AQA A Level Chemistry复习笔记2.3.2 Chemical Properties of Group 7

Halogens: Oxidising Agents

 

  • Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge

Eg.   Ca (s) + Cl2 (g) → CaCl2 (s)  consisting of Ca2+ and 2Cl- ions

  • Halogens are therefore oxidising agents:
    • Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
    • Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)

     

  • The oxidising power of the halogens decreases going down the group (the halogens get less reactive)
  • This can be explained by looking at their electronegativities:

 

2.3-Group-17-Electronegativity-Halogens

The electronegativity of the halogens decreases going down the group

 

  • The electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
  • The decrease in electronegativity is linked to the size of the halogens
  • Going down the group, the atomic radii of the elements increase which means that the outer shells get 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 (their oxidising power) therefore decreases going down the group

2.3-Group-17-Trend-Oxidising-Power

With increasing atomic size of the halogens (going down the group) their electronegativity, and therefore oxidising power, decreases

 

  • The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions
  • A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
    • Eg. The addition of chlorine water to a solution of bromine water:

     

Cl2 (aq) + 2NaBr (aq) → 2NaCl (aq) + Br2 (aq)

    • The chlorine has displaced the bromine from solution as it is more reactive which can be summarised in the following ionic equation:

Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)

Reaction with Hydrogen

  • Halogens react with hydrogen gas to form hydrogen halides
  • Due to the decrease in reactivity of the halogens going down the group, the reactions between halogen and hydrogen gas become less vigorous
  • The table below shows a summary of the reaction between the halogen and hydrogen gas

 

Reaction between Halogen & Hydrogen Gas

2.3-Group-17-Table-1_Reaction-with-Hydrogen

Hydrogen Halides

  • Thermal stability refers to how well a substance can resist breaking down when heated
    • A substance that is thermally stable will break down only at high temperatures

     

  • The hydrogen halides formed from the reaction of halogen and hydrogen gas decrease in thermal stability going down the group
  • The decrease in thermal stability can be explained by looking at the bond energies of the hydrogen-halogen bond
    • Going down the group, the atomic radius of the halogens increases
    • The overlap of its outer shell with a hydrogen atom therefore gives a longer bond length
    • The longer the bond, the weaker it is, and the less energy required to break it

     

  • As the bonds get weaker, the hydrogen halogens become less stable to heat going down the group

 

2.3-Group-17-Thermal-Stability-Trend

The thermal stability of the hydrogen halide decreases going down the group as their bonds become weaker due to the increased atomic radius of the halogens

Halide Ions: Reducing Agents

  • Halide ions can also act as reducing agents and donate electrons to another atom
  • The halide ions themselves get oxidised and lose electrons
  • The reducing power of the halide ions increases going down the group
  • This trend can be explained by looking at the ionic radii of the halide ions

2.3-Group-17-Electron-Arrangement-in-Halide-Ions

The diagram shows that going down the group the ionic radii of the halide ions increases

 

  • Going down the group, the halide ions become larger
  • The outermost electrons get further away from the nucleus
  • The outermost electrons also experience more shielding by inner electrons
  • As a result of this, the outermost electrons are held less tightly to the positively charged nucleus
  • Therefore, the halide ions lose electrons more easily going down the group and their reducing power increases

2.3.2-Group-17-Reducing-Power-Trend-1

The reducing power of the halide ions increases going down the group

Exam Tip

Halogen is a derived from a greek word meaning salt-maker, a good reminder of their reaction with metals to form salts.

 

转载自savemyexams

 

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