HALOGENS
Halogens (salt formers) are found in group VII of the periodic table. The halogens are the most reactive nonmetals. They have seven electrons in their outermost shells and so ionize to form univalent negative ions. They exist as diatomic molecules. They are coloured and They form electrovalent compounds with metals.
In the group are chlorine, fluorine, bromine, iodine and astatine.
ELECTRONIC CONFIGURATION OF HALOGENS
The halogens have one electron short of the noble gas structure in their electronic configuration (i.e. they contains seven electrons in their outermost shells), and the readiness to complete the octet arrangement by receiving an electron makes the halogens very reactive.
The electronic configurations of the halogens are shown below:
Fluorine = 9: 1s2 2s2 2p5
Chlorine = 17: 1s2 2s2 2p6 3s2 3p5
Bromine = 35: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5
Iodine = 53: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5
PHYSICAL PROPERTIES OF THE HALOGENS
1. They are univalent, and readily accept one electron from other atoms to form ionic compounds (especially from metals e.g Na & K). They also share electrons with themselves or with non-metals to form covalent compounds.
2. They exist as diatomic molecules.
3. Fluorine and chlorine are gases, bromine is a liquid and iodine a solid.
4. The halogens are coloured, with typical penetrating odour. The colours deepen down the group. Fluorine is pale-yellow, chlorine is greenish- yellow, bromine is red and iodine is violet.
5. They are volatile substances. Their volatility decreases down the group.
6. All the halogens except fluorine, dissolve to some extent in water, fluorine reacts with water to give oxygen and hydrogen fluoride.
CHEMICAL PROPERTIES OF THE HALOGENS
The halogens are very reactive elements. Their reactivity decreases down the group. Fluorine is the most reactive halogen. They are also strongly electronegative. Their Electronegativity decreases down the group.
1. As oxidizing agents. Halogens are strong oxidizing agent. They readily accept electrons. The oxidizing power decreases down a group.
2. Reaction with metals: Halogens react with metals to form ionic compounds.
2Na(s) + F2(g) → 2NaF(s)
3. Reaction with hydrogen: Fluorine explodes with hydrogen even in the dark, chlorine reacts slowly in the dark but explode in bright sunlight, bromine reacts with hydrogen in the presence of platinum catalyst, while iodine reacts partially with hydrogen on heating. Example
H2(g) + Cl2(g) → 2HCl(g)
Stability of the hydrogen halides decreases down the group. Hydrogen fluoride is a liquid with a boiling point of 19OC. The other hydrogen halides are gases.
4. Reaction with water: Fluorine reacts vigorously with water to give off oxygen gas.
Chlorine reacts very slowly with water to give a mixture of hydrochloric acid and oxochlorate (I) acid
Cl2(g) + H2O → HOCl + HCl
which later decomposes to give hydrochloric acid and oxygen gas.
HOCl(aq) → HCl(aq) + O2(g)
The oxygen gas given off by the oxochlorate (I) acid is responsible for the bleaching action of moist chlorine gas and chlorine water.
H2O(g) + Cl2(g) → HCl(aq) + HOCl(aq)
CHLORINE
Chlorine is the most important member in the halogen family. It does not occur as free element in nature because it is too reactive. It is usually found in combined state as chlorides.
LABORATORY PREPARATION OF CHLORINE
1. By the oxidation of concentrated HCl with strong oxidizing agent such as MnO2 or KMnO4
MnO2(s)+ 4HCl(aq)→MnCl2(aq)+ 2H2O(l) + Cl2(g)
2. By heating concentrated H2SO4 with a mixture of NaCl and MnO2
2NaCl(s) + MnO2(s) + 2H2SO4(aq) →Na2SO4(aq) + MnSO4(aq) + H2O(l)+ Cl2(g)
INDUSTRIAL PREPARATION
Chlorine is manufactured industrially by the electrolysis of brine and molten NaCl, MgCl2 or CaCl2
PHYSICAL PROPERTIES
1. Chlorine is a greenish-yellow gas with unpleasant chocking smell.
2. It is a poisonous gas.
3. It is about 2.5 times denser than air.
4. It is liquefied under a pressure of about 6atm.
5. It is moderately soluble in water.
CHEMICAL PROPERTIES
1. It is very reactive and forms electrovalent compound with metals and a single covalent bond compounds with non-metals.
2Na(s) + Cl2(g) → 2NaCl(s)
Cl2(g) + H2(g) → 2HCl(g)
2. It displaces other halogens from solution of their acids and salts
Cl2(g) + NaI(aq) → 2NaCl(aq) + I2(g)
3. It combines directly with other elements except oxygen, nitrogen carbon and the noble gases; to form chlorides
Ca(s) + Cl2(g) → CaCl2(s)
4. It displaces hydrogen from its compounds due to its strong affinity for hydrogen
C10H12(l) + 8Cl2(g) →10C(s) + 16HCl(g)
5. It is a powerful oxidizing agent: it oxidizes green Fe2+ to yellow or brown Fe3+
2FeCl2(aq) + Cl2 →2FeCl3(aq)
6. It is a bleaching agent: The bleaching action of chlorine is due to its ability to react with water to form oxochlorate (I) acid which decomposes to release oxygen which in turn oxidizes the dye to form a colourless compound.
H2O(l) + Cl2(g) → HCl(aq) + HOCl(aq)
HOCl(aq) → HCl(aq) + [O]
Dye + [O] → [Dye + O]
Colored Colourless
7. It reacts with hot concentrated NaOH solution to give a mixture of sodiumtrioxochlorate (V) and sodium chloride.
6NaOH + 3Cl2(g) → NaClO3(aq) + 5NaCl(aq) + H2O(l)
hot concentrated Sodium trioxochlorate (V)
With cold dilute solution of NaOH, a pale yellowish mixture of sodiumoxochlorate (I) and sodium chloride is formed.
2NaOH(aq) + Cl2(g) → NaOCl(aq) + NaCl(aq) + H2O(l)
cold dilute sodium oxochlorate(I)
8. It reacts with CaOH solutions to produce bleaching powder
Ca(OH)2(aq) + Cl2(g) → CaOCl2.H2O(s)
Bleaching powder
TEST FOR CHLORINE
1. It turns wet blue litmus paper pink and then bleaches it.
2. It turns damped starch-iodide paper blue black.
Chlorine turns starch-iodide paper blue black because it displaces iodine from the iodide. The iodine liberated then turns the starch blue.
USES OF CHLORINE
1. It is a powerful germicide (due to its oxidizing nature).
2. It is used as a bleaching agent for cotton, wool, pulp etc.
3. It is used in the manufacture of polyvinyl chloride (PVC) and synthetic rubber.
4. It is used in the manufacture of organic compounds like trichloromethan (CHCl3), and tetrachloromethane (CCl4
5. It is used in producing KClO3, for making matches and fireworks.
6. It is used for making NaClO3, a weed killer.
7. It is used for making acidified NaClO solution a domestic antiseptic.
COMPOUNDS OF CHLORINE
HYDROGEN CHLORIDE
Hydrogen chloride exists as a gas at room temperature. It dissolves readily in water to form hydrochloric acid. It occurs in traces in the air as industrial by-product and is considered as an air pollutant; but it can be easily washed down as acid rain since it is very soluble in water.
LABORATORY PREPARATION
The gas is prepared by the action of hot concentrated H2SO4 on any soluble chloride. Example 2NaCl(s) + H2SO4(aq) → Na2SO4(aq) + 2HCl(g)
Note: NaHSO4 is first formed at a lower temperature and later at higher temperature HCl gas is formed. The gas is dried by passing it through concentrated H2SO4 in another flask and collected.
INDUSTRIAL PREPARATION
Pure HCl gas can be produced in large scale by direct combination of hydrogen and chloride gas obtained from the electrolysis of brine.
H2(g)+ Cl2(g)→2HCl(g)
PHYSICAL PROPERTIES
1. Pure HCl gas is colourless and has sharp irritating smell
2. It turns damp blue litmus paper red
3. It is about 1.25times denser than air
4 It is very soluble in water, hydrochloric acid
5. It dissolved readily in non-polar solvent like chloroform and toluene.
when HCl is dissolved in nonpolar solvents, the solution does not conduct electricity and has no acidic properties because hydrogen chloride which is a covalent molecule does not ionize when it dissolves in non-polar solvents. But it dissolves in water and ionizes. The ions formed in aqueous solution are responsible for the acidic property and conductivity of its aqueous solution.
6. It forms misty fumes in moist air because it dissolves in the moisture to form tiny droplets of HCl acid
CHEMICAL PROPERTIES
1. Combustion: - Hydrogen does not support combustion
2. It combines directly with NH3 producing dense white fumes of ammonium chloride
HCl(g) + NH3(g) → NH4Cl(s)
3. It reacts with electropositive metals to form their respective chloride displacing hydrogen gas
Zn(s) +2HCl(g) → ZnCl2(s) + H2(g)
TEST FOR HYDROGEN CHLORIDE
1. Place a gas rod that has been dipped in ammonia solution over the gas jar containing the unknown gas, if a dense white fumes forms on the glass rod, then the gas is hydrogen chloride gas.
2. Few drops of silver trioxonitrate (V) is added to the gas jar containing the unknown gas and shaken. If white precipitate of silver chloride is observed, then the gas is hydrogen chloride gas
CHLORIDES
Chlorides are normal salts formed when metallic ion replace the hydrogen ion in hydrochloric acid. Soluble Chlorides are prepared by neutralization reaction while insoluble are prepared by double decomposition method.
All Chlorides are soluble in water with except, AgCl, HgCl2, PbCl2
PROPERTIES
1. Chlorides are stable to heat, that is, they are not decomposed by heat.
They are recovered from solution by evaporation to dryness and sometimes by crystallization.
2. They react with hot concentrated tetraoxosulphate (VI) acid to produce hydrogen chloride gas.
2NaCl(s)+H2SO4(aq) →Na2SO4(aq)+2HCl(g)
and in the presence of a strong oxidizing agent, chlorine is produced.
ZnCl2(s) + KMnO4(s)+ 2H2SO4(aq) → ZnSO4(aq)+ K2SO4(aq) + 2MnO2(aq) + 2H2O(l)+Cl2(g)
TEST FOR CHLORIDES
Add some Few drops of AgNO3(aq) to a solution of the sample in a test tube, if it forms a white precipitate, now acidify the solution by adding dilute trioxonitrate acid if the white precipitate remains but readily dissolves in excess NH3(aq) solution then a chloride ion is present.
OBJECTIVE QUESTIONS
1. The bleaching action of chlorine is through the process
a. Hydration
b. Hydrolysis
c. Reduction
d. Oxidation
2. When chlorine is passed through a sample of water, the pH of the water sample would be
a. <7
b. =7
c. >7
d. 0
3. Halogens generally react with metals to form
a. Alkalis.
b. Acids.
c. Bases.
d. Salts.
4. Potassium chloride solid does not conduct electricity because
a. It is a covalent compound
b. Stong cohesive forces make its ions immobile
c. Strong cohesive forces make its molecules immobile
d. Each of Potassium and chlorine ions has a noble gas structure.
5. Chlorine water is used as a bleaching agent because it is
a. An acidic solution
b. An alkaline solution
c. An oxidizing agent
d. A reducing agent
6. Which of the following halogens is a liquid at room temperature
a. Iodine
b. Chlorine
c. Bromine
d. Fluorine.
7. Chlorine, bromine and iodine belong to the same group and
a. Are gaseous at room temperature
b. Form whit precipitate with AgNO3(aq)
c. React violently with hydrogen without heating.
d. React with alkali
8. Which of the following chlorides is insoluble in water?
a. AgCl
b. KCl
c. NH4Cl
d. ZnCl2
9. Which of the following statements about chlorine and iodine at room temperature is correct
a. Chlorine is a gas and Iodine as solid
b. Chlorine is a liquid and iodine is a gas.
c. Chlorine and iodine are gases
d. Chlorine is solid and iodine is a liquid.
10.
THEORY QUESTIONS
1. Draw and label a diagram to illustrate the preparation and collection of a dry sample of chlorine gas in the laboratory.
1b. state two use of chlorine
2a. Write the equation for reaction between chlorine gas and
i. Concentrated NaOH
Ii. Dilut NaOH
b. Write the electronic configuration of the following atoms/ions: Cl, F-, Br.
C. Give three physical properties of the halogens
2a. Explain one laboratory preparation of dry chlorine gas.
b. Name the method of collection of chlorine gas and explain why it can be collected by the method.
3a. Mention three physical properties of chlorine.
b. Using balanced equations, state THREE chemical properties
C. Explain why hydrogen chloride in toluene does not conduct electricity but its aqueous solution conducts electricity.
4a. Describe a test for a soluble chloride.
bGive three uses of chlorine gas.
C. State TWO physical and TWO chemical properties of hydrogen chloride gas
5a. An unknown gas is colourless, has an irritating smell, fumes in moist air and turns blue litmus paper red; describe how you will confirm the gas to be hydrogen chloride gas.
b. A solid chloride E which sublimed on heating reacted with an alkali F to give a choking gas G. G turned moist red litmus paper blue. Identify E,F and G.
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