Question

What is the total enthalpy change when 100.0 g of solid NH3 at -150.0 degrees Celcius is changed to gas at 100 degrees Celcius?

I've done this problem several times and keep getting the answer of 223 kj, but in class we were told the correct answer is 237 kj. Please show all your steps/work so I can see what I'm doing wrong. Thank you!

Answer 1

Mass of ammonia = 100 g = 0.1 kg

Initial temperature = - 150 oC

Final temperature = 100 oC

Melting point of ammonia = -77.73 °C

Boiling point of ammonia = -33.34 °C

Enthalpy of sublimation of ammonia, H sub = 332.17 kJ/ kg

Enthalpy of vaporization of ammonia, H vap = 1369 kJ/ kg

Heat Capacity of liquid ammonia, Cp = 4.7 J/g-K

Heat Capacity of gas ammonia, C = 2.06 J/g.K

Enthalpy change from - 150 oC to - 77.73 oC:

H1 = m*c*T

= 100 * 4.7 * (-77.73 + 150)

= 100 * 4.7 * 72.27

= 33.97 kJ

Enthalpy change in melting from solid to liquid at -77.73 oC:

H2 = m * H sub

= 0.1 kg * 332.17 kJ/ kg

= 33.217 kJ

Enthalpy change from - 77.73 oC to -33.34 oC:

H3 = m*c*T

= 100 * 4.7 * (-33.34 + 77.73)

= 100 * 4.7 * 44.39

= 20.86 kJ

Enthalpy change in heating from liquid to gas at -33.34 oC:

H4 = m * H vap

= 0.1 kg * 1369 kJ/ kg

= 136.9 kJ

Enthalpy change from -33.34 oC oC to 100 oC:

H5 = m*c*T

= 100 * 2.0 * (100 + 33.34)

= 100 * 2.06 * 133.34

= 27.46 kJ

Total enthalpy change, H = 33.97 + 33.217 + 20.86 + 136.9 + 27.46

= 252.4 kJ