GROUP VIIA & GROUP VIIIA ELEMENTS
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) The halogen with the highest electronegativity is __________. (Fluorine)
(ii) HI is _________ agent. (reducing agent)
(iii) The only halogen acid which is not a strong acid is ________. (HF)
(iv) Tincture of _______ is a common antiseptic. (iodine)
(v) The halogen with the lowest melting and boiling points is __________. (Fluorine)
(vi) Bleaching powder is prepared from ______ and ______. (Ca(OH)2 / Cl2)
(vii) ________ is the halogen which causes burns on the skin which heal slowly. (Bromine)
(viii) ________ is used for making unshrinkable wool. (Bleaching powder)
(ix) A mixture of ________ and _______ is used for breathing by the sea divers. (80%He / 20% O2)
(x) Noble gas used to fill fluorescent tubes is _______. (Argon/Krypton)
Q.02: Indicate True or False:
(i) HF is used for etching glass. (TRUE)
(ii) HI is weaker reducing agent as compared to HF. (FALSE)
CORRECT: HI is stronger reducing agent as compared to HF.
(iii) Bleaching powder is completely soluble in water. (TRUE)
(iv) The formula of perchloric acid is HCIO2. (FALSE)
CORRECT: The formula of perchloric acid is HCIO3.
(v) On warming, aqueous KOCl disproportionate as follows: 3KOCl
(vi) a-particles emitted by radioactive elements are ions of radon. (FALSE)
CORRECT: a-particles emitted by radioactive elements are ions of Helium.
(vii) Radon is the only one of the noble gases that is radioactive. (TRUE)
(viii) The molecules of the noble gases are all monoatomic. (TRUE)
(ix) Argon is used to fill electric bulbs. (TRUE)
(x) The noble gas which is present in the largest amount in atmosphere is krypton. (FALSE)
CORRECT: The noble gas which is present in the largest amount in atmosphere is Argon.
Q.03: Multiple choice questions.
(i) Which of the following hydrogen halides is the weakest acid in solution?
(a) HF
(b) HBr
(c) HI
(d) HCl
ANWSER: (a) HF
EXPLANATION: HF is the weakest of all halogen acids because of strong hydrogen bonding among its molecules which decreases the proton (H+) donating capacity of the acid.
(ii) Chlorine heptaoxide (Cl2O7) reacts with water to form:
(a) Hypochlorous acid
(b) Chloric acid
(c) Perchloric acid
(d) Chlorine & oxygen
ANWSER: (c) Perchloric acid
EXPLANATION: Chlorine heptaoxide is the anhydride of perchloric acid. Thus, it can react with water to form perchloric acid again as: Cl2O7 + H2O ® 2HClO4
(iii) Hydrogen bond is the strongest between the molecules of:
(a) HF
(b) HCl
(c) HBr
(d) HI
ANWSER: (a) HF
EXPLANATION: Fluorine is the smallest and the most electronegative element of all halogens. Its E.N. difference is maximum with hydrogen. So, the bond between H—F is the strongest among all other halogen halides.
(iv) Which halogen will react spontaneously with Au(s) to produce Au3+?
(a) Br2
(b) F2
(c) I2
(d) Cl2
ANWSER: (d) Cl2
EXPLANATION: [Cl] produced by aqua regia can dissolve Au and other noble metals. Thus, Au is converted into Au3+ in the form of its chloride.
(v) The anhydride of HClO4 is:
(a) ClO3
(b) ClO2
(c) Cl2O5
(d) Cl2O7
ANWSER: (d) Cl2O7
EXPLANATION: The removal of one H2O from two HClO4 molecules, produces one molecule of chlorine heptoxide (Cl2O7). Thus, it is the anhydride of perchloric acid.
(vi) Bleaching powder may be produced by passing chlorine over?
(a) Calcium Carbonate
(b) Hydrated calcium sulphate
(c) Anhydrous calcium sulphate
(d) Calcium hydroxide
(e) Magnesium hydroxide
ANWSER: (d) Calcium hydroxide
EXPLANATION: Bleaching powder can be produced by the action of chlorine on dry slaked lime as: Ca(OH)2 + Cl2 ® Ca(OCl)Cl + H2O
(vii) Which is the strongest acid?
(a) HClO
(b) HClO2
(c) HClO3
(d) HClO4
ANWSER: (d) HClO4
EXPLANATION: HClO4 is the strongest of all oxyacids of chlorine because it has greater number of oxygen atoms, out of which three are coordinated to the central Cl atom, and their strong polarizing effect make the terminal H-atom highly electropositive which can easily be ionized and donated.
(viii) Which halogen occurs naturally in positive oxidation state?
(a) Fluorine
(b) Bromine
(c) Chlorine
(d) Iodine
ANWSER: (d) Iodine
EXPLANATION: Iodine is the least electronegative halogen, so has weaker hold on its valence electrons. That’s why, it exists in positive oxidation state when bonded with highly electronegative element like oxygen etc.
(ix) An element that has a high I.E. and tends to be chemically inactive would most likely to be:
(a) An alkali metal
(b) A transition element
(c) A noble gas
(d) A halogen
ANWSER: (c) A noble gas
EXPLANATION: This is because noble gases are present in the extreme right of the periodic table, with maximum nuclear charge and minimum atomic size. They have completely filled valence shells or complete octet, so they have the highest I.E. values as well as are chemically inactive.
(x) Which of the followings represents the correct electronic configuration of the outermost energy level of an element of zero (VIIIA) group in the ground state?
(a) s2p2
(b) s2p4
(c) s2p5
(d) s2p6
ANWSER: (d) s2p6
EXPLANATION: Noble gases have complete octet. It means they have completely filled valence s & p orbitals. Therefore, their valence shell electronic configuration should be ns2, np6.
Q.04: What is bleaching powder? How it is prepared commercially? Give its uses.
Bleaching Powder: “Calcium chlorohypochlorite or calcium oxychloride (CaOCl2) is commonly called bleaching powder.” It is a yellowish white powder with strong smell of chlorine. It is prepared by the action of chlorine on dry slaked lime.
Ca(OH)2 + Cl2 ⟶ Ca(OCl)Cl + H2O
Q.05: (a) Discuss the oxides of chlorine.
ANSWER: Consult the text book at page 84 — 85.
(b) What are disproportionation reactions? Explain your answer with an example.
ANSWER:
Disproportionation Reactions: “A reaction in which a species (molecule, atom or ion) is simultaneously oxidized and reduced is called a disproportionation reaction.” The reactions of chlorine with cold and hot NaOH solutions are examples of such reactions. e.g.,
2NaOH(aq) + Cl2(g) ⟶ NaCl(aq) + NaClO(aq) + H2O(aq)
Zero oxidation state of chlorine in Cl2 has changed to -1 in chloride and +1 in hypochlorite ion. So, Cl2 is reduced as well as oxidized.
Q.06: Discuss the system of nomenclature used for oxyacid of halogens. Support your answer with examples.
ANSWER: Consult the textbook at page 86 — 87.
Q.07: (a) How the halogen acids are ionized in water?
ANSWER: In water, hydrogen halides give hydrofluoric, hydrochloric, hydrobromic and hydroiodic acids. Their strength as acids depends upon their extent of ionization. i.e.,
HX(g) + H2O(ℓ) ⟶ H3O+(aq) + X–(aq)
Hydrofluoric acid is a weak acid due to its limited ionization. The other three acids are stronger acids due to their greater ionization. The acidic strength of these acids increases in the following order: HF < HCl < HBr < HI
(b) Why HF is weaker acid than HCl?
ANSWER: HF is a weaker acid as compared to HCl because its proton (H+) donating capacity is less than that of HCl. This is due to very strong covalent bonding between H and F, and very strong hydrogen bonding between HF molecules. HCl, on the other hand, is a stronger acid than HF, because its proton donating capacity is greater than that of HF. This is due to relatively weaker H-Cl bond, and weaker dipole-dipole forces between HCl molecules as compared to H-bonds in HF.
Q.08: In the following sets, arrange the substances in order of the property indicated. Give reasons:
(a) Increasing acidic character HCIO, HCIO2, HClO3, HClO4
ANSWER: The acidic strength of oxyacids of halogens increases with the increase in the oxidation state of the halogens. Therefore, the increasing order of the acidic strength of oxyacids of chlorine is: HClO < HClO2 < HClO3 < HClO4
(b) Increasing oxidizing power F2, Cl2 , Br2 , I2
ANSWER: The oxidizing power of halogens decreases with the increase in atomic number. Therefore, the order of decreasing oxidizing power of halogens is: F2>Cl2>Br2>I2
Q.09: What happens when bleaching powder reacts with the following reagents dil. H2 SO4, excess of conc. H2 SO4 , NH3 , HI and CO2 .
ANSWER: (1) With dilute H2SO4, bleaching powder gives hypochlorous acid.
2CaOCl2 + H2SO4(dil.) ⟶ CaSO4 + CaCl2 + 2HClO
(2) When excess conc. H2SO4 is added to bleaching powder, Cl2 gas is given out.
CaOCl2 +(excess) H2SO4(conc.) ⟶ CaSO4 + H2O + Cl2↑
(1) With NH3: 3CaOCl2 + 2NH3 ⟶ 3CaCl2 + 3H2O + N2
(2) With CO2: CaOCl2 + CO2 ⟶ CaCO3 + Cl2
Q.10: Discuss the various commercial uses of halogens and their compounds.
ANSWER:
Commercial Uses of Halogens & Their Compounds:
(1) Uses of Fluorine:
(i) Fluorine is used for the preparation of freons. Freon is the commercial name of low molecular mass fluorochlorocarbons, CCl2F2, CClF3. These are being used as refrigerants and aerosol propellants.
(ii) Fluorine is used to prepare Teflon (-CF2 — CF2)n. It is a polymerized tetrafluoro ethylene compound. It is a valuable plastic which resists the action of oxidants, acids and alkalies.
(a) Corrosion-proof parts of machinery are made of it.
(b) It is used for coating the electrical wiring.
(c) Teflon is also used as a non-stick coating for cooking pans.
(iii) Halothane is used as an anaesthetic.
(iv) Fluorides in toothpastes build a protective coating on teeth.
(2) Uses of Chlorine:
(i) Chlorine is used in the manufacture of bleaching powder.
(ii) It is used as a disinfectant in swimming pools and water treatment plants.
(iii) A number of antiseptics, insecticides, weedkillers and herbicides are manufactured from chlorine.
(iv) It is also used in the manufacture of hydrochloric acid, which is the cheapest industrial acid.
(v) Chlorine is also used in the manufacture of polyvinyl chloride (PVC) plastics.
(vi) Chloroform and carbon tetrachloride are prepared from chlorine which are used as solvents.
(3) Uses of Bromine:
(i) Ethylene dibromide (C2H4Br2) is added to leaded gasoline to save the engine from lead oxide and lead sulphate deposits.
(ii) Bromine is also used as fungicide.
(iii) Silver bromide is used in photography.
(4) Uses of Iodine:
The major applications of iodine are in pharmaceutical industry.
(i) It is used as disinfectant and germicide.
(ii) Tincture of iodine and iodex are popular preparations of iodine.
(iii) Diet with insufficient iodide ions leads to an enlargement of the thyroid (Goiter). To ensure the presence of iodide ion in the diet, sodium or potassium iodide is added to the common salt which is known as iodized salt.
Q.11: What are noble gases? Explain their inertness on the basis of their electronic configuration.
ANSWER:
Noble Gases: “The elements of group VIIIA i.e., Helium, Neon, Argon, Krypton, Xenon and Radon are called noble gases.” They are inert because their valence shells are closed octets (except He). They have very high ionization energies so, usually, they do not react.
Q.12: Write notes on the followings: (i) Oxyfluorides of xenon (ii) Applications of noble gases
ANSWER:
(i) Oxyfluorides of Xenon:
Xenon Oxytetrafluoride, XeOF4 is also formed by a rapid reaction of XeF6 with silica (quartz).
2XeF6 + 6SiO2 ⟶ 2XeOF4 + SiF4
XeOF4 is a colourless volatile liquid. It can be kept in nickel vessel. It reacts with water to give XeO3.
2XeOF4 + 2H2O ⟶ XeO3 + 4HF
Xenon Oxydifluoride, XeOF2 is obtained when xenon reacts with oxygen difluoride in an electric discharge.
Xe + F2O ⟶ XeOF2
(ii) Applications of Noble Gases:
(1) Helium is used in weather balloons, in welding and in traffic signal light.
(2) A mixture of 80% helium and 20% oxygen is used for breathing by the sea divers.
(3) Helium is used as a cooling medium for nuclear reactors.
(4) Neon is largely used in making neon advertising signs, in high voltage indicators and TV tubes.
(5) Neon and helium arc is used in making glass lasers.
(6) Argon is used in electric light bulbs, in fluorescent tubes, in radio tubes, and in Geiger counters (used to detect radioactivity).
(7) Argon is also used for arc welding and cutting. (8) Krypton is used to fill fluorescent tubes and in flash lamps for high-speed photography.
(9) Xenon is used in bactericidal lamps.
(10) Radon being radioactive is used in radiotherapy for cancer and for earth quake prediction.
Q.13: Short questions.
(i) What is “Iodized Salt”?
ANSWER:
Iodized Salt: “The common salt containing small amount of sodium or potassium iodide is called iodized salt.” Iodized salt is used to ensure the presence of iodide ion in the diet because its deficiency leads to the enlargement of thyroid gland, a disease called goiter.
(ii) What are Freons and Teflon?
ANSWER:
Freons: Freon is the commercial name of low molecular mass fluorochlorocarbons, CCl2F2, CClF3.
Uses: These are being used as refrigerants and aerosol propellants.
Teflon: Teflon is a polymerized tetrafluoro ethylene (-CF2-CF2-) compound. It is a valuable plastic which resists the action of oxidants, acids and alkalies.
Uses:
(1) Corrosion-proof parts of machinery are made of it.
(2) It is used for coating the electrical wiring.
(3) Teflon is also used as a non-stick coating for cooking pans.
(iii) Arrange the following ions in order of increasing size: F– , CI– , I– , Br–
ANSWER:
The arrangement of halide ions (X–) in increasing order of their sizes: F– < CI– < Br– < I–
(iv) Why iodine has metallic luster?
ANSWER: Iodine is a greyish black solid, which gives metallic luster. This is because iodine has greater atomic size, so its valence electrons are relatively loose. When light falls on iodine crystals, the loose electrons of the surface molecules absorb incident light and excite to higher energy levels. When they de-excite to lower energy levels, they release the absorbed energy, which appears in the form of metallic luster.
(v) Which halogen sublimes to violet vapours?
ANSWER: Iodine, a metallic-appearing shining greyish black solid, sublimes to violet vapours on heating.
(vi) Which halogen is used as an antiseptic?
ANSWER: The solution of iodine in alcohol which is called tincture of iodine is used as antiseptic on wounded skin. A number of antiseptics are also manufactured from chlorine.
(vii) Which halogen is used in water treatment to kill bacteria?
ANSWER: Chlorine is used as disinfectant in swimming pools and water treatment plants to kill bacteria.
(viii) Name the gas, which is used for earthquake prediction.
ANSWER: Radon is used for earth quake prediction.
(ix) Name the gas, which is used in bactericidal lamps
ANSWER: Xenon is used in bactericidal lamps.