F322: Enthalpy Changes

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Enthalpy

  • "The heat energy stored in a chemical system" (Symbol = H).
  • Enthalpy change = △H.
  • Can be measured by temperature changes in reactions.
  • Can be exothermic or endothermic. [Next cards]
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Exothermic Reactions

  • Reactants lose energy to the surroundings.
  • The temperature increases.
  • Common examples:
    -Combustion of fuels.
    -Respiration.
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Endothermic

  • Reactants gain energy from surroundings.
  • Temperature decreases.
     
  • Common examples:
    -Thermal decomposition (needs heat).
    -Photosynthesis. 
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Bonds

  • In a chemical reaction, bonds are broken in the reactants (endothermic [needs energy]), and then in the products bonds are made (exothermic [releases energy]).
  • If more energy is needed to break bonds than the energy released = Endothermic overall.
  • If more energy is released when bonds are made than bonds broken = Exothermic.
  • Strength of bond is shown by its bond enthalpy. 
  • Average Bond Enthalpy = The enthalpy change when breaking 1 mole of a given type of bond in the molecules of a gaseous species.
    E.g. H2 -> 2H    △H = +436 kJmol-1
  • You can use a table of these (e.g. C-H = 413 etc.) to work out whether a reaction is exothermic or endothermic by taking away the products enthalphy from the reactant's; if the number is negative then the reaction is exothermic; if positive, endothermic. 
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Standard Enthalpy Changes

  • Symbol = △Hθ 
  • Standard conditions are:
    -298K (25 degrees).
    -100KPa (1 atm)
    -1 mol dm-3 for concentration of solutions.
  • △Hfθ = The enthalpy change when 1 mole of a substance is made from its elements in their standard states under standard conditions.
  • △Hcθ = The enthalpy change when 1 mole of a substance is reacted with oxygen under standard conditions.
  • △ Hrθ = The enthalpy change when chemicals react in the molar quantities as expressed in the reaction equation, under standard conditions.
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Calculating Enthalpy Changes

  • Q =                                   -m                                         c                                 △T 
    Enthalpy change    mass of liquid heated      4.18 (specific heat capacity)     change in temp.
  • The use of experimental data to calculate the enthalpy change.
  • The values calculated experminentally will be lesx than theoretical due to heat loss.
  • Loses heat:
    -Through material of container.
    -Through open top.
  • Fixed by:
    -Use insulating material.
    -Add a lid.
     
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Hess' Law

  • "The enthalpy change of a reaction is independant of the route taken, providing the initial and final conditions are the same".
  • Simple methods:
    -If for △Hf = Sum of enthalpies of products - sum of enthalpies of reactants.
    -If for △Hc = Sum of enthalpies of reactants - sum of enthalpies of products.
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