Thermodynamics:
Problem-Solving Example
Thermodynamics Gateway Page
In this module:
Problem 1
Problem 2
Problem 3

How much water is required to dissipate the heat released when 1 kg of phosphorus is condensed in step 2?

P4 (g) P4 (s, red)

To perform this calculation, start by calculating the heat flow when 1 kg of phosphorus is cooled and condensed. Assume it starts at 1450 ºC and ends up at 25 ºC. Gaseous phosphorus is deposited (the opposite of sublimed) at 590 ºC. The heat capacities of gaseous and solid phosphorus are 0.5420 and 0.2328 J/g ºC, respectively.

Step1: Identify the system and surroundings.

Step 2: Identify and assign signs to all the kinds of energy and work entering or leaving the system.

Step 3: Predict the units your answer should have.

Step 4: Predict the approximate magnitude of your answer.

Answer:


Correct! Click on the next button to do the second part of the problem.

See the calculation below. Click on the next button to do the second part of the problem.
q1=m cgas DT=0.5420 J/g ºC • (590 - 1450) ºC • 1000 g/kg
   = -466.12 kJ/kg
q2=m csolid DT=0.2328 J/g ºC • (25 - 590) ºC • 1000 g/kg
   = -131.532 kJ/kg
q=q1 + q2=-598 kJ/kg phosphorus

While numerically correct, your answer has the wrong number of significant digits. Try again.
Be careful with your units.
That is incorrect. Try again.
Remember to include units with your answer.
Since the phosphorus is losing heat, the value of q for the system must have a negative sign.