Anonymous asked in Science & MathematicsChemistry · 1 decade ago

chem hw help? thanks:)?

find the molar enthalpy of vaporization for a substance, given that 3.21 mol of the substance absorbs 28.4 kJ. of energy as heat when the substance changes from a liquid to a gas.

thank you!

2 Answers

  • 1 decade ago
    Favorite Answer

    Heat of Vaporization


    The heat of vaporization is the heat that is absorbed to transform a substance from its liquid state to its vapor, that is, to boil or evaporate the liquid substance completely. In common usage, the heat of vaporization is used in place of the more precise term the enthalpy of vaporization, which has the symbol ()Hvap. The enthalpy of vaporization is the heat of vaporization for vaporizing one mole of the substance under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process. A heat of vaporization for a substance is only valid for conversion of the pure liquid to the pure gaseous state of the substance.

    Your specific question...

    Hv is measured as J/mol or kJ/mo

    For you,

    Hv = kJ/mol

    Hv of substance =28.4/3.21

    Hv = 8.847 kJ/mol

    More from Wikipedia...

    Enthalpy of vaporization

    From Wikipedia, the free encyclopedia

    Temperature-dependency of the heats of vaporization for Water, Methanol, Benzene, and Acetone

    The enthalpy of vaporization, (symbol ΔvH), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas.

    It is often measured at the normal boiling point of a substance; although tabulated values are usually corrected to 298 K, the correction is often smaller than the uncertainty in the measured value.

    The heat of vaporization is temperature-dependent, though a constant heat of vaporization can only be assumed for small temperature ranges and below Tr<<1.0. The heat of vaporization diminishes with increasing temperature and it vanishes completely at the critical temperature (Tr=1) because above the critical temperature the liquid and vapor phases don't coexist anymore.


    Values are usually quoted in J/mol or kJ/mol (molar heat of vaporization), although kJ/kg or J/g (specific heat of vaporization), and older units like kcal/mol, cal/g and Btu/lb are sometimes still used, among others.

    Latent heat

    From Wikipedia, the free encyclopedia

    In thermochemistry, latent heat is the amount of energy in the form of heat released or absorbed by a chemical substance during a change of state (i.e. solid, liquid, or gas), or a phase transition.[1][2]

    The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden. The term has now been replaced by "enthalpy of transformation".

    Two latent heats (or enthalpies) are typically described: latent heat of fusion (melting), and latent heat of vaporization (boiling). The names describe the direction of heat flow from one phase to the next: solid → liquid → gas.

    The change is endothermic, i.e. the system absorbs energy, when the change is from solid to liquid to gas. It is exothermic (the process releases energy) when it is in the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains more water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

    Latent heat equation

    The equation for latent heat is:


    Q is the amount of energy released or absorbed during the change of phase of the substance (in joules),

    m is the mass of the substance,

    L is the specific latent heat for a particular substance (J kg-1).

    In other words, specific latent heat is found when energy is divided by mass.

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  • Anonymous
    1 decade ago

    damn.... how did i pass chem

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