If water vapour is assumed to be a perfect gas, molar enthalpy change for vapourisation of 1 mol of water at 1bar and 100°C is 41kJ mol -1 . Calculate the internal energy change, when

(i) 1 mol of water is vaporised at 1 bar pressure and 100°C.

(ii) 1 mol of water is converted into ice.

Molar enthalpy equation for an ideal or perfect gas is

 

 

 is the molar enthalpy = 41kJ mol -1

 is the internal energy change - to be calculated

 is the no. of moles created or absorbed during the chemical reaction

R is the gas constant = 8.314 J mol -1 K-1

T is temperature in K = 100 degree C = 373 K

 

The internal energy change, when

(i) 1 mol of water is vaporised at 1 bar pressure and 100C.

Let us consider the reaction,

 

 

 =  

  = 3-2

  = 1

Substituting in molar enthalpy equation for an ideal or perfect gas, the imternal energy change is,

 

 

The internal energy change, when

 (ii) 1 mol of water is converted into ice.

Let us consider the reaction,

 

 

 =  

   = 1-1

  = 0

Substituting in molar enthalpy equation for an ideal or perfect gas, the imternal energy change is,

 

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