Question

P4B.9 The vapour pressure, p, of nitric acid varies with temperature as follows: 0/°C 0 20 40 50 70 80 90 100 p/kPa 1.92 6.38 17.7 27.7 62.3 89.3 124.9 170.9 Determine (a) the normal boiling point and (b) the enthalpy of vaporization of nitric acid.

Answer 1

Write the expression for Clausis clapeyron equation as follows: In (p)= - + ... (1) where, p is vapor pressure, AH is heat of vaporisation, Ois temperature, Ris gas constant and k is integral constant. (6) Draw the graph between In (p) vs by using the given data as follows:

(oC) |p(kPa) 1/8 in (p) 이 1.92 0.003663) 0.652325 6.38 0.003413 1.853168 17.7 0.003195 2.873565 27.7 0.003096 3.321432 62.3 0.002915 4.131961 89.3 0.002833 4.492001 124.9 0.002755 4.827513 170.9 0.002681 5.141079 y = -4565.x + 17.42 In(p) Series1 — Linear (Series1) O 0.001 0.002 0.003 0.004 1/8

From the graph, the straight line is y = - 4565x + 17.42 compare the above equation with equation (1) k = 17.42 slope. - AH yay -4565 AH = 4565xR AH = 4565x8.314 Jim ol AH = 37953J/mol Therefore, the enthalpy of vaporisation is 37953J/mol (a) At normal boiling point, the pressure is equal to atmospheric pressure. p= 1 atm Convert atm to kPa as follows: D = 1 am 101.325 kPa p = a 1 atm p= 101.325 kPa Substitute the correpsoni dng values in equation (1) and calculate e 37953J/mol In (101.325) = -2 - +17.42 8.314J/molkx 4.618 = 4565 +17.42 4565 = 17.42-4.618 4565 v 9 = 7802 8 = 356.6K 8 = 356.6-273 °C 0 = 83.6 °C Therefore, the normal boiling point is 183.6 °C]