1 characteristic properties of the halogens 2 group viia elements include fluorine chlorine ...
TRANSCRIPT
1
Characteristic Characteristic Properties of Properties of the Halogensthe Halogens
2
• Group VIIA elements include
fluorine
chlorine
bromine
iodine
astatine
IntroductionIntroduction
halogens
(Salt producers)
3
• Astatine
chemistry not much known
radioactive
the total amount present in the Earth's crust is probably
less than 30 g at any one time.
IntroductionIntroduction
4
• Halogens are p-block elements
outermost shell electronic configuration of ns2np5
5
one electron short of the octet structure
• Halogens are p-block elements
6
• In the free elemental state
IntroductionIntroduction
they complete their octets by sharing their single unpaired p-electrons
7
they either
gain an additional electron to form halide ions
or
When halogens react with other elements
share their single unpaired p-electrons to form
single covalent bonds
8
highest among the elements in the same period
have a high tendency to attract electrons
strong oxidizing agents
High Electronegativity / Electron AffinityHigh Electronegativity / Electron Affinity
9
-1 is the most common oxidation state of halogens in their compounds
Ionic : NaF, NaCl, NaBr, NaI
Covalent : HF, HCl, HBr, HI
High Electronegativity / Electron AffinityHigh Electronegativity / Electron Affinity
10
Variable Oxidation StateVariable Oxidation State
All halogens (except fluorine) can expand their octet of electrons by utilizing the
vacant,
energetically low-lying
d-orbitals.
11
“ Electrons-in-boxes” diagrams of the electronic configuration of a halogen atom of the ground state and various
excited states
12
H
O
Cl
The half-filled orbital(s) overlap(s) with those of more electronegative atoms (e.g. O)
positive oxidation state (+1, +3, +5, +7)
H
O
Cl
O
H
O
Cl
O
O
H
O
Cl
O
O
O
+1+3
+5
+7
13
Oxidation state
of halogenIon / Compound
–1
F– Cl– Br– I–
HF HCl HBr HI
OF2
0 F2 Cl2 Br2 I2
+1
Cl2O Br2O
HOCl HOBr
OCl– OBr–
+3HClO2
ClO2–
Various oxidation states of halogens in their ions or compounds
14
Oxidation state
of halogenIon / Compound
+4 ClO2 BrO2
+5HClO3 HBrO3 I2O5
ClO3– BrO3
– HIO3
IO3–
+6 Cl2O6 BrO3
+7
Cl2O7 H5IO6
HClO4 HIO4
ClO4– IO4
–
Various oxidation states of halogens in their ions or compounds
15
• Fluorine (1)
the most electronegative
element
only one unpaired p electron
available for bonding
oxidation state is limited to –1
16
• Fluorine (1)
cannot expand its octet
no low-lying empty d orbitals
available
the energy required to promote
electrons into the third
quantum shell is very
high
Absence of HFO, HFO2,
HFO3, HFO4
17
Variation in Physical PropertiesVariation in Physical Properties
1. Melting point / boiling point down the group
HalogenMelting point
(C)
Boiling point
(C)
Fluorine
Chlorine
Bromine
Iodine
Astatine
–220
–101
–7.2
114
302
–188
–34.7
58.8
184
380
18
Variations in melting point and boiling point of the
halogens
19
Variation in Physical PropertiesVariation in Physical Properties
1. Melting point / boiling point down the group
The molecular size down the group
The electron cloud is more easily polarized
Induced dipoles are formed more easily
Stronger London dispersion forces
20
2. Colour becomes darker down the group
Haloge
nF2 Cl2 Br2 I2
ColourPale
yellow
Greenis
h
yellow
Reddish
brown
Violet
black
21
Appearances of halogens at room temperature and pressure: chlorine
chlorine
22
Appearances of halogens at room temperature and pressure: bromine
bromine
23
Appearances of halogens at room temperature and pressure: iodine
iodine
24
Colour Colour
• All halogens
coloured
the absorption of radiation in the visible light region of the electromagnetic spectrum
The colour is due to the unabsorbed radiation in the visible light region
25
Colour Colour
• Fluorine atom
has the smallest size
absorbs the radiation of relatively high frequency (i.e. blue light)
appears yellow (the unabsorbed radiation)
26
Colour Colour
• Atoms of other halogens
larger sizes
absorb radiation of lower frequency
27
Colour Colour
• Iodine
absorbs the radiation of relatively low frequency (i.e. yellow light)
appears violet
28
Q.1The colour of astatine is black.
29
Colour Colour
• Halogens
different colours when dissolved in different solvents
30
HalogenColour
in pure form in water in 1,1,1-trichloroethane
F2 Pale yellow Pale yellow Pale yellow
Cl2 Greenish
yellowPale yellow Yellow
Br2 Reddish
brown Yellow Orange
I2 Violet black
Yellow (only
slightly soluble)
Brown in
KI(aq)
Violet
Colours of halogens in pure form and in solutions
31
Colour Colour
• Halogens
non-polar molecules
not very soluble in polar solvents (such as water)
but very soluble in organic solvents (such as 1,1,1-trichloroethane)
32
Colours of halogens in water:(a) chlorine; (b) bromine; (c) iodine
(a) (b) (c)
33
Colours of halogens in 1,1,1-trichloroethane:(a) chlorine; (b) bromine; (c) iodine
(a) (b) (c)
34
3. Electron Affinity
down the group
Halogen F Cl Br I At
E.A.
kJ/mol1-322 -349 -335 -295 -270
35
The number of electron shells and size of atoms down the group
The nuclear attraction for the additional electron down the group
Electron affinity from Cl to I
36
Atoms of fluorine have the smallest size among the halogens
The addition of an extra electron to the small quantum shell(n=2) results in great repulsion among the electrons.
Fluorine has a lower electron affinity than Cl and Br.
37
Halogen F Cl Br I At
Electronegativit
y4.0 3.0 2.8 2.5 2.2
4. Electronegativity
down the group
38
The number of electron shells and size of atoms down the group
The nuclear attraction for the bonding electrons down the group
Electronegativity down the group
39
Fluorine has the highest electronegativity because it is the most reactive elements.
The electronegativity of fluorine is arbitrarily assigned as 4.0.
40
Variation in Chemical Variation in Chemical PropertiesProperties
Reactivity : F2 > Cl2 > Br2 > I2
React by gaining electrons
Oxidizing power : F2 > Cl2 > Br2 > I2
41
1. Reactions with Sodium
• All halogens
combine directly with sodium to form sodium halides
the reactivity decreases down the group from fluorine to iodine
42
• Fluorine
react explosively to form sodium fluoride
2Na(s) + F2(g) 2NaF(s)
1. Reactions with Sodium
43
• Chlorine
reacts violently to form sodium chloride
2Na(s) + Cl2(g) 2NaCl(s)
1. Reactions with Sodium
44
• Bromine
burns steadily in bromine vapour to form sodium bromide
2Na(s) + Br2(g) 2NaBr(s)
1. Reactions with Sodium
45
• Iodine
burns steadily in iodine vapour to form sodium iodide
2Na(s) + I2(g) 2NaI(s)
1. Reactions with Sodium
46
Na(s) + X221
NaX(s)ofH
Na+(g) + X2(g)21
oatmHovapH
I.E.
Na+(g) + X(g)
21
B.E. Na+(g) + X(g)E.A.
olatticeH
Vigor of reaction depends on
1.The activation energy (endothermic)
2.The lattice energy (exothermic) Activation energy
47
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
F has an exceptionally low B.E. & zero o
vapH
F is the most reactive
(g)
48
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
The lattice enthalpy of NaF is most negative
49
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
Cl has zero ovapH
Cl is more reactive than Br & I
(g)
50
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
Lattice enthalpy :
NaCl > NaBr > NaI
51
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
(s)/(l)
Br is more reactive than I
ovapH : Br2(l) < I2(s)
52
Na(s) + X221
NaX(s)ofH
Na+(g) + X(g)
Na+(g) + X2(g)21
oatmHovapH
I.E.
21
B.E. Na+(g) + X(g)E.A.
olatticeH
Lattice enthalpy :
NaBr > NaI
53
Q.2(a)
Variation: bond enthalpy decreases from Cl2 to I2Reason : The size of atoms and thus the bond
length between atoms increases down the group.
The shared electron pair is getting further away from the bonding
nuclei. weaker bond and lower B.E.
F2 has an exceptionally small B.E. because the F atoms are so small that the repulsive forces between lone pairs on adjacent bonding atoms become significant.
54
Q.2(b)
The lattice enthalpy becomes less negative down the group.
It is because the anionic radius, r- , increases down the group.
rrHolattice
1
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F2 reacts explosively even in the dark at 200C
Cl2 reacts explosively in sunlight
Br2 reacts moderately on heating with a catalyst
I2 reacts slowly and reversibly even on heating
2.1 Reactions with hydrogen
X2 + H2(g) 2HX(g)
56
Q.3 Explain the extreme reactivity of fluorine in terms of the bond enthalpies of F–F and H–F bonds.
Fluorine has an exceptionally small F-F bond enthalpy.
Thus, the activation energy of its reaction with hydrogen is also exceptionally small.
Hydrogen fluoride has the highest bond enthalpy among the hydrogen halides.
Thus, the formation of HF from H2 and F2 is the most exothermic.
The energy released from the reaction further speeds up the reaction.
F2 + H2(g) 2HF(g)
57
Chlorine removes hydrogen completely from turpentine(C10H16)
C10H16(l) + 8Cl2(g) 10C(s) + 16HCl(g)
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Q.4The cotton wool bursts into flames and the gas jar is filled with dark smoke (of carbon) and white fumes (of HCl)
HCl gives dense white fumes with ammonia.
59
2.2 Reactions with phosphorus
F2 + P PF5 Cl2 + P PCl3 + PCl5Br2 + P PBr3
I2 + P PI3
F2 is the strongest oxidizing agent, it always oxidizes other elements to their highest possible oxidation states.
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2.2 Reactions with phosphorus
F2 + P PF5 Cl2 + P PCl3 + PCl5Br2 + P PBr3
I2 + P PI3
Br2 and I2 are NOT strong enough to oxidize P to its highest possible oxidation state.
61
2.3 Reactions with xenon
Fluorine reacts directly with all non-metals except nitrogen, helium, neon and argon. It will even react with diamond and xenon on heating.
C(diamond) + 2F2 CF4
Xe + F2 XeF2
Xe + 2F2 XeF4
Xe + 3F2 XeF6
62
2.3 Reactions with xenon
It is because(a) Xenon can expand its octet by utilizing vacant, low-lying d-orbitals.
63
By VB Theory,
To form two Xe-F bonds in XeF2, a 5p electron in Xe has to be promoted to a 5d orbital.
Xe
5s 5p
F
2s 2p
Xe*
5s 5p
5d
64
By VB Theory,
To form four Xe-F bonds in XeF4, two 5p electrons in Xe have to be promoted to two 5d orbitals.
Xe
5s 5p
5d
Xe**
5s 5p
5d
65
By VB Theory,
To form six Xe-F bonds in XeF6, three 5p electrons in Xe have to be promoted to three 5d orbitals.
Xe
5s 5p
5d
Xe***
5s 5p
5d
66
2.3 Reactions with xenon
The gap between np and nd sub-shells down the group, thus,
Tendency to form bonds down the group : -
Xe > Kr > Ar > Ne > He
the promotion of electrons from np sub-shell to nd sub-shell becomes easier down the group.
67
Xe
5s 5p
5d
Xe***
5s 5p
5d
Also, the energy released by forming more single bonds outweighs the energy required for promoting 5p electrons to 5d orbitals.
68
3 Reactions with other reducing agents
I2 is the weakest oxidizing agents among the halogens.
69
3.1 All halogens(except I2) oxidize Fe2+ to Fe3+
Half reactionStandard electrode
potential (V)
Cl2(aq) + 2e– 2Cl–(aq)
Br2(aq) + 2e – 2Br–
(aq)
Fe3+(aq) + e– Fe2+(aq)
I2(aq) + 2e– 2I–(aq)
+1.36
+1.07
+0.77
+0.54
X2(aq) + 2Fe2+(aq) 2X(aq) + 2Fe3+
(aq)( X = F, Cl, Br) 0Eo
cell
70
3.1 All halogens(except I2) oxidize Fe2+ to Fe3+
Half reactionStandard electrode
potential (V)
Cl2(aq) + 2e– 2Cl–(aq)
Br2(aq) + 2e – 2Br–
(aq)
Fe3+(aq) + e– Fe2+(aq)
I2(aq) + 2e– 2I–(aq)
+1.36
+1.07
+0.77
+0.54
0Eocell I2(aq) + 2Fe2+(aq) No reaction
71
3.2 All halogens(except I2) oxidize S2O32
to SO42
4X2(aq) + S2O32(aq) + 5H2O(l) 8X(aq) + 10H+(aq) +
2SO42(aq)
(X = F, Cl, Br)
I2(aq) + 2S2O32(aq) 2I(aq) + S4O6
2(aq)
S
O
OO-
S
SS
O
O
O-
S
O
OO-
O-+5
+5 +60
0
S
S
OO-
O-+4
0
Used in iodometric titration
72
(i) 2I(aq) + 2Fe3+(aq) I2(aq) + 2Fe2+(aq) (excess) (unknown)
Determination of [Fe3+(aq)] by iodometric titration
Using starch as indicator
(ii) I2(aq) + 2S2O32(aq) 2I(aq) + S4O6
2(aq) (standard solution)
1:1n:n 232
3 OSFe
73
4 Displacement reactionsCl2(aq) + 2Br(aq) 2Cl(aq) +
Br2(aq)Cl2(aq) + 2I(aq) 2Cl(aq) + I2(aq)
Br2(aq) + 2I(aq) 2Br(aq) + I2(aq)
I2(aq) + I(aq) I3(aq)
(yellow) (brown)
More reactive
Less reactive
74
4 Displacement reactions
Cl2(aq) + 2I(aq) 2Cl(aq) + I2(aq)
Br2(aq) + 2I(aq) 2Br(aq) + I2(aq)
I2(aq) + I(aq) I3(aq)
(yellow) (brown)
What would be observed if an excess of Cl2(aq) or Br2(aq) is added to I(aq)? The solution turns cloudy and a black solid settles at the bottom
75
Aqueous
solution
Halogen added
F2 Cl2 Br2 I2
F– No reaction No reactionNo
reaction
No
reaction
Cl–
A pale
yellow
solution is
formed (Cl2
is formed)
No reaction
No
reaction
No
reaction
Reactions of halide ions with halogens
76
Aqueous
solution
Halogen added
F2 Cl2 Br2 I2
Br–
A yellow
solution
is formed
(Br2 is
formed)
A yellow
solution
is formed
(Br2 is formed)
No reactionNo
reaction
I–
A yellowish
brown
solution is
formed
(I3 is formed)
A yellowish
brown
solution is
formed
(I3 is formed)
A yellowish
brown
solution is
formed (I3
is formed)
No
reaction
Reactions of halide ions with halogens
77
Q.5Shake hexane or 1,1,1-trichloroethane with the two solutions respectively.
The one that turns the organic layer violet is I3
(aq).
The one that turns the organic layer orange or brown is Br2(aq).
78
1,1,1-trichloroethane
Br2 I2
Br2(aq) I3(aq)
If hexane is used, the upper layer will be the organic layer
79
Disproportionation is a chemical change in which oxidation and reduction of the same species (which may be a molecule, atom or ion) take place at the same time.
5. Disproportionation
80
A. Reactions with Water
HOCl : chloric(I) acid or hypochlorous acid
Chlorine water
a mixture of hydrochloric acid and chloric(I) acid
81
• Chlorate(I) ion, OCl is also known as hypochlorite ion
unstable
decomposes when exposed to sunlight or high
temperatures to give chloride ions and oxygen
2OCl–(aq) 2Cl–(aq) + O2(g)
A. Reactions with Water
82
• Chlorate(I) ion
bleaches by oxidation
A. Reactions with Water
Cl2(aq) + H2O(l)2H+(aq) + Cl–(aq) +
OCl–(aq)OCl–(aq) + dye Cl–(aq) + (dye + O)
coloured colourless
83
• Bromine
only slightly soluble in water
mainly exists as molecules in saturated bromine
water
A. Reactions with Water
84
• When the solution is diluted
hydrolysis takes place
hydrobromic acid and bromic(I) acid (hydrobromous acid) are formed
Br2(l) + H2O(l) HBr(aq) +
HOBr(aq)
A. Reactions with Water
85
• Bromate(I) ion, OBr
also unstable
bleaches dyes by oxidation
OBr–(aq) + dye coloured
Br–(aq) + (dye + O)
colourless
A. Reactions with Water
86
• Iodine
does not react with water
only slightly soluble in water
A. Reactions with Water
87
• Fluorine
reacts vigorously with water to form hydrogen fluoride and oxygen
A. Reactions with Water
Being the strongest oxidizing agent, F2 undergoes reduction rather than disproportionation with water.
2F2(g) + 2H2O(l) 4HF(aq) + O2(g)0 1
88
Chlorine reacts similarly at high temperature or when exposed to light
2Cl2(aq) + 2H2O(l) 2HCl(aq) + 2HOCl(aq)2HOCl(aq) 2HCl(aq) + O2(g)
Heat or light
Overall :2Cl2(aq) + 2H2O(l) 4HCl(aq) + O2(g)
Heat or light
A. Reactions with Water
89
• All halogens react with aqueous alkalis
• All halogens (except F2) undergoes disproportionation with alkalis• In general,
Reactivity decreases down the group
B. Reactions with Alkalis
90
The products formed depend on
1. Temperature
2. The type of halogen reacted
3. The concentration of alkali used
B. Reactions with Alkalis
91
Effect of temperature
(a)At lower temperatures,
X2 Cl2 Br2 I2
T1 / C 20 0 <0
X2(aq) + 2OH(aq) XO(aq) + X(aq) + H2O(l)
T10 +1 1
B. Reactions with Alkalis
92
Effect of temperature
(a)At higher temperatures,
3XO(aq) XO3(aq) + 2X(aq)
T2
XO ClO BrO IO
T2 / C 70 20 0
+1 +5 1
B. Reactions with Alkalis
93
(2) 3XO(aq) XO3(aq) + 2X(aq)
T2
(1) X2(aq) + 2OH(aq) XO(aq) + X(aq) + H2O(l)
T1
Overall reaction : 3(1) + (2)3X2(aq) + 6OH(aq) XO3
(aq) + 5X(aq) + 3H2O(l)
T2
B. Reactions with Alkalis
94
3XO(aq) XO3(aq) + 2X(aq)
T2
XO ClO BrO IO
T2 / C 70 20 <0
On moving down the group,
1. stability of XO decreases ClO > BrO > IO
2. stability of XO3 increases ClO3
< BrO3 <
IO3
B. Reactions with Alkalis
95
3X2(aq) + 6OH(aq) XO3(aq) + 5X(aq)
+ 3H2O(l)At lower pH (when acid is added),
the equilibrium position shifts to the left and the reversed process predominates.
XO3(aq) + 5X(aq) + 6H+(aq) 3X2(aq) +
3H2O(l)This reaction (when X=I) is often used to prepare standard iodine solution for iodometric titrations
B. Reactions with Alkalis
96
• Dissolving a known quantity of KIO3(s) in excess KI(aq) and dilute H2SO4 generates a known amount of I2(aq)
KIO3(aq) + 5KI(aq) + 6H+(aq) 3I2(aq) + 3H2O(l) +
6K+(aq)
• The iodine produced can be used to standardize thiosulphate solution
3I2(aq) + 6S2O32(aq)
6I(aq) + 3S4O62(aq)
B. Reactions with Alkalis
97
• This known amount of iodine generated can also be used to oxidize reducing agents (of unknown concentrations) such as SO3
2(aq) and ascorbic acid (vitamin C)• The excess iodine can be determined by back titration with sodium thiosulphate solution
I2(aq) + 2S2O32–(aq)
2I–(aq) + S4O62–
(aq)
B. Reactions with Alkalis
98
Effect of concentration of alkali
(a) At higher concentrations, XO3(aq)
is the major product.
(b) At lower concentrations, XO(aq) is the major product.
B. Reactions with Alkalis
99
B. Reactions with Alkalis
In general,
Halogens react with cold, dilute alkali to give halate(I) ions, halide ions and water
X2(aq) + 2OH XO(aq) + X(aq) + H2O(l)
Halogens react with hot, concentrated alkali to give halate(V) ions, halide ions and water.
3X2(aq) + 6OH XO3(aq) + 5X(aq) +
3H2O(l)
100
2F2 + 2OH(aq) OF2(aq) + 2F(aq) + H2O(l)very
dilute
20C
2F2 + 4OH(aq) O2(aq) + 4F(aq) + 2H2O(l)concentrat
ed
70C
0
0 1
1
12
2
+2
0
Being the strongest oxidizing agent, F2 undergoes reduction rather than disproportionation with alkalis.
B. Reactions with Alkalis
101
Variation in chemical properties of Variation in chemical properties of halideshalides
A Comparative studyA Comparative study
1. Reactions with conc. sulphuric acid
2. Reactions with conc. phosphoric acid
3. Reactions with silver ion
102
• Concentrated sulphuric acid
non-volatile (b.p. ~330C)
oxidizing
Reactions with ConcentratedReactions with ConcentratedSulphuric(VI) AcidSulphuric(VI) Acid
103
KF(s) + H2SO4(l) KHSO4(s) + HF(g)
KCl(s) + H2SO4(l) KHSO4(s) + HCl(g)
warmwarm
non-volatile
volatile
Warming is required to speed up the reaction and to drive out the volatile acids
Fluoride and chloride : -
104
KF(s) + H2SO4(l) KHSO4(s) + HF(g)
KCl(s) + H2SO4(l) KHSO4(s) + HCl(g)
warmwarm
acid salt
Acid salt rather than normal salt is formed because HSO4
is a relatively weak acid
A convenient way to prepare HCl in the laboratory
105
KF(s) + H2SO4(l) KHSO4(s) + HF(g)
KCl(s) + H2SO4(l) KHSO4(s) + HCl(g)
warmwarm
Observation : -
White fumes are produced
Confirmatory test : -
Dense white fumes appear with NH3(aq)
106
Bromide: -
-1 0
oxidation
+6 +4
reduction
KBr(s) + H2SO4(l) KHSO4(s) + HBr(g)warm
2HBr(g) + H2SO4(l) SO2(g) + Br2(g) + 2H2O(l)
warm
107
(1) KBr(s) + H2SO4(l) KHSO4(s) + HBr(g)
(2) 2HBr(g) + H2SO4(l) SO2(g) + Br2(g) + 2H2O(l)
Bromide: -
Overall reaction : 2(1) + (2)
Not suitable for preparing HBr
warm
warm
2KBr(s) + 3H2SO4(l)
2KHSO4(s) + SO2(g) + Br2(g) + 2H2O(l)warm
108
• A brown gas
is evolved on
warming
• A pungent
smell is
detected
• A brown colour is
observed when
adding hexane
Br2
• It turns orange
dichromate solution
green
SO2
Confirmatory Test
• Dense white fumes
are formed with
aqueous ammonia
HBr• White fumes
are formed
Br–
Produ
ctObservation
Halid
e
2KBr(s) + 3H2SO4(l) 2KHSO4(s) + SO2(g) + Br2(g) + 2H2O(l)
109
iodide: -
KI(s) + H2SO4(l) KHSO4(s) + HI(g)warm
2HI(g) + H2SO4(l) SO2(g) + I2(g) + 2H2O(l)-1 0+6 +4
warm
8HI(g) + H2SO4(l) H2S(g) + 4I2(g) + 2H2O(l)-1 0+6 -2
warm
HI is strong enough to reduce sulphur to its lowest possible oxidation state
110
KI(s) + H2SO4(l) KHSO4(s) + HI(g) (1)warm
2HI(g) + H2SO4(l) SO2(g) + I2(s) + 2H2O(l) (2)
warm
8HI(g) + H2SO4(l) H2S(g) + 4I2(s) + 2H2O(l) (3)
warm
Overall reaction = 10(1) + (2) + (3)10KI(s) + 12H2SO4(l)
10KHSO4(s) + SO2(g) + H2S(g) + 5I2(s) + 4H2O(l) No suitable for preparing HI
111
10KI(s) + 12H2SO4(l)
10KHSO4(s) + SO2(g) + H2S(s) + 5I2(s) + 4H2O(l) Observation : -
A bad egg smell is detected
Confirmatory test : -
It turns lead(II) ethanoate paper black
(CH3COO)2Pb + H2S PbS(s) + 2CH3COOH
112
10KI(s) + 12H2SO4(l)
10KHSO4(s) + SO2(g) + H2S(s) + 5I2(s) + 4H2O(l) Observation : -
Confirmatory test : -
A violet colour is observed when added to hexane
Violet fumes are formed and
condense when cooled to give a black solid
113
Conclusion : -
Reducing power : HI > HBr > HCl > HF
Increases down the group
114
Interpretation:-
Consider the reaction,
2H–X + H2SO4 X–X + SO2 + 2H2O
The feasibility of the reaction depends on
1. the strength of H–X bond to be broken
the stronger the bond, the less feasible is the rx
2. the strength of X–X bond to be formed
the stronger the bond, the more feasible is the rx
115
2H–X + H2SO4 X–X + SO2 + 2H2O
The feasibility of the reaction depends on
1. the strength of H–X bond
the stronger the bond, the less feasible is the rx
2. the strength of X–X bond
the stronger the bond, the more feasible is the rxThe reaction with HF is least feasible because
1. H-F bond is the strongest
2. F-F bond is exceptionally weak due to repulsion between lone pairs of bonding atoms.
116
H-X B.E.(kJ mol1) X-X B.E. (kJ mol1
H-Cl 432 Cl-Cl 244
H-Br 366 Br-Br 192
H-I 298 I-I 152
On moving down the group,
both H–X bonds and X–X bonds become weaker
117
2H–X + H2SO4 X–X + SO2 + 2H2O
The strength of H-X bond is more important
Since two H-X bonds have to be broken for each X-X bond formed.
Reactivity : H-Cl < H-Br < H-I
118
Reactions with Phosphoric Acid Reactions with Phosphoric Acid
non-volatile
volatile
H3PO4(l) + HX(g) no reactionless
oxidizingSuitable for preparing HX from solid halids
NaCl(s) + H3PO4(l) NaH2PO4(s) + HCl(g)
NaBr(s) + H3PO4(l) NaH2PO4(s) + HBr(g)
NaI(s) + H3PO4(l) NaH2PO4(s) + HI(g)
warm
warm
warm
119
Halid
e ionObservation
Produ
ct
Confirmatory test
of
the product
Cl– White fumes are
formed on
warming
HCl Dense white fumes
are formed with
aqueous ammoniaBr– HBr
I– HI
NaCl(s) + H3PO4(l) NaH2PO4(s) + HCl(g)
NaBr(s) + H3PO4(l) NaH2PO4(s) + HBr(g)
NaI(s) + H3PO4(l) NaH2PO4(s) + HI(g)
warm
warm
warm
120
Reactions with Silver Ions Reactions with Silver Ions
• Aqueous solutions of chlorides, bromides and iodides
give precipitates when reactingwith acidified silver nitrate
solution
121
Reactions with Silver Ions Reactions with Silver Ions
Ag+(aq) + Cl–(aq) AgCl(s) white ppt
Ag+(aq) + Br–(aq) AgBr(s) pale yellow ppt
Ag+(aq) + I–(aq) AgI(s) yellow ppt
122
AgI(s)AgBr(s)AgCl(s)
Colour intensity down the group
123
Reactions with Silver Ions Reactions with Silver Ions
Silver nitrate solution should be acidified with nitric acid
(a) to remove interfering ions like
SO32 or CO3
2
They may form white ppt with Ag+
124
Reactions with Silver Ions Reactions with Silver Ions
2H+(aq) + SO32–(aq) SO2(g) + H2O(l)
2H+(aq) + CO32–(aq) CO2(g) +
H2O(l)
125
Silver nitrate solution should be acidified with nitric acid
(b)to avoid the formation of black ppt of Ag2O in alkaline solution.
2Ag+(aq) + 2OH(aq) Ag2O(s) + H2O(l)
126
The solubility(in water) of AgX down the group
AgF >> AgCl > AgBr > AgI
Ksp/mol2 dm6 1.61010 7.71013 1.51016
soluble insoluble
127
Q.7
On moving down the group,
the size of the halide anions The electron cloud of the anions becomes more easily polarized by Ag+ The halides become more covalent and
less ionic
The halides become less soluble in polar solvents like water
128
Reactions with Silver Ions Reactions with Silver Ions
The reaction can be used as a test to show the presence of halide ions.
Different halides give ppt with different colours.
Sometimes ambiguous.
Confirmatory tests are needed.
129
Two confirmatory tests for halides
1.Adding NH3(aq) to the AgX ppt
2.Exposing AgX ppt to sunlight
130
AgX(s) dissolve in NH3(aq) due to the formation of soluble complex ions.
AgCl(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Cl(aq)
AgBr(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Br(aq)
AgI(s) + 2NH3(aq) No reactionSolubility in NH3(aq) down the group
131
• When exposed to sunlight
light2AgCl(s) 2Ag(s) + Cl2(g)
2AgBr(s) 2Ag(s) + Br2(l)light
2AgI(s) No reactionlight
silver bromide turns yellowish grey
silver iodide remains yellow
silver chloride turns grey
132
Ion
Action of
acidified
AgNO3
solution on
halides
Confirmatory test of the
product
Effect of
adding
aqueous
ammonia
Effect of
exposure
to sunlight
Cl–A white ppt is
formed
The white ppt
dissolves
The solution
turns grey
Br–
A pale yellow
ppt is formed
The pale yellow
ppt slightly
dissolves
The solution
turns
yellowish grey
I–A yellow ppt is
formed
The yellow ppt
does not
dissolve
The solution
remains yellow
Action of acidified silver nitrate solution on halides
133
Anomalous Behaviour of Anomalous Behaviour of Hydrogen FluorideHydrogen Fluoride
1. Hydrogen fluoride has abnormally high boiling point and melting point amongthe hydrogen halides HX HF HCl HBr HI
b.p./C 19.5 85 66.4 35
134
Formation of the extensive intermolecular hydrogen bonds among hydrogen fluoride
molecules
• Molecules of all other hydrogen halides
held together by weak van der Waal’s forces only
135
• The acid strength of hydrogen halides decreases in the order:
HI > HBr > HCl >> HF
2.2. Acidic Properties of Hydrogen Acidic Properties of Hydrogen HalidesHalides
136
Hydrogen
halide
Acid
dissociation
constant,
Ka (mol dm–3)
Degree of
dissociation in 0.1
M solution (%)
Acid
strength
HF
HCl
HBr
HI
5.6 × 10–4
1 × 107
1 × 109
1 × 1011
8.5
92
93
95
Low
Strong
Strong
Very strong
Acid dissociation constants of hydrogen halides and their degrees of dissociation in 0.1 M solutions
137
In dilute (e.g. 0.1M) solution,
HF is the weakest acid among all the hydrohalic acids
HF(l) + H2O(l) H3O+(aq) + F–(aq)
Ka = 5.6 × 10–4 mol dm–3
138
H
O
H
H + F O H
H
H
F
H-bond
or H3O + F H3OF
Very stable ion pair
Freedom of F & H3O+ greatly (a drop in entropy of the system) due to H-bond formation
Effective concentration of F & H3O+ greatly
Thus, Ka & pH
139
In concentrated solution,
HF is the strongest acid among all the hydrohalic acids
140
H FF O H
H
H
F>
Strength of H-bond:-
2. HF is in excess in concentrated solution F ions combine with excess HF rather than with H3O+
free H3O+ & pH
excess
H3OF(aq) + HF(aq) H3O+(aq) + HF2
(aq)
141
For other HX acids,
acidity as concentration
It is due to the significant interaction between X and H3O+ at high concentrations
the effective concentration of H3O+
For HF, interaction between F and H3O+ is significant even at low concentrations due to the smaller size of F.
142
3. Pure, anhydrous liquid HF is ionic due to the formation of HF2
and H2F+ ions
Self ionization : -
2HF(l) H2F+(l) + F(l)
HF(l) + F(l) HF2(l)
Overall : -
3HF(l) [H2F]+[HF2](l)
143
Stabilized by resonance
Two identical H – F bonds
F H F FHF
144
• Heating the solid potassium hydrogen difluoride
reverses the reaction
a convenient way to obtain anhydrous hydrogen
fluoride
KF(s) + HF(l) KHF2(s)heat
145
Uses of fluorine and its compoundsSodium hexafluorosilicate, Na2SiF6, is used in water fluoridation.
F, being isoelectronic to OH, can replace the OH in the tooth enamel, making it less soluble in acidic solutions.
146
Uses of fluorine and its compoundsMolten cryolite, Na3AlF6
Lowers the temperature (2517C 1000 C) needed for extracting Al from Al2O3 by electrolysis.
147
Uses of fluorine and its compoundsConvert U to UF6
Separate 235UF6 from 238UF6 by diffusion for use in nuclear reactors.
The heavier 238UF6 diffuses a bit slower, making the separation possible.
148
Uses of fluorine and its compoundsConc. HF(aq) is used in etching glass
(e.g. making scales/graduation marks on glassware)
CaSiO3(s) + 6HF(aq)
CaF2(aq) + SiF4(aq) + 3H2O(l)(Glass)
149
Uses of fluorine and its compounds• The glass object to be etched
coated with wax or a similar acid-proof material
cutting through the wax layer to expose the glass
apply hydrofluoric acid
150
Uses of fluorine and its compounds
A glass is etched by hydrofluoric acid
151
Uses of fluorine and its compoundsMaking fluorocarbon compounds
Used as refrigerants, aerosol propellants, anaesthetics and
fire-fighting agents(BTM, BCF)
PTFE (teflon) used in electrical insulation, coating on surface of non-stick saucepans, etc.
152
Uses of fluorine and its compoundsHydrazine/fluorine mixtures are excellent rocket fuels
N2H4(g) + 2F2(g) N2(g) + 4HF(g)
H = -1166 kJ mol1 (extremely exothermic)Due to the strong NN and H-F bonds
153
Uses of fluorine and its compoundsExtraction of fluorine
Electrolyte : KF(s) dissolved in pure HF(l)
Anode : graphite
Cathode : steel
154
Q.8(a)
Anode : 2HF2 2HF + F2 + 2e
Cathode : 2H2F+ + 2e 2HF + H2
Overall :
2HF2 + 2H2F+ 4HF + F2 + H2
155
8.(b) Overall :
2HF2 + 2H2F+ 4HF + F2 + H2
6HF 4HF + F2 + H2
2HF F2 + H2
KF is added to increase the conductivity of the electrolyte.
KHF2 > HF or [H2F][HF2]
156
Q.8(c)
OH- (from H2O) rather than HF2 is
oxidized at the anode
2F2(g) + 2H2O(l) 4HF(aq) + O2(g)
vigorous reaction
Also, F2 reacts vigorously with water.
157
8.(d)
At high temperatures, fluorine produced can react vigorously with the electrodes, air, etc.
158
Uses of Chlorine and its compounds
Polyvinyl chloride, PVC
making electrical insulation, bottles, floor tiles, table cloth, shower curtain, etc.
159
CH2=CH2 + Cl2 CH2Cl – CH2Cl
CH2Cl – CH2Cl CH2=CHCl + HClheat
CH2
CH
Cl
n
n(CH2=CHCl)
160
Making chlorine bleach
Cl2(g) + 2NaOH(aq) NaCl(aq) + NaOCl + H2O(l)
Disinfectant in sterilizing water and sewage treatment.
Extraction of bromine from sea water
Cl2(g) + 2Br(aq) 2Cl(aq) + Br2(aq)
161
Uses of Bromine and its compounds
Manufacture of 1,2-dibromoethane to remove Pb from petrol engine
Pb(C2H5)4, TEL : anti-knock agent added to petrol engine to prevent premature ignition.
TEL decomposes to give Pb that may cause damage to the engine
CH2Br-CH2Br + Pb(C2H5)4 PbBr2volatile and emitted to air easily
Air pollutant
162
AgBr is used in black-and-white photography
exposure to light
2AgBr(s) 2Ag(s) + Br2(l)coated on film
black
The excess AgBr(s) is removed as soluble complex ion.
AgBr(s) + 2S2O32(aq) [Ag(S2O3)2]3(aq) +
Br(aq) hypo
163
Uses of Iodine and its compounds
Making iodine tincture (antiseptic)
I2 in alcohol or KI(aq)
Radioactive iodine-131 as tracer in medical diagnosis
Iodide is used to make iodized table salt for preventing development of goitre.
164
Laboratory preparation of halogens(except F2)
conc. H2SO4
MnO2 +
NaCl
165
2NaCl + MnO2 + 2H2SO4 Na2SO4 + MnSO4 +
2H2O + Cl2
conc. H2SO4
MnO2 +
NaCl
-1
0
+4
+2
Free from HCl and H2O
NaCl + H2SO4 HCl + NaHSO4
To remove HCl
To dry Cl2
166
Laboratory preparation of halogens(except F2)
conc. HCl
MnO2
167
Laboratory preparation of halogens(except F2)
conc. HCl
MnO4
168
The END