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CBSE Class 11-science Answered

Discuss the application of the first law of thermodynamics to Boiling process :
Asked by Topperlearning User | 04 Jun, 2014, 01:23: PM
Expert Answer

Boiling process : When a liquid is heated, it starts boiling at a particular temperature called its boiling point.  At its boiling point (pressure is kept constant), the unit mass of the liquid requires a definite amount of energy to change from the liquid to the vapour state.  The amount of heat supplied is known as the latent heat of vaporisation of the liquid.

Consider a liquid of mass m at its boiling point.  Let P be the external pressure and L be the latent heat of vaporisation.

Therefore, Amount of heat required to convert the whole of the liquid into vapour, dQ =mL.  From the first law of thermodynamics,

dQ = dU + dW

dU = Uf - Ui and dW = PdV = P(Vf - Vi)

where the subscripts i and f denote the initial and final values respectively.

Therefore, mL = (Uf - Ui) + P(Vf -Vi)

or (Uf - Ui) = mL - P(Vf - Vi)

Thus, knowing m, L, Vf, Vi, the increase in the internal energy can be calculated.

Answered by | 04 Jun, 2014, 03:23: PM
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