thermodynamics chemistry

[warilad]

plz help me for this question,

Suggest how the internal energy of van der waals gas vary with volume at constant pressure?

 

[Shahroz95]

is this question from the past papers?

 

[beacon_of_light]

That's an isobaric change or a process occurring at constant pressure.
Apply first law of thermodynamics.
Do you want a relation b/w Volume and temperature?
A general overview: If the volume of gas increases, the gas expands. Some of the work is done against the atmosphere during expansion so energy is required for that. Thereby, this energy is provided by internal energy. The internal energy of the remaining gas molecules decreases, thereby, the temperature of the gas falls. Similarly, the temperature of the gas would rise during compression!
Hope that helps

 

[warilad]

owh thanks for this info..but how about if the van der waals gas vary with volume at constant temperature?

 

[beacon_of_light]

At constant temperature, that's an isothermal change. Since the temperature of the gas remains constant, the change is internal energy is zero. The work done on or by the gas is done at the expense of the energy input. If energy is supplied to the system, work is done by the system(expansion) . If energy is taken out of the system, work is done on the gas(compression). The whole of the energy input is used to do work against the surroundings and no part of energy is expended in internal energy. Thereby, temperature remains constant!

 

[warilad]

wow thanks a lot..now i understand the law..means van der waals is vary with the first law of thermodynamics..am i right? are u lecturer? so good your chemistry

 

[beacon_of_light]

lol no I am a student!

You have to modify the first law according to the conditions such as isochoric, adiabatic or isothermal.

thanks

 

[warilad]

owh..could you solve this question?
1) A sample consisting of 1.5 mol of perfect gas molecules, initially at 230kPa and 315K undergoes a reversible adiabatic expansion until its pressure reaches 170kPa. Given that its molar constant-pressure eat capacity is 280 J/K/mol,, calculate the final volume, final temperature and the work done.

2) A system consists of 2.0mol CO2 (perfect gas) at 25C is confined to a cylinder of cross-section 10cm2 at 10atm. The gas expands adibatically and irreversibly against a constant pressure of 1.0atm. Calculate w, q, change in internal energy, change in enthalpy energy, cange in temperature when the piston has moved 20cm.

i try many times already but cant get the answer. Could you plz help me solve the problem

 

[beacon_of_light]

For question # 1 : Try this method.
Find initial volume using ideal gas equation, PV=nRT . Then find the final volume by using P1V1 = P2V2 . Find the work done by using W =P . delta or change in volume. Some other equations can also to be used but they aren't included in A-level's syllabus. Basic concepts are important.

For question#2: For work done, calculate the initial volume by PV=nRT. The final volume is 10x20 ...do convert into SI units... find change in volume... then P x change in V ... q=0 because the change is adiabatic... change in internal energy is equal to the work done by the gas...