Enthalpy

Enthalpy

What is Enthalpy?

When a process takes place at constant pressure, the heat absorbed or released is equal to the Enthalpy change. Enthalpy is sometimes known as “heat content”, but “enthalpy” is an interesting and unusual word, so most people like to use it. Etymologically, the word “entropy” is derived from the Greek, meaning “turning” and “enthalpy” is derived from the Greek meaning “warming”. As for pronunciation, Entropy is usually stressed on its first syllable, while enthalpy is usually stressed on the second. Enthalpy(H) is nothing but the sum of the internal energy(U) and the product of pressure(P) and volume(V).

Enthalpy H can be written as,

H = U + pV

Where, H = Enthalpy of the system

U = Internal energy of the system

p = Pressure of the system

V = Volume of the system

Enthalpy is not measured directly, however, the change in enthalpy (ΔH) is measured, which is the heat added or lost by the system. It is entirely dependent on the state functions T, p and U.

Enthalpy can also be written as

\(\Delta H=\Delta U+\Delta PV\)

At constant temperature, for the process heat flow(q) is equal to the change in enthalpy, this is represented as

\(\Delta H=q\)

Enthalpy Units

The Enthalpy is expressed as,

H = Energy/Mass

Any physical quantity can be represented by dimensions. The arbitrary magnitudes allocated to the dimensions are called units. There are two types of dimensions namely primary or fundamental and secondary or derived dimensions.

Primary dimensions are: mass, m; length, L; time, t; temperature, T

Secondary dimensions are the ones that can be derived from primary dimensions such as velocity(m/s2 ), pressure (Pa = kg/m.s2 ).

There are two unit systems currently available SI (International System) and USCS (United States Customary System) or English system. We, however, will use SI units exclusively in this course. The SI units are based on the decimal relationship between units.

SI Prefixes:

Factor Name symbol
1012 Tera T
109 Giga G
106 Mega M
103 Kilo k
102 Hecto h
10-2 Centi c
10-3 Milli m
10-6 Micro \(\mu\)
10-9 Nano n
10-12 Pico p

What is Thermodynamics?

Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter. First law of thermodynamics: one of the most fundamental laws of nature is the conservation of energy principle.
It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. The second law of thermodynamics: energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. Whenever there is an interaction between energy and matter, thermodynamics is involved. Some examples include heating and air‐conditioning systems, refrigerators, water heaters, etc.
Learn more about enthalpy, laws of thermodynamics and lot more. At BYJU’S we also provide free CBSE  Sample Papers and Study Notes for better understanding of the topics.

Practise This Question

The enthalpy change for the reaction of 50.00 ml  of ethylene with 50.00 ml of H2 at 1.5 atm pressure is ΔH = -0.31 kJ. The value of ΔE will be