This article is on the P Block Class 12 Notes of Chemistry. The notes on P Block of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the P Block chapter of Class 12 for the students so that they can give a quick glance of the chapter.
This chapter of Class 12 has been divided into four articles. This article (Part 3) includes the Group 17 elements. The second article (Part 1) is on Group 15 elements. The third article (Part 2) is on Group 16 elements. At last, the fourth article (Part 4) is on Group 18 elements.
P block Elements Part 3
Group 17 Elements
F, Cl, Br, I and At (astatine) are the elements of group 17. These are collectively known as halogens. Astatine is a radioactive element. Their general valence shell configuration is ns2np5.
Fluorine and chlorine are fairly abundant while bromine and iodine are less. Fluorine is present as insoluble fluorides (fluorspar CaF2, cryolite Na3AlF6 and fluoroapatite 3 Ca3 (PO4)2 . CaF2). It is present in small quantities in the soil, river water plants and bones and nth of animals.
Seawater contains chlorides, bromides, and iodides of sodium, potassium, magnesium and but is mainly sodium chloride solution (2.5% by mass). The deposits of dried up seas and beds of inland lakes contain sodium chloride and carnallite, KCl. MgCl2 6H2O and calcium chloride CaCl2.
Bromides occur in sea water and salt lakes as bromides of alkali and alkaline earth metals, NaBr, KBr, MgBr2, etc.
Iodide occurs in certain forms of marine life in their systems. Iodine occurs, in various seaweeds (up to 0.5% by mass) and in Chile saltpetre (up to 0.2% as sodium iodate (NaIO3).
Halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear charge. Atomic and ionic radii increase from fluorine to iodine.
Halogens have high ionisation enthalpy because they have very little tendency to lose an electron. On moving down the group, ionisation enthalpy decreases due to the increase in atomic size.
Electron Gain Enthalpy:
Halogens have maximum negative electron gain enthalpy in the corresponding periods because have only one electron less than stable noble gas configurations. On moving down the group, it becomes less negative. However, negative electron gain enthalpy of fluorine is less than that of chlorine. It is due to the small size of the fluorine atom. As a result of this, there are strong interelectronic repulsions and the incoming electron (in 2p orbital) does not experience much attraction.
Halogens are coloured due to the absorption of radiations in visible region which results in excitation of valence shell electrons to higher energy level. Smaller dissociation enthalpy of F2 in comparison to Cl2 shows that there is relatively large electron-electron repulsion between the lone pairs in F2 molecule due to its small size.
Halogens exhibit -1 oxidation state. However, Cl, Br, I exhibit +1, +3, +5 and +7 oxidation also. F exhibits only -1 oxidation state because it has no vacant d-orbitals in its valence shell.
Halogens are the strong oxidising agent, it is due to ready acceptance of an electron. F2 is the strongest oxidising halogen and it oxidises other halide ions in solution or even in the solid phase.
Halogens form HX type hydrogen halides with hydrogen. Thermal stability and bond dissociation enthalpy decreases down the group from HF to HI but acidic strength increases from HF to HI. Their boiling points also increase from HCl to HI. The high boiling point of HF is due to the intermolecular H-bonding.
OF2 and O2F2 both are strong fluorinating agents. O2F2 is used in removing Pu as PuF6 from spent nuclear fuel. Cl, Br and I form oxides in which halogens exist as +1 to +7 states.
Chemical properties of group 17 elements are as follows
Oxidation States and Trends in Chemical Reactivity:
All the halogens show —1 oxidation state. Other halogens excluding fluorine show + 1, + 3, +5 and +7 oxidation states because of gradual promotion of np-electrons to nd-electrons.
The fluorine atom has no d-orbital in its valence shell and therefore cannot expand its octet and it shows only —1 oxidation state. The higher oxidation states of chlorine, bromine, and iodine are present in interhalogens (when the halogens are in combination with the small and highly electronegative F and O atoms), oxides and oxoacids.
Compounds in lower oxidation state are ionic whereas in higher oxidation states are covalent. All halogens are strong oxidising agents. F2 is the strongest oxidising agent and it oxidises other halide ions in solution or even in solid state. A halogen oxidises halide ions of higher atomic number.
Fluorine oxidises all the halides.
F2 + 2X– —> 2F– + X2 [X = CI, Br, I]
Chlorine oxidises only bromide and iodide.
Cl2 +2X– —> 2Cl– + X2 [X =Br, I]
Bromine oxidises only iodide.
Br2 +2I– —->2Br– + I2
The oxidizing ability of halogens decreases from fluorine to iodine.
F2 > O2 > Br2 > I2
The decreasing oxidizing ability of halogens in the aqueous solution down the group can be illustrated by their E° values
Which are dependent on the parameters indicated below
½ X2 (g) —½∆diss Ho—> X (g) —∆egHo –> X– (g) —∆hydHo –> X– (aq)
Fluorine oxidizes water to oxygen.
2F2 (g) + 2H2O(l) —-> 4HF (aq) +O2 (g)
Chlorine and bromine undergo disproportionation reaction with water and gives hvdrohalic and hypohalous acids.
X2 (g)+ H20 (/) —> HX(ag)+HOX [X =C1 or Br]
The reaction of iodine with water is non-spontaneous. Iodine can be oxidised by oxygen in the acidic medium.
4I– (aq) + 4H+ (g) + O2 (aq) —-> 2I2 (s)+2H2O (l)
It is a greenish yellow gas with a pungent and suffocating odour. It is manufactured by Deacon’s process or by the electrolysis of brine (liberated at the anode). It is a powerful bleaching agent and bleaching action is due to oxidation (in the presence of moisture). It is used in sterilising drinking water and in the preparation of poisonous gases such as phosgene (COCl2), tear gas (CCl3NO2), mustard gas (ClCH2CH2SCH2CH2Cl), etc.
MnO2 + 4HCl —> MnCl2 + Cl2 + 2H2O
2KMnO4 +16HCl—> 2KCl+ 2MnCl2 + 8H2O + 5Cl2
Deacon’s process 4HCl + O2 —CuCl2–> 2Cl2 + 2H2O
2Fe+ 3Cl2 —> 2FeCl3
P4 + 6Cl2 –> 4PCl3
S8 + 4O2 –> 4S2Cl2
H2 + Cl2 —> 2HCl
H2S + Cl2 —> 2HCl+S
8NH3 (excess) + 3Cl2 —> 6NH4Cl + N2
NH3 + 3Cl2 (excess) —> NCl3 + 3HCl
2NaOH + Cl2 —> NaCl + NaOCl +H2O
6NaOH + 3Cl2 —-> 5NaCl + NaClO3 +3H2O
Uses of Chlorine:
It is used in the manufacture of dyes, drugs and some organic compounds like DDT, carbon tetrachloride refrigerants, etc and for bleaching wood pulp, cotton and textiles.
It is used in sterilising drinking water, in the extraction of gold and platinum and in the preparation of poisonous gases such as phosgene, tear gas (CC13NO2) and mustard gas (ClCH2CH2SCH2CH2Cl).
Compounds of Halogens:
Hydrogen Chloride (HCl):
Bleaching action Coloured substance + [O] —> Colourless substance
An aqueous solution of hydrogen chloride is hydrochloric acid. A mixture of conc. HC1 and conc HNO3 (3:1 ratio) is known as aqua-regia which is used to dissolve noble metals such as Au, Pt, etc. When conc. HCl reacts with finely powdered iron, ferrous chloride is formed. Liberation of H2 prevents the formation of ferric chloride.
It is used in the manufacture of chlorine, NH4Cl and glucose (from corn starch), and in medicine and as a laboratory reagent. It is also used for extracting glue from bones and purifying bone black.
Oxacids and Halogens:
Fluorine forms only one oxoacid, i.e., HOF due to high electronegativity and small size of F. The other halogens form several oxoacids such as HOX, HOXO, HOXO2, HOXO3.
(i) Order of strength of oxoacids of the same halogen is HClO4 > HClO3 > HClO2 > HOCl
(ii) Oxidising power of oxoacids of chlorine is HClO4 < HClO3 < HClO2 < HClO
Halogens combine amongst themselves to form a number of compounds known as interhalogen compounds of the types XX’, XX’3, XX’ 5 and XX’7 (X = larger size halogen and X’ is smaller size halogen). These are more reactive than halogens except fluorine. On the basis of VSEPR theory, XX’3 compounds have the bent T-shape, XX’5 compounds are square pyramidal and XX’7 has pentagonal bipyramidal structure.
This article has tried to highlight all the important points of P Block Elements in the form of notes for class 12 students in order to understand the basic concepts of the chapter. The notes on P Block Elements have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.
Check Part 1 of the Chapter Here: Group 15 Elements – p block elements
Check Part 2 of the Chapter Here: Group 16 Elements – p block elements
Check Part 4 of the Chapter Here: Group 18 Elements – p block elements