How can heat change be measured under constant pressure?

In summary: Under constant volume they are identical.After all, in the former, the amount of energy released (if the reaction inside releases energy) can be calculated by multiplying the change in volume with the outside pressure, and this value of energy, as far as I know, should be equal to the energy of the change in temperature inside the latter system.This only applies if the system is an ideal gas and the reaction takes place while it is in contact with a thermal reservoir at the same temperature. Otherwise you cannot assume that the pressure of the gas will remain constant.Heat is not a state variable, i.e., it is not a property of a system, it is the energy that is thermally exchanged between two systems. This follows from the fact that
  • #1
TheExibo
55
1
So enthalpy is the heat content of a system at constant pressure. Enthalpy change is equal to the heat absorbed or evolved by the system at constant pressure. If my understanding is correct, a system whose temperature goes up will return back to that starting temperature if pressure is kept constant (i.e., its volume is allowed to expand). Therefore, is enthalpy, or heat change, measured in terms of the expansion?

Additionally, would enthalpy change and internal energy change (which is equal to the heat absorbed or evolved by the system at constant volume) not have the same values? After all, in the former, the amount of energy released (if the reaction inside releases energy) can be calculated by multiplying the change in volume with the outside pressure, and this value of energy, as far as I know, should be equal to the energy of the change in temperature inside the latter system.
 
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  • #2
Heat is not a state variable, i.e., it is not a property of a system, it is the energy that is thermally exchanged between two systems. This follows from the fact that ##\delta Q## is not an exact differential.
 
  • #3
Enthalpy change is not $\Delta U+P\Delta V$. It is $$\Delta U+\Delta (PV)$$. So, even at constant volume, if the pressure changes, the enthalpy change is not equal to the internal energy change.
 
  • #4
The relationship between enthalpy ##H## and internal energy ##U## is ##H=U+PV##.
In a reversible process we have ##dU=dQ+dW=dQ-PdV##, and we have ##dH=dQ+VdP##.
So ##\Delta H = \Delta Q + \int_{P_1}^{P_2} V dP##. If the pressure is indeed constant, this becomes ##\Delta H=\Delta Q##, which is then independent of volume.
Similarly ##\Delta U = \Delta Q - \int_{V_1}^{V_2} PdV##. If the volume is constant, this becomes ##\Delta U=\Delta Q##, which is independent of pressure.
In both cases the change in enthalpy respectively in internal energy is simply the heat that is applied to the system. Keep in mind though that these are different processes with different final states.
 
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  • #5
TheExibo said:
So enthalpy is the heat content of a system at constant pressure.

No, it isn't. Enthalpy is fedined as

[itex]H = U + p \cdot V[/itex]

TheExibo said:
Enthalpy change is equal to the heat absorbed or evolved by the system at constant pressure.

Yes that's correct and the reason for the definition of enthalpy. Calorimetirc measurements of internal energy would require constant volume which is much harder to achieve.

TheExibo said:
If my understanding is correct, a system whose temperature goes up will return back to that starting temperature if pressure is kept constant (i.e., its volume is allowed to expand).

I'm afraid your understanding is not correct.

TheExibo said:
Therefore, is enthalpy, or heat change, measured in terms of the expansion?

Change of enthalpy is measured in terms of exchanged heat under constant pressure. Expansion is just a possible side effect of the constant pressure and irrelevant for the measurement.

TheExibo said:
Additionally, would enthalpy change and internal energy change (which is equal to the heat absorbed or evolved by the system at constant volume) not have the same values?

No. Under constant pressure they differ by the volumetric work.
 

Related to How can heat change be measured under constant pressure?

1. How is heat change measured under constant pressure?

Heat change can be measured under constant pressure using a calorimeter. A calorimeter is a device that is designed to measure the heat released or absorbed during a chemical reaction or physical change. It works by measuring the change in temperature of a known amount of substance, typically water, as heat is transferred to or from the system.

2. What is specific heat capacity and how is it related to heat change measurement?

Specific heat capacity is a measure of the amount of heat required to raise the temperature of a substance by 1 degree Celsius. It is related to heat change measurement because it allows us to calculate the amount of heat released or absorbed by a substance using the equation Q = m x c x ΔT, where Q is the heat change, m is the mass of the substance, c is the specific heat capacity, and ΔT is the change in temperature.

3. Can heat change be measured under constant pressure in a closed system?

Yes, heat change can be measured under constant pressure in a closed system. This is because in a closed system, the pressure remains constant while the volume may change. This allows us to use the equation Q = ΔH, where Q is the heat change and ΔH is the change in enthalpy, to measure the heat change under constant pressure.

4. How does a bomb calorimeter measure heat change under constant pressure?

A bomb calorimeter is a type of calorimeter that is used to measure the heat of combustion of a substance. It works by placing the substance in a sealed container, or "bomb," surrounded by water. The substance is then ignited, causing it to combust and release heat, which is absorbed by the surrounding water. The change in temperature of the water can then be used to calculate the heat change under constant pressure.

5. Are there any limitations to measuring heat change under constant pressure?

One limitation to measuring heat change under constant pressure is that it assumes that the pressure remains constant throughout the process. In reality, pressure may change due to factors such as changes in volume or the release of gases. Additionally, the heat capacity of the calorimeter itself must be taken into account in order to obtain accurate measurements. Other factors such as heat loss to the surroundings may also affect the accuracy of the measurement.

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