Group 17 (halogens) Flashcards

1
Q

Colour trend

A
  • gets darker as you go down the group
    Fluorine- pale yellow gas
    Chlorine- green/yellow gas
    Bromine- orange/brown liquid
    Iodine- grey/black solid, purple vapour
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2
Q

Trends down the group

A
  • gets darker
  • volatility decreases
  • bond strength decreases
  • bond enthalpy decreases (exception is fluorine)
  • oxidising power decreases
  • reducing power increases
  • reactions become less vigorous
  • thermal stability decreases
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3
Q

Volatility

A
  • how easily a substance can evaporate
  • a volatile substance will have a low melting and boiling point
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4
Q

Trend of volatility

A

going down the group the boiling/melting point increases, therefore volatility of the halogens decreases

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5
Q

Bond strength trend- decreases going down the group, because

A
  • halogens are diatomic molecules in which covalent bonds are formed by overlapping their orbitals
  • in a covalent bond, the bonding pair of electrons is attracted to the nuclei on either side and it is this attraction that holds the molecule together
  • going down the group, the atomic size increases therefore the bonding pair of electrons is further away from the nucleus and are therefore less strongly attracted towards it

*the bigger the atom, the weaker the covalent bond

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6
Q

Bond enthalpy trend- decreases going down the group

A
  • bond enthalpy is the heat needed to break 1 mole of a covalent bond
  • the higher the bond enthalpy, the stronger the bond
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7
Q

Exception is fluorine: it has a smaller bond enthalpy than chlorine and bromine, because:

A
  • fluorine is so small that when 2 atoms of fluorine get together their lone pairs get so close that they cause significant repulsion which counteracts the attraction between the bonding pair of electrons and 2 nuclei (causes a decrease in bond strength)

*weak bond, therefore less energy is needed to break bond, therefore lower enthalpy

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8
Q

Describing volatility in terms of instantaneous dipole-induced dipole forces

A
  • halogens are simple molecular structures with weak van der Waals forces between the diatomic molecules, caused by instantaneous dipole-induced dipole forces
  • the more electrons, the stronger the id-id forces, therefore a higher melting and boiling point
  • as it gets more difficult to separate the molecules, the volatility of the halogens decrease going down the group
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9
Q

Oxidising power- decreases going down the group (related to electronegativities), because:

A
  • the atomic radii increases, which means the outer shell gets further away from the nucleus
  • an “incoming” electron will therefore experience more shielding from the attraction of the positive nuclear charge
  • the halogens ability to accept an electron (oxidising power) therefore decreases
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10
Q

Displacement reactions

A

when a more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen

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11
Q

Example of a displacement reaction
(the addition of chlorine water to a solution of bromine water)

A

CI2 (aq) + 2NaBr (aq) = 2NaCI (aq) + Br2 (aq)
- the chlorine has displaced the bromine from the solution as it is more reactive

  • summarised in the following equation
  • remove element that has the same oxidation number in reactants and products (in this case sodium)
    CI2 (aq) + 2Br- (aq) = 2CI- (aq) + Br2 (aq)
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12
Q

Reaction with the elements and hydrogen

A

H2 (g) + X2 (g) = 2HX (g)
- due to the decrease in reactivity of the halogens going down the group, the reactions become less vigorous

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13
Q

Thermal stability

A
  • how well a substance can resist breaking down when heated
  • a substance that is thermally stable will break down at high temperatures
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14
Q

Thermal stability trend of halogen halides- going down the group the thermal stability decreases, because:

A
  • their bonds become weaker due to the increased atomic radius of the halogens
  • the longer the bond length, the weaker it is and the less energy is needed to break it
  • as the bonds get weaker, the hydrogen halides become less stable to heat going down the group
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15
Q

Reducing power trend: increases going down the group (related to ionic radii)

A
  • the halide ions become larger
  • the outermost electrons get further away from the nucleus and therefore experience more shielding by inner electrons
  • as a result, the outermost electrons are held less tightly to the positive charged nucleus
  • therefore the halide ions lose electrons more easily going down the group and their reducing power increases
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16
Q

Reaction of halide ions with silver ions & ammonia

A
  • halide ions can be identified in an unknown solution by dissolving the solution in nitric acid
  • then adding a silver nitrate solution
  • then ammonia solution
17
Q

Halide ions reacting with the silver nitrate solution (X- is the halide ion)

A

AgNO3 (aq) + X- (aq) = AgX (s) + NO3- (aq)

18
Q

General equation

A

Ag+ (aq) + X- (aq) = AgX (s)
- if the unknown solution contains halide ions it will form a precipitate of the silver halide

19
Q

Ionic equation

A

X- (aq) + AgNO3 (aq) = NO3 (aq) + AgX (s)

20
Q

Table to results in notes

A
21
Q

Reaction of halide ions with concentrated sulfuric acid

A

in notes

22
Q

Reaction of halide ions with chlorine (disproportionation rxn)

A

1) Chlorine in cold alkali (15 degrees Celsius)
2) Chlorine in hot alkali (70 degrees Celsius)

23
Q

Chlorine in cold alkali (15 degrees Celsius)

A

CI2 + 2NaOH = NaCI + NaCIO + H2O

24
Q

Chlorine in hot alkali (70 degrees Celsius)

A

3CI2 + 6NaOH = 5NaCI + NaCIO3 + H2O

25
Q

Chlorine in water purification (disproportionation rxn as the chlorine is both oxidised and reduced)

A

CI2 (aq) + H2O (I) = HCI (aq) + HCIO (aq)
0 -1 +1

26
Q

Chloric acid dissociation

A

HCIO (aq) = H+ (aq) + CIO- (aq)
- Chloric(I)acid sterilises water by killing bacteria
- product acts as a sterilising agent