# Is enthelpy change during isothermal process zero?

• RUSI16
Therefore, even though the temperature may remain constant during a phase transformation, the enthalpy can still change due to changes in pressure or state of aggregation. The enthalpy of vaporization or fusion represents the amount of energy required to change the state of a substance at a given pressure and temperature. In summary, while enthalpy change may be zero during an isothermal process, the enthalpies of vaporization or fusion are still significant as they represent the amount of energy required for a phase transformation to occur.
RUSI16
is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.

RUSI16 said:
is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.
H = U +PV so dH =dU + PdV + VdP. For an ideal gas, dU =0 for an isothermal process, so dH = PdV +VdP.

For melting ice which occurs at constant T, there may be negligible change in P and V, but is ΔU =0?

AM

RUSI16 said:
is enthalpy change during isothermal process zero? if it is zero then what is the significance of enthalpies like enthalpy of vapourization or fussion since during phase transformation temperature remains constant.
You answered your own question. Enthalpy is not just a function of temperature. It is also a function of pressure, state of aggregation, and chemical composition.

## 1. What is enthalpy change during an isothermal process?

Enthalpy change during an isothermal process refers to the amount of heat energy absorbed or released by a substance while its temperature remains constant. This means that there is no change in the internal energy of the substance, and the energy is solely used to perform work.

## 2. Why is enthalpy change during an isothermal process considered to be zero?

The enthalpy change during an isothermal process is considered to be zero because the temperature of the substance remains constant. This means that there is no change in the internal energy of the substance, and all the energy is used to perform work instead of increasing the temperature.

## 3. Is enthalpy change during an isothermal process always zero?

No, enthalpy change during an isothermal process is not always zero. It is only zero when the temperature of the substance remains constant. If there is a change in temperature, then the enthalpy change will not be zero.

## 4. How is enthalpy change related to heat transfer during an isothermal process?

Enthalpy change is directly related to heat transfer during an isothermal process. This is because the enthalpy change represents the amount of heat energy absorbed or released by the substance during the process. The higher the enthalpy change, the more heat is transferred.

## 5. Can enthalpy change during an isothermal process be negative?

Yes, enthalpy change during an isothermal process can be negative. This occurs when the substance releases heat energy during the process. In this case, the enthalpy change will have a negative value, indicating an exothermic reaction.

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