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Enthalpy help

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    • Thread Starter

    The standard enthalpy change of combustion of ethanol is -1371 kjmol-1. Calculate the mass of ethanol needed to heat 500cm3 of water by 25c

    can someone help??

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    Just quoting in Fox Corner so she can move the thread if needed
    (Original post by Fox Corner)

    First I'd calculate the mass of water you have. Density = mass/volume, so mass = density * volume.

    The density of water is 1000kgm-3, so we convert the volume of water, 500cm3, into m3, which is 500 x 10-6 or 5 x 10-4 m3. Multiplied together, this gives a mass of about 0.5kg.

    From here, you can calculate the energy needed to heat the water by 25 degrees using Q=mc\Delta T where Q is the energy, m is the mass of water, c is the specific heat capacity of water, and \Delta T is the change in temperature.

    Q = 0.5 * 4180 * 25 = 52250 J = 52.25 kJ

    Now, you know that the energy provided when combusting 1 mole of ethanol is 1371 kJ. We only need 52.25 kJ, which by dividing means we need about 0.038 moles of ethanol.

    Finally, you can use n = \frac {m}{m_R} rearranged to m = nM_r where M_r is the relative molecular mass of ethanol, about 46.

    m = 0.038 * 46 = 1.75g of ethanol

    I think this is right anyway. Hope it helps!

    Have a look at 'chemistry student teacher' on youtube. He has a couple of good videos on enthalpy that actually goes through similar calculations have a watch ans itll help im sure!!!

    (Original post by tomlfc)
    The standard enthalpy change of combustion of ethanol is -1371 kjmol-1. Calculate the mass of ethanol needed to heat 500cm3 of water by 25c

    can someone help??
    First of all I'd find out what one mole of ethanol weighs. C2H5OH = 46g.

    After that you would associate this mass with the energy created when it burns as the enthalpy change of combustion is the energy change when mole of a substance burns in oxygen.
    So 46g ---> -1371kj

    Now I would do an Eh=CmDeltaT calculation to find out the energy need to increase the temperature of 500cm^3 of water by 25 degrees celsius. Note the mass of water has to be in litres when doing the calculation.
    Eh=4.18x0.5x25 = -52.25kj

    Then you would calculate the associated mass using the enthalpy of combustion.

    -1371kj ---> 46g
    -1kj ---> (46/1371)
    -52.25kj---> (46/1381)x52.25
    -52.25kj---> 1.75g

    1.75g is needed to heat the 500cm^3 of water by 25 degrees celsius.

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