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Revision:AQA AS Chemistry Unit 2

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > AQA AS Chemistry Unit 2

AQA AS Chemistry Unit 2

Contents

Halogens

Reaction with concentrated Sulphuric acid

  • Sodium Chloride
    • \displaystyle \mathsf{NaCl + H_2SO_4} \longrightarrow \mathsf{HCl + NaHSO_4}
  • Sodium Bromide
    • \displaystyle \mathsf{NaBr + H_2SO_4} \longrightarrow \mathsf{HBr + NaHSO_4}
    • then
    • \displaystyle \mathsf{2NaBr + 2H_2SO_4} \longrightarrow \mathsf{2H_2O + Br_2 + SO_2 + Na_2SO_4}
  • Sodium Iodide
    • \displaystyle \mathsf{NaI +H_2SO_4} \longrightarrow \mathsf{HI +NaHSO_4}
  • then
  • \displaystyle \mathsf{8NaI + 5H_2SO_4} \longrightarrow \mathsf{4I_2 + H_2S + 4H_O + 4Na_2SO_4}

Chlorine Water

  • Disproportionation – chlorine is simultaneously oxidised to a Cl- ion and reduced to a ClO- ion
    • \displaystyle \mathsf{Cl_2 + H_2O} \leftrightarrow \mathsf{HCl + HClO}

Trends in chemical properties

  • chlorine will displace bromine and iodine
  • bromine will displace iodine but not chlorine
  • iodine will displace neither chlorine or bromine
  • Oxidising Power
    • \displaystyle \mathsf{F_2 > Cl_2 > Br_2 > I_2}
  • Reactions with Cl2
    • Halide ion
      1. Br-
      2. I-
    • Observations
      1. yellow/brown
      2. brown colour, black ppt
    • Conclusion
      1. Br2 displaced
      2. I2 displaced
  • Reactions with Br2
    • Halide ion
      1. Cl-
      2. I-
    • Observations
      1. no change
      2. brown colour, black ppt
    • Conclusion
      1. Cl2 not displaced
      2. I2 displaced
  • Reactions with I2
    • Halide ion
      1. Cl-
      2. Br-
    • Observations
      1. no change
      2. no change
    • Conclusion
      1. Cl2 not displaced
      2. Br2 not displaced

Trend in electronegativity

  • decreases as atomic number increases
    • Because
    • Atomic number increases
    • attraction for bonding electron pair in covalent bond increase
    • increase number of electron shells
    • shielding increases
    • electrons less strongly attracted
    • atomic radius increases

Trend in boiling point

  • all exist as diatomic molecules
  • attraction = weak intermolecular forces
  • van der Waals forces
  • increase with size of molecule

Tests

  • Tests for halide ions
  • Reagents
    • Silver nitrate solution
      • Fluoride ions
        • Silver fluoride is soluble
        • no ppt formed
      • Chloride ions
        • White ppt formed
        • soluble in dilute ammonia
      • Bromide ions
        • cream ppt formed
        • soluble in concentrated ammonia
      • Iodide ions
        • yellow ppt formed
        • insoluble in ammonia solution
    • followed by ammonia solution


Energetics

\displaystyle q = mc\delta T

  • \displaystyle \mathsf{heat\ energy\ (joules)} = \mathsf{mass \times heat\ capacity \times the\ change\ in\ temperature}
  • \displaystyle \mathsf{enthalpy\ change\ (kJ\ mol_{-1} )} = \frac{-q}{(1000 \times \mathsf{number\ of\ moles})}


Standard enthalpy of formation

  • enthalpy change when 1 mol of a compound is formed from its elements under standard conditions
  • all reactants and products in their standard states

Standard enthalpy of combustion

  • enthalpy when 1 mol of a substance is completely burned in oxygen
  • under standard conditions and all reactants and products in their standard states

Standard enthalpy of neutralisation

  • enthalpy change that accompanies the neutralisation of an acid by a base to form 1 mol of H2O
  • under standard conditions

Standard conditions

  • 298 K
  • 100kPa
  • 1mol dm-3


Redox

Oxidation

  • loss of electons
  • addition of oxygen

Reduction

  • gain of electrons
  • loss of oxygen

Reactions which involve both oxidation and reduction

Oxidising agent is reduced – an electron pair acceptor

Reducing agent is oxidised – an electron pair donor


Extraction of metals

Titanium

  • Batch process
  • Argon
    • Inert atmosphere
    • to prevent contamination of metal with oxygen or nitrogen
  • \displaystyle \mathsf{TiO_2 + 2C + 2Cl_2} \longrightarrow \mathsf{TiCl_4 + 2CO}
  • \displaystyle \mathsf{TiCl_4 + 4Na} \longrightarrow \mathsf{4NaCl + Ti}
  • economic factors
    • chlorine and sodium have to be produced
    • high temperatures involved
    • argon atmosphere has to be contained

Aluminium

  • Bauxite
  • \displaystyle \mathsf{Al_2O_3}
  • bauxite dissolves in molten cryolite – to reduce costs from trying to melt just bauxite
  • At cathode
  • \displaystyle \mathsf{Al^{3+} + 3e^-} \longrightarrow \mathsf{Al}
  • At anode
  • \displaystyle \mathsf{2O^{2-}} \longrightarrow \mathsf{4e^- + O_2}

Iron

  • In blast furnace
    • haematite
      • \displaystyle \mathsf{Fe_2O_3}
    • coke
      • \displaystyle \mathsf{C}
    • limestone
      • \displaystyle \mathsf{CaCO_3}
    • hot air
  • \displaystyle \mathsf{2C + O_2} \longrightarrow \mathsf{2CO}
  • \displaystyle \mathsf{Fe_2O_3 + 3CO} \longrightarrow \mathsf{2Fe + 3CO_2}
  • \displaystyle \mathsf{2Fe_2O_3 + 3C} \longrightarrow \mathsf{4Fe + 3CO_2}
  • Impurities
    • \displaystyle \mathsf{SiO_2}
    • \displaystyle \mathsf{CaCO_3 + SiO_2} \longrightarow \mathsf{CO_2 + CaSiO_3}
      • \displaystyle \mathsf{CaSiO_3} is slag
    • sulphur impurities
      • \displaystyle \mathsf{Mg + S} \longrightarrow \mathsf{MgS}
    • Phosphorus
    • Manganese
  • Making steel
    • O2 under pressure blown on molten iron
    • carbon oxidised to carbon monoxide

Economic factors and recycling

  • Purity required
  • Energy costs
  • Cost of reducing agent
    • manufacturing chlorine and sodium
    • electrolysis of brine water
  • Recycling aluminium
    • cheaper to recycle aluminium than mine bauxite and electrolysis
    • electrolysis needs high temperature and large electricity
    • high resistance to corrosion
  • Recycling of iron
    • Earths reservoirs of ore are saved
    • energy of mining ore, transportation, smelting is greater than recycling


Comments

  • Suitable for: AS Level Chemistry, Unit 2, AQA Board.
  • Written by: Sycren with some sections by Fointy
  • From here.
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