Name: Absolute Zero and Third Law of Thermodynamics
Uploaded: 2019-07-19
Duration: 20 min
Description: In this video, you will learn what absolute zero and the third law of thermodynamics is. You will also learn about the significance of the third law of therm...

Question

Explain the term enthalpy. How is it related to the internal energy of the system?

Accepted Answer

The energy stored within the substance or the system that is available for conversion into heat is called the heat content or enthalpy of the substance or the system.

According to first law of thermodynamics, we know that

$q = ∆ U - w$ .

Putting, $w = - P ∆ V$ .

$q_{p} = ∆ U + P ∆ V$ .

Suppose when the system absorbs $q_{p}$ joules of heat, its internal energy increases from $U_{1}$ to $U_{2}$ and the volume increases from $V_{1}$ to $V_{2}$ . Then, we have

$∆ U = U_{2} - U_{1}$ and $∆ V = V_{2} - V_{1}$ .

$q_{p} = (U_{2} - U_{1}) + P (V_{2} - V_{1})$ .

$q_{p} = (U_{2} + {PV}_{2}) - (U_{1} + {PV}_{1})$ .

The thermodynamic quantity $U + PV$ is called the heat content or enthalpy of the system and is represented by the symbol $H$ , i.e., the enthalpy may be defined mathematically by the equation,

$H = U + PV$ .

Enthalpy change accompanying a process may also be defined as the sum of the increase in internal energy of the system and the pressure-volume work done, i.e., the work of expansion.

$∆ H = ∆ U + P ∆ V$ .

Explain the term enthalpy. How is it related to the internal energy of the system?

Important Questions on Thermodynamics

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