Halogen Symbol
Fluorine F
Chlorine Cl
Bromine Br
Iodine I
Astatine At
General Physical Properties of Halogens
1
2. Poor conductors of heat and electricity (because they are non-metals, have no free ions or electrons that can move and carry electric current).
3. They are of low solubility in water.
Chlorine
Physical properties of Chlorine
1) Denser than air (collected by downward delivery)
2) Yellowish green in color
3) Soluble in organic solvent as CCl4 forming a pale yellow solution.
4) A poisonous choking smelling gas.
5) Slightly soluble in water.
6) Bleaches litmus solution
Test for Chlorine gas :
Test: Approach a damp litmus paper:
Observations: If the paper is blue litmus, it turns red then bleaches. If the paper is red litmus, it bleaches.
Collection of Chlorine :
1. Downward delivery ( chlorine, 3.2 g/L, is denser than air, 1.28 g/L)
2. In a gas syringe.
Uses of Chlorine :
1) As a disinfectant in swimming pools.
2) In sterilizing water.
3) In manufacture of hydrogen chloride and PVC plastic.
4) As a bleaching agent.
Manufacture of Chlorine by Electrolysis
Industrially, chlorine is produced by the electrolysis of brine (concentrated sodium chloride solution). Chlorine gas evolves at the anode while at the cathode hydrogen gas is produced.
Reactivity of Halogens
Fluorine is the most reactive halogen. It is the most reactive non-metal.
The reactivity of the halogens decreases as we move down the group.
This happens because as we go down the group, the atoms get bigger, and the added electron is further away from the nucleus and it is less attracted to the nucleus thus it is harder for the atom to gain an electron.
Chemical Properties of Halogens
1) Reaction with hydrogen
2) Displacement reactions of Halogens
3) Reaction of halogens with metals
When halogens or group VII elements react with metals, they produce a wide variety of salts. a.
Reaction of halogens with iron
a. Reaction of halogens with iron
b. Reaction of halogens with sodium
Halogens react with sodium metal to form a white solid of sodium halide.
Uses of Halogens
1. Fluorides in toothpaste help prevent tooth decay
2. Fluorine compounds used in making plastics like Teflon (the non-stick surface on pans)
3. Chlorofluorocarbons (CFC) are used as propellants in aerosols and refrigerants. (They are now being replaced because of their damaging effect on the ozone layer).
4. Chlorine is used as a disinfectant to purify water
5. Chlorine compounds are used as household bleaches.
6. Hydrochloric acid is widely used in industry
7. Bromine compounds are used in making pesticides.
8. Silver bromide is the light-sensitive film coating on photographic films
9. Iodine solution is used as an antiseptic
Hydrogen chloride and hydrochloric acid
Both of them have the formula HCl Hydrogen chloride is a colorless gas at room temperature while hydrochloric acid is the solution of the gas in water.
If hydrogen chloride gas is dissolved in methylbenzene (an organic solvent known as toluene) the solution does not show the acidic properties
The table below lists the properties of solution of hydrogen chloride in methylbenzene and water.
The following deductions in the table below can be made from the above results:
The hydrogen chloride molecules do not ionize when dissolved in methylbenzene, but they do when dissolved in water.
Hydrochloric acid
Hydrogen chloride gas dissolves in water to form hydrochloric acid that completely dissociates in water to form hydrogen ions and chloride ions
Physical properties of hydrochloric acid
1. acidic
2. turns blue litmus red
Chemical properties of hydrochloric acid
1. reacts with reactive metals as magnesium, zinc and iron to form metal chloride in solution and liberate hydrogen gas
magnesium + hydrochloric acid → magnesium chloride + hydrogen
2. reacts with metal carbonates to form metal chloride in solution, water and carbon dioxide.
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
3. reacts with bases (metal oxides and metal hydroxides) to form a metal chloride in solution and water
Copper (II) oxide + hydrochloric acid → copper (II) chloride + water
Halogens and Photography
In photography, the film in a camera is coated with a film consisting of silver salts. These salts are silver chloride, AgCl or silver bromide, AgBr or silver iodide, AgI. When light hits the film, the silver ions in the salts are reduced by gaining electrons in a photochemical reaction:
Ag+ + e - → Ag
Light energy speeds up this reduction process. When the film is developed to produce negatives, these show the dark and light patches of the pictures taken. The darker areas contain most silver, the lighter contain least silver formed. The silver salts need to be kept in the dark but even then they very slowly change to silver over time. This is why photographic films have expiration dates.
A sheet of white paper was covered with silver (I) bromide and the experiment below was carried out.
Rate of reaction depends on the intensity or amount of light. The more the light, the more silver bromide is changed to black silver and the card darkens more. The part of the paper that is not covered received the most light so it is most black. The part of the paper that was covered with thick card didn’t receive any light so it stayed white.
A similar experiment would be a piece of paper coated with a layer of silver (I) chloride.
When the cross was removed, the following was the result
