Is delta H=0 for isothermal process?

In summary, the conversation discusses the concept of delta H, which is the change in enthalpy of a system. It is calculated by adding the change in internal energy (delta U) and the change in pressure and volume (delta (PV)). However, for an isothermal process, delta U is 0 and delta (PV) is also 0, resulting in a delta H of 0. This may seem counterintuitive, as work is still being done by the system, but the energy gained through work is equal to the energy lost through the process, keeping the internal energy and enthalpy constant. Q (heat) and H (enthalpy) are different concepts, with Q being a process function and H being a
  • #1
sachin123
121
0
1.>Consider a gas in a vessel with a piston on top.Let it expand to a greater volume.
So,
delta H=delta U+delta(PV)
but delta U is 0 as it is isothermal.
now,as the number of moles of gas remains constant,delta(PV) is nR(delta T) which is again 0.
SO delta H is 0.

Thats what my book says.

BUt,my questions is,when work is done by the gas on the piston,without change in internal energy,then it should have absorbed some energy from the surroundings.(conservation of energy)
Correct?
then shouldn't delta H be a non zero value for an isothermal process?

please help.Thank You
 
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  • #2
Your assumptions are correct but your conclusion is not. Why do you think the system expands? It does work to expand and the energy the system got to do that is the change in Q, but the system lost all the energy it got by doing work and the internal energy stayed constant through the process, hence H is constant also.
 
  • #3
Then what's the difference between Q and H?aren't they the same?though their definitions differ,I have not understood any difference between them.Can you clarify it?
 
  • #4
H (enthalpy) is a state quantity, that is when you go in a circle (from one state, to state two, to state three to state four and end again in state one) then H is the same before and after the circle but Q (heat) is a process function and is dependent on how you do things.
 
  • #5
Thank you ojs,
so finally ,delta H is 0 for an isothermal process right?and my assumption that q and H are the same was wrong.
 

Related to Is delta H=0 for isothermal process?

1. What is delta H and what does it represent in thermodynamics?

Delta H, also known as enthalpy change, is a measure of the change in energy of a system during a process. In thermodynamics, it represents the heat added to or removed from a system at constant pressure.

2. What does it mean for delta H to equal zero in an isothermal process?

In an isothermal process, the temperature of the system remains constant. Therefore, if delta H equals zero, it means that no heat is being added or removed from the system, and the internal energy of the system remains the same.

3. Can delta H ever be negative in an isothermal process?

No, delta H cannot be negative in an isothermal process. This is because the temperature remains constant, so the internal energy of the system cannot decrease or increase.

4. How is delta H related to the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted. This is represented by the equation: ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done on the system. In an isothermal process, where temperature remains constant, ΔU is equal to zero. Therefore, ΔH = Q, meaning that delta H is directly related to the heat added to the system.

5. Why is delta H=0 for an isothermal process considered a reversible process?

In a reversible process, there is no net change in the state of the system or the surroundings. In an isothermal process where delta H equals zero, the system remains in a constant state, and no heat is transferred between the system and surroundings. This makes it a reversible process.

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