagram and description is
required), properties (reducing strength, acidic nature and solubility)
and uses of haloacids (HCl, HBr and HI)

3. Position of halogens in periodic table
• Flourine(9), chlorine(17), bromine(35), iodine(53) and astatine(85) are
commonly called halogens.
• Fluorine is also called super halogen due to extra ordinary reactivity.
• The meaning of halogen is “salt forming”. They readily react with
alkali metals and alkaline earth metals to form halide salts.
• They are located to the group VIIA or group 17 in the periodic table.

5. Comparison among halogens

7. General characterstics of halogens
1. Electronic configuration: all have seven electrons (ns2 np5) in their outermost shell.
2. They exist as diatomic molecules(F2, Cl2, Br2, I2) containing single covalent bond.
3. Fluorine and chlorine exist as gas. Bromine as liquid and iodine is volatile solid at room
temperature
4. The m.p and b.p increases with the increase in atomic number due to increase in
Vanderwaal’s forces. So the order is F2<Cl2<Br2<I2
5. Fluorine is most electronegative element. Electronegativity value decreases down the
group. So the order is F>Cl>Br>I
6. Chlorine has the highest electron affinity(E.A) value. The higher E.A value of chlorine
over fluorine is fluorine is small in size. Consequently, higher Inter elctronic repulsion
in fluorine decreases E.A
7. They are strong oxidizing agent as they have high tendency to attract electrons.
Oxidizing power decreases down the group as F2>Cl2>Br2>I2
8. They are less soluble in water and their solubility decreases with increase in Z (atomic
number)

8. Comparative study on preparation
Halogens are prepared by the oxidation of halide ions. Oxidizing agent like
MnO2 is used in this process. Since, fluoride ions are most difficult to be
oxidized, F2 cannot be prepared by this method. Instead, F2 can be prepared
by the electrolysis of molten mixture of KHF2 and HF.
KHF2  HF + KF, KF ⇄ K
+
+F
− , HF⇄ 𝐻
+
+𝐹
−
On electrolysis, hydrogen is evolved at the cathode and fluorine at the anode
At cathode, 2H++2e-H2
At anode, 4F-2F2 + 4e-
Overall reaction for the preparation of Cl2, Br2 and I2
2NaX(s) + 3H2SO4 + MnO2(s)
∆
𝑀𝑛𝑆𝑂4 𝑠 + 2 𝑁𝑎𝐻𝑆𝑂4 + 2𝐻2𝑂 + 𝑋2
(where, X = Cl, Br, I)

9. Laboratory preparation of chlorine: chlorine is prepared in laboratory by heating a mixture of
NaCl, MnO2, and conc. H2SO4
2NaCl + 3H2SO4 + MnO2
∆
2NaHSO4 + MnSO4 + Cl2 + 2H2O
Laboratory preparation of bromine: Bromine is prepared in laboratory by heating a mixture of
NaBr, MnO2, and conc. H2SO4
2NaBr + 3H2SO4 + MnO2
∆
2NaHSO4 + MnSO4 + Br2 + 2H2O
Laboratory preparation of iodine: Iodine is prepared in laboratory by heating a mixture of KI,
MnO2, and conc. H2SO4
2KI + 3H2SO4 + MnO2
∆
2KHSO4 + MnSO4 + I2 + 2H2O

10. Chemical properties
1. With H2O: fluorine readily and vigorously decomposes H2O into oxygen and
ozone.
Chlorine and bromine reacts with water to give hypohalous acids. These acids are
unstable.
Cl2 + H2O  HCl + HOCl (hypochlorous acid)
2HOCl  2HCl + O2
Br2 + H2O  HBr + HOBr (hypobromous acid)
2HOBr  2HBr + O2
Iodine has very little action with water, but dissolves in aqueous solution of KI.
I2 + KI  KI3. Solution of Iodine in potassium iodide(I2 + KI +H2O +C2H5OH) is called
tincture of iodine which is used as antiseptic for the prevention of microbial
growth in wounds

11. Chemical properties
2. With alkali: F2 reacts with cold and dilute NaOH to give Oxy diflouride and
and with hot and conc NaOH to give oxygen. However, other halides reacts
with dilute and conc NaOH to give following reactions.
2F2 + 2NaOH(cold and dilute) OF2 + 2NaF + H2O
2F2 + 4NaOH(hot and conc) O2 + 2H2O
Cl2 + 2NaOH(cold and dilute) NaCl + H2O + NaClO(sodium hypochlorite)
3Cl2+ 6NaOH(hot and conc) 5NaCl + 3H2O + NaClO3 (Sodium chlorate)
Br2 + 2NaOH(cold and dilute) NaBr + H2O + NaBrO(sodium hypobromite)
3Br2+ 6NaOH(hot and conc) 5NaBr + 3H2O + NaBrO3 (Sodium bromate)
I2 + NaOH(cold and dilute) NaI + HOI(hypoiodous acid)
3I2 + 6NaOH(hot and conc) 5NaI + 3H2O + NaIO3(sodium iodate)

12. Chemical properties
3. With ammonia
8NH3(excess) + 3Cl2  N2 + 6 NH4Cl
2NH3 + 6Cl2 (excess) 2NCl3(nitrogen trichloride) + 6HCl
8NH3(excess) + 3Br2  N2 + 6 NH4Br
2NH3 + 6Br2 (excess) 2NBr3(nitrogen tribromide) + 6HBr
With iodine, a brown ppt. of nitrogen triodide monoammoniate is
formed
5NH3 + 3I2 3NH4I + NI3.NH3

13. Chemical properties
4. Oxidizing nature: halogens are good oxidizing agent. Their oxidizing power is in the order: F2 > Cl2
> Br2 > I2
i. Mutual displacement: Flourine displaces other halogens from their aqueous salt
F2 + 2NaCl  2NaF + Cl2
Chlorine displaces Br2 and I2 from their salt solution.
Cl2 + 2KBr 2KCl + Br2
Cl2 + 2KI 2KCl + I2
Bromine displaces iodine from iodide salt
Br2 + 2KI 2KBr + I2
ii. With H2S: H2S is oxidized to sulphur
H2S + Cl22HCl + S
iii. With SO2: SO2 is oxidized to sulphuric acid
SO2 + 2H2O + Cl2  2HCl + H2SO4
iv. With Na2SO3: Na2SO3 is oxidized to Na2SO4
Na2SO3 + H2O + Cl2  2HCl + Na2SO4

14. Chemical properties
v. With Na2S2O3(sodium thiosulphate):
Na2S2O3 + H2O + Cl2  2HCl + Na2SO4 + S
2Na2S2O3 + I2Na2S4O6(sodium tetrathionate) + 2NaI
5. Bleaching action
Bleach: to remove/clean/to make white by removal of color.
Halogens, especially chlorine changes colored compounds to colorless which is called
bleaching.
F2: due to high reactivity and strong oxidizing agent, it destroys the substance to be
bleached. So, it can’t be used as bleaching agent.
I2: Since, weakest oxidizing agent, iodine has no bleaching action.
Br2 is mild bleaching agent and Cl2 is best bleaching agent among all halogens. It bleaches
the colored substance in presence of moisture and and sunlight. In such conditions,
nascent oxygen is liberated that oxidizes coloring substance(eg: hair) to colorless product.

15. Chemical properties
Chemistry of bleaching
Cl2 + H2O
𝑆𝑢𝑛𝑙𝑖𝑔ℎ𝑡
𝐻𝐶𝑙 + 𝐻𝐶𝑙𝑂
HClO  HCl + O(nascent oxygen)
Colored substance + O  colorless substance.
• Cl2 bleaches color by oxidation and it is permanent process.
• Cl2 can’t bleach printed material and pencil marks because carbon
present doesn’t react with chlorine. Also, dry chlorine can’t bleach
the matter.

16. Uses of Halogens(Cl2, Br2 and I2)
Uses of Cl2
• As disinfectant and germicide for sterilization of drinking water. Eg: Piyush for jar
water treatment
• As a good bleaching agent
• Used in the metallurgy of silver, gold and platinum
• Used in the preparation of poisonous gases like: phosgene, mustard gas, tear gas
Uses of Br2
• AgBr is used in the manufacture of photographic films
• It is used in the manufacture of dyes, drugs
• Bromine water is used in the detection of unsaturation in the organic
compounds.

17. Uses of Halogens
Uses of I2
• As antiseptic (tincture of iodine) and analgesic (iodex)
Antiseptic; prevent microbial growth or infection
Analgesic; pain relieving or decreasing
• In the treatment of goiter(enlargement of thyroid gland)
• Silver iodide is also used to make photographic films.

18. Test for Cl2, Br2 and I2
Test for Cl2
• It is greenish yellow gas with irritating smell
• It turns starch iodide paper (paper+KI+starch) to blue
Cl2 + 2KI  2KCl + I2
I2 + starch  blue
Test for Br2
• It is pungent smelling dark red liquid.
• It turns starch iodide paper to blue
Br2 + 2KI  2KBr + I2
I2 + starch  blue
Test for I2
• It has violet vapors when heated
• It turns moist starch paper to blue
I2 + starch  blue

19. Comparative study on preparation of haloacids
(HCl, HBr, HI)
• Haloacids or hydrogen halides may be prepared by direct combination of elements.
H2 + X2  2HX (where X = Cl, Br, I)
However, the rate of reaction varies. So this method isn’t practicable in lab
Acidic strength: HF < HCl < HBr < HI. Due to H-bonding in HF, it has high bond
dissociation energy. So it liberates less number of H+
• HCl is prepared by heating common salt with concentrated sulphuric acid.
NaCl + H2SO4
ℎ𝑖𝑔ℎ 𝑡𝑒𝑚𝑝
𝑁𝑎2𝑆𝑂4 + 2𝐻𝐶𝑙 (𝑔)
• HBr is prepared by the action of bromine on moist red phosphorous .
P4 + 6Br2 + 12H2O  4H3PO3 + 12HBr
• HI is prepared by the action of iodine on moist red phosphorous
P4 + 6I2 + 12H2O  4H3PO3 + 12HI

20. Properties
Solubility:
HCl, HBr and HI are highly soluble in water giving hydronium ions and respective halides ions.
Solubility is related to dipole moment. Higher the dipole moment, higher the solubility in water. Since, the F is
the most electronegative element, HF has highest dipole moment.
Dipole moment: HF > HCl > HBr > HI
Solubility order: HF > HCl > HBr > HI
HX + H2O H3O+(hydronium ion) + X- (where X = Cl, Br, I)
Acidic nature: in aqueous solution, haloacids ionizes to give proton.
HX + H2O  H3O+ + X-
Acidic strength: HI > HBr > HCl. As the size of halogen increases , H-X bond becomes weaker and ionizes easily
in water. So HI is strongest acid. While HF is weakest one due to poor ionization of HF caused by intermolecular
hydrogen bonding among the HF molecules.
• Reaction with bases: gives salt and water
HX + NaOH  NaX + H2O (X = Cl, Br, I)
• Reaction with metal: electropositive metals like Zn, Na, Fe, Al etc react with HX to liberate H2 gas
Zn + 2HX  ZnX2 + H2
Na + 2HX  2NaX + H2

21. Properties
• Reducing strength: reducing property depends upon the dissociation energy. Higher the bond
dissociation energy, more stable is the halogen acid and hence weaker is reducing agent.
Bond dissociation energy increases in the order: HI<HBr<HCl<HF
Reducing power is in reverse order: HI>HBr>HCl>HF
Reducing properties of HCl
a) With KMnO4; it decolorizes KMnO4 solution
2KMnO4 + 16HCl  2KCl + MnCl2 + 8H2O + 5Cl2
b) With MnO2: it decolorizes reddish brown color of MnO2 to colorless
MnO2 + 4HCl  MnCl2 + 2H2O + Cl2
c) With H2O2: It reduces H2O2 to H2O.
H2O2 + 2HCl  2H2O + Cl2
Reducing properties of HBr and HI: they are strong reducing agent as they easily oxidized to Br2 and
I2 respectively. They reduce MnO2 to MnX2, H2SO4 to SO2, HNO3 to NO2, decolorize acidified KMnO4
solution, convert acidified K2Cr2O7 solution to green.
MnO2 + 4HX  MnX2 + 2H2O + X2(where X = Br and I)
2KMnO4(Pink) + 3H2SO4 + 10HX  K2SO4 + 2MnSO4(colorless)+ 5H2O + X2 (X= Br and I)

22. Properties (continue…)
K2Cr2O7(orange)+4H2SO4+6HXK2SO4+Cr2(SO4)3(green)+7H2O+3X2
H2SO4 + 2HX  2H2O + SO2+ X2
Note: HI reduces CuSO4 solution and FeCl3 solution but HBr and HCl
can’t.
2CuSO4 + 4HI  CuI2 + 2H2SO4
2CuI2  Cu2I2 + I2
(Brown ppt.)
2FeCl3 + 2HI  2FeCl2 + 2HCl + I2

23. Uses of haloacids
• HCl is used in the preparation of aqua regia.
• For manufacture of Cl2, metal chlorides
• HBr is used in the preparation of bromides like AgBr which is used in
photography
• Used as reducing agent
• Used as laboratory reagent
• Used in preparation of iodides and iodine.

24. Dipole moment order/trend in Haloacids
Decreases in the order: HF > HCl > HBr > HI
WHY: since the electronegativity of halogen decreases from fluorine to
Iodine, HF has highest dipole moment value.
NOTE: percentage of ionic character, polarity order is same as dipole
moment order.

25. Bond length trend in Haloacids
Increases in the order; HF < HCl < HBr < HI
WHY: the H-X bond in halogen acid/haloacid is formed by the overlap
of s orbital of hydrogen and p orbital of halogen. As we move down the
group from F to I, the size of P orbital increases. As a result, HI has
longest bond length.
Note: bond strength is inversely proportional to bond length
So bond strength order: HF > HCl > HBr > HI

26. Boiling point order in Haloacids
HF has exceptionally high boiling point due to the presence of H-
bonding. Among other haloacids, boiling point increases from HCl to HI
due to increase in size of halogen (due to increasing Vanderwaal force
of attraction).
So b.p increases in the order; HCl < HBr < HI < HF
NOTE: elements/molecules having low b.p are more volatile and vice
versa.
NOTE: Higher the boiling point of chemical, the chemical is thermally
more stable.

27. Oxyacids of halogens

28. Oxyacid of halogens
Acidic strength of oxyacid
• Acidic strength of oxyacids of same halogen increase with increase in
oxidation number of halogen or simply with increase in number of oxygen
atom.
HClO4 > HClO3 > HClO2 > HClO
• Acidic strength of oxyacids of different halogens in same oxidation state
decreases with increase in size
HOCl > HOBr > HOI
Oxidizing power
Oxidizing power of oxyacids of same halogen decrease with increase of
oxidation state of halogen. HClO> HClO2 > HClO3 > HClO4

29. Homework Questions
1. Bromine is manufactured commercially from Carnallite. What is the chemical formula of carnallite?
2. HF is least volatile while HCl is most volatile among haloacids. Give reason.
3. How HBr is prepared in laboratory?
4. Write any two uses of chlorine and bromine
5. Write short notes on bleaching action of chlorine?
6. What happens when HCl is treated with KMnO4 solution?
7. Write the action of HI with
a) CuSO4 solution
b) Acidified K2Cr2O7
8. HF is weaker acid. Why?
9. Give three reactions to show the oxidizing action of chlorine.
10. Write laboratory preparation of chlorine, bromine and iodine describing the principles with no diagram and
description. Write their reaction with a). NaOH b). NH3
11. Define haloacids. Arrange the haloacids in the increasing order of boiling point and explains that order.
12. A test tube contains a solution of one of the following salts: NaCl, NaBr, and NaI. Describe a single test that
can distinguish among these salts.
13. Is this reaction feasible: 2NaF+Cl2 ⟶ 2NaCl+F2. Explain with reason.

30. MCQ
1. Which is most reactive?
a. F b. Cl c. Br d. I
2. Deacon’s process is used in the manufacture of
a. Chlorine b. freons c. Bromine d. bleaching powder
3. Which halogen is liquid at room temperature?
a. F b. Cl c. Br d. I
4. The halogen that displace other three halogens from their compound
a. F b. Cl d. Br d. I
5. Which of the following statement(s) is true?
a. Chlorine has highest electron affinity value
b. Bromine is extracted commercially from carnallite
c. HCl is strongest acid among haloacids
d. Both a and b
6. Tincture of iodine , that is mixture of I2, KI, C2H5OH and H2O, is
a. Antiseptic b. analgesic c. disinfectant d. depressant

31. MCQ
7. Which of the following reactions are not possible?
a. 2NaCl+F2 ⟶ 2NaF+Cl2
b. 2NaCl+Br2 ⟶ 2NaBr+Cl2
c. 2NaF+Cl2 ⟶ 2NaCl+F2
d. 2NaBr+Cl2 ⟶ 2NaCl+Br2
a). a and d b). Both a and c c). Both b and c d). a,b and c