AS Unit 2 AQA Chemistry Revision Notes

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Chemistry Unit 2 AS
Group 7, the halogens
Physical properties, fluorine is a pale yellow gas, chlorine a greenish gas, bromine a red-brown liquid and iodine a black solid
o Fluorine is small and leads to the repulsion between non-bonding electrons because they are so close together
o The atoms get bigger as we go down the group because each element has one extra filled main level of electrons compared with
the one above it
Halogen Atomic number Electron Electronegativity Atomic radius nm Melting point K Boiling Point K
Arrangement
Fluorine 9 [He]2s22p5 4.0 0.071 53 85
Chlorine 17 [Ne]3s23p5 3.0 0.099 172 238
Bromine 35 [Ar]3d104s24p5 2.8 0.114 266 332
Iodine 53 [Kr]4d105s25p5 2.5 0.133 387 457
Electronegativity
This is a measure of the ability of an atom to attract electrons, or electron density towards itself within a covalent bond
o This depends on the attraction between the nucleus and bonding electrons in the outer shell, which depends on the balance
between the nuclear charge and the distance between the nucleus and the bonding pairs plus the shielding effect of inner shells
of electrons
Melting and boiling points
Melting and boiling points involve weakening and breaking van der Waals forces only, the covalent bonds in the halogen molecules stay
intact
o The lower the boiling point the more volatile the element
Chemical reactions of the halogens
The oxidising ability of the halogens increases as we go up the group
Displacement reactions
Halogens will react with metal halides in solution in such a way that the halide in the compound will be displaced by a more reactive halogen
o The sodium ions are spectator ions, the 2 colourless starting materials react to produce the red-brown colour of bromine
In this redox reaction the chlorine is acting as an oxidising agent by removing electrons from the Br- and so oxidising
2Br- to Br2 (The oxidation number of the bromine increases from -1 to 0)
A halogen will always displace the ion of a halogen below it in the periodic table
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Reactions of halide ions
Halide ions act as reducing agents as the give away electrons and become halogen molecules
o The increase in reducing ability is linked to the size of the ions, so the larger the atom the more easily it loses an electron which is
because the electron is lost from the outer shell which is further from the nucleus as the ion gets larger
Reactions of sodium halides with concentrated sulphuric acid
The products produced reflect the reducing powers of the halide…read more

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Silver fluoride No precipitate
Silver chloride White ppt Dissolves in dilute ammonia
Silver bromide Cream ppt Dissolves in concentrated
ammonia
Silver iodide Pale yellow ppt Insoluble in concentrated
ammonia
Uses of chlorine
Reaction with water
o Chlorine reacts with water in a reversible reaction to form chloric (I) acid (HClO) and hydrochloric acid
o In this reaction, the oxidation number of one of the chlorine atoms increases from 0 to +1 and that of the other decreases from 0 to
-1
This type of redox…read more

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The elements have two electrons in an outer s-orbital.…read more

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Oxidation is loss of electrons so in all their reactions the Group 2 metals are oxidised
o The metals go from oxidation state 0 to oxidation state +2, these are redox reactions
Reaction with water
o With water we see the same trend in reactivity, the metals get more reactive as we go down the group
The basic reaction is as follows, where M is any Group 2 metal
Magnesium hydroxide is milk of magnesia and is used in indigestion remedies to neutralise excess stomach…read more

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The solution is first acidified with nitric or hydrochloric acid, then barium chloride solution is
added to the solution is added to the solution under test and if a sulphate is present a white
precipitate of barium sulphate is formed
The addition of acid removes a carbonate ions as carbon dioxide, barium carbonate is
also a white insoluble solid
Metal Extraction
Sulphide ores to oxides
o Before reduction sulphide ores are usually converted to oxides by heating them in air a process called roasting for…read more

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Other metals
o Manganese us produced by the reduction of its oxide with carbon
o Copper is reduced in a similar way. The oxide is then heated with oxide
Some copper ores are converted into solutions containing Cu2+ ions by spraying copper mining waste with dilute acid in
the presence of a bacterium.…read more

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Tungsten - reduction with hydrogen
o It is used in light bulb filaments as it has a high m.p.…read more

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Solubility
The polar bonds are not polar enough to make the Haloalkanes soluble in water
o The main intermolecular forces of attration are dipole-dipole attractions and van der Waal forces
Haloalkanes mix with hydrocarbons so they can be used as dry cleaning fluids and to remove oily stains
Boiling Points
Depends on the number of carbon atoms and halogen atoms
o Boiling point increases with increased chain length
o Boiling point increases as we go down the halogen group
These are both caused by increased…read more

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It has a lone pair of electrons which it can use to form a covalent bond
o The lone pair is situated on an electronegative atom
Therefore a nucleophile is a species that has a lone pair of electrons with which it can form a bond by donating its electrons to an electron
deficient carbon atom, some common nucleophiles are, :-OH , :NH3 , :-CN
o They will replace the halogens in a haloalkane these reactions are called Nucleophilic substitutions and they all follow essentially…read more

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