Matt's Question: Heat Transfer and Entropy Change for H2/N2 Mixture

[Synergyx 26]

Hey everyone, I'm new to the forums here and just have a question.

Homework Statement

A piston-cylinder device contains a mixture of .2kg of H₂ and 1.6kg of N₂ at 100kPa and 300K. Heat is now transferred to the mixture at constant pressure until the volume is doubled. Assuming constant specific heats at the average temperature, determine (a) the heat transfer and (b) the entropy change of the mixture

The Attempt at a Solution

My multiple attempts included using equations from examples in the chapter. The only thing with the example is that its with air and not H₂ or N₂. I don't know if this really makes a difference but I've tried m C deltaT to find the heat and gotten an answer of around 29kJ but my understanding of the problem is that I'm looking for the flow of the heat transfer in kJ/min which involves using the partial pressure and specific humidity. The main thing I'm lacking in that process is the I have no flow in kg/min. Any help would be much appreciated.

Matt

 

[Mapes]

Hi Synergyx 26, welcome to PF. Please show your calculations if you'd like others to comment; right now there's no way to know what you've tried.

What assumptions can you make about the gases involved that might help you to extract a temperature change?

Humidity and flow are not relevant factors in this problem.

 

[Mene]

,

Thanks for your question. It's great to see you using examples from the chapter to try and solve this problem. The fact that the example uses air instead of H2 and N2 should not make a significant difference, as long as you are using the correct equations and values for the specific heats.

To solve this problem, you will need to use the First Law of Thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. In this case, the work done by the system can be assumed to be negligible, so the change in internal energy will be equal to the heat added.

To find the heat transfer, you can use the equation Q = mCΔT, where Q is the heat transfer, m is the mass of the mixture, C is the specific heat capacity, and ΔT is the change in temperature. Since the specific heat capacity is constant, you can use the average temperature of the mixture, which is given as 300K.

To find the mass of the mixture, you can use the ideal gas law, PV = mRT, where P is the pressure, V is the volume, m is the mass, R is the gas constant, and T is the temperature. You can solve for m and substitute in the given values to find the mass of the mixture.

To find the entropy change, you will need to use the Second Law of Thermodynamics, which states that the total entropy change of a system and its surroundings must be greater than or equal to zero. You can use the equation ΔS = Q/T, where ΔS is the change in entropy, Q is the heat transfer, and T is the absolute temperature. Again, you can use the average temperature of the mixture to find the change in entropy.

I hope this helps you solve the problem. Let me know if you have any further questions or if you need clarification on any of the steps. Good luck!