Question

Calculate the enthalpy change when 1.15ng of ice at -55.0c is converted to steam at 175.00c.

Answer 1

mass of water = 1.15 x 10^-9 g

moles of water = 81 / 18 = 4.5

-15.0 °C ----------> 0.0 °C ------------> 100.0 °C -------------> 175.0 °C

                 1           2          3             4              5

in this process 5 conversions are required.

Q1 = m Cp dT = 1.15 x 10^-9 x 2.09 x (0 + 15) = 3.60 x 10^-8 J

Q2 = n x delta H fusion = 6.39 x 10^-11 x 6.01 x 10^3 = 3.84 x 10^-7 J

Q3 = m Cp dT = 1.15 x 10^-9 x 4.18 x (100 - 0) = 4.81 x10^-7 J

Q4 = n x delta H vap = 6.39 x 10^-11 x 40.7 x 10^3 = 2.60 x 10^-6 J

Q5 = m Cp dT = 1.15 x 10^-9 x 2.01 x (175 - 100) = 1.73 x 10^-7 J

total heat = Q1 + Q2 + Q3 + Q4 + Q5

= 3.67 x 10^-6 J

enthalphy change = 3.67 x 10^-9 k J / (1.15 x10^-9 / 18 )

= 57.4 kJ / mol