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    why are high pressures and high temps needed for the polymerization of polythene and polypropene
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    (Original post by SmileyGurl13)
    why are high pressures and high temps needed for the polymerization of polythene and polypropene
    Temp to overcome the activation energy and pressure to drive the equilibrium to the RHS
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    (Original post by charco)
    Temp to overcome the activation energy and pressure to drive the equilibrium to the RHS
    Could you please explain this in more detail, positive rep is available
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    (Original post by SmileyGurl13)
    Could you please explain this in more detail,
    This reaction, like all reactions is controlled by kinetic and thermodynamic factors.

    nCH2=CH2(g) --> -[-CH2-CH2-]n-(s)

    The best way to improve the rate of a reaction is to heat it up. This overcomes the activation energy barrier which makes the reactrion too slow.

    Thermodynamic control of reactions is due to a combination of the entropy change and the enthalpy change, via Gibbs free energy.

    ΔG = ΔH - TΔS

    In a polymerisation you are changing a massive amount of gas into a small amount of polymer. There is a huge loss of entropy, i.e. ΔS is negative and the term -TΔS isd positive and gets more positive as the temperature increases. This makes the reaction LESS thermodynamically favourable as the temperature increases.

    Hence the only reason for increasing the temperature is kinetic control and increasing the rate of the reaction.

    Pressure increase favours the direction of fewer moles of gas (Le Chatelier) which of course is to the RHS. Hence very high pressures.
 
 
 
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