When does the change in enthelpy=change in internal energy?

In summary, ΔU=ΔH is justified for processes such as combustion in a bomb calorimeter when the number of moles of gas remains constant. However, in cases where the number of moles does not change, like in an irreversible expansion of a perfect gas, ΔH may still differ from ΔU due to a change in temperature. This justification can be applied whenever a reaction is given and the change in molar internal energy is known, as long as the moles of gas remain constant on both sides of the equation. This also implies that the temperature of a sample in a bomb calorimeter remains constant. Furthermore, using the equation H = U + PV, it can be seen that ΔH = ΔU whenever Δ
  • #1
LogicX
181
1
Sometimes in my book, a problem justifies ΔU=ΔH for a process, such as combustion in a bomb calorimeter, by saying that since the number of moles of gas doesn't change, they are equal.

In other questions, the number of moles doesn't change (such as an irreversible expansion of a perfect gas) but still, ΔH is different from ΔU because there is a change in temperature, so ΔH= ΔU + Δ(nRT)= ΔU + nRΔT

When do you use the first justification? Only in a bomb calorimeter? Any time I am given a reaction and the change in molar internal energy for that reaction where the moles of gas is the same on both sides of the equation? Does this mean that the temperature of a sample in a bomb calorimeter is constant?
 
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  • #2
H = U + PV, so ΔH = ΔU whenever Δ(PV) = 0.
 

1. What is the definition of enthalpy and internal energy?

Enthalpy is a measure of the total heat content of a system, including both the internal energy and the work done by or on the system. Internal energy, on the other hand, is the total energy contained within a system, including kinetic and potential energies of its particles.

2. How are enthalpy and internal energy related?

The change in enthalpy is equal to the change in internal energy plus the product of pressure and volume. This can be expressed as the equation: ΔH = ΔU + PΔV.

3. When does the change in enthalpy equal the change in internal energy?

The change in enthalpy equals the change in internal energy when the system is at constant pressure and there is no work being done on or by the system. This is known as an isobaric process.

4. Can the change in enthalpy and change in internal energy ever be different?

Yes, the change in enthalpy and change in internal energy can be different if the system is not at constant pressure or if work is being done on or by the system. In these cases, the change in enthalpy will also take into account the work done by or on the system.

5. How is the change in enthalpy and change in internal energy measured?

The change in enthalpy and change in internal energy can be measured using calorimetry, which involves measuring the heat transfer between the system and its surroundings. The change in enthalpy can also be calculated using the equation ΔH = q + w, where q is the heat transferred and w is the work done by or on the system.

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