Question

Calculate the enthalpy change, ΔH, for the process in which 14.3 g of water is converted from liquid at 4.0 ∘C to vapour at 25.0 ∘C . For water, ΔvapH = 44.0 kJ mol−1 at 25.0 ∘C and Cs = 4.18 J g−1 ∘C−1 for H2O(l). Express your answer to three significant figures and include the appropriate units.

Answer 1

The following process involves different Heat

i) Q1 = Heat to raise the temperature of water from 4 to 25 ∘C

Q1 = Mass of water X change in temprature X specific heat of water

Q1 = 14.3 X (25-4) X 4.18

Q1 = 1255.25 Joules = 1.255 KJ

ii) conversion of water to vapours = Q2

Q2 = Moles of water X Delta H vapurization

Moles = Mass of water / Mol wt = 14.3 / 18 = 0.794 moles

Q2 = 0.794 X 44 = 34.936 KJ

Total heat or enthalpy change = Q1 + Q2 = 34.936 + 1.255 = 36.191 KJ