Question

A process takes place at constant temperature and pressure. Then:
A. $\Delta H=\Delta E$
B. $\Delta H=T\Delta S$
C. $\Delta H=0$
D. $\Delta S=0$

Answer 1

Enthalpy is dependent on various state functions like temperature, pressure and internal energy. The SI unit of enthalpy is joule.

Complete step by step answer:
- As we know that enthalpy, represented by (H) and internal energy represented by (U) depends upon temperature. So, if temperature is constant then enthalpy and internal energy will also be constant.
- As, enthalpy and internal energy are constant, so there is no change. So, enthalpy and internal energy will be equal to zero, that is: $\Delta H=0$ and also $\Delta U=0$
So, the enthalpy will be equal to zero.
- Hence, we can say that correct option is (C), that is $\Delta H=0$

Additional Information:
- The enthalpy is the sum of the pressure, volume and internal energy, the formula of enthalpy is given as: H=U+PV,
where, H is the enthalpy, U is the internal energy, P is the pressure and V is the volume.
- As we know that the total enthalpy of the system cannot be measured directly, because there are components in internal energy that are not known.
- The enthalpy is independent of its pressure and is dependent only on its temperature. That in turn correlates to its internal energy.
So, the correct answer is “Option C”.

Note: - By measuring the change in enthalpy we can determine whether a reaction is exothermic (release of heat, where there will be negative change in enthalpy) or endothermic (heat absorbed, where there will be positive change in enthalpy).
-It is also used to calculate the heat of reaction in a chemical reaction.