## What is Enthalpy?

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 |

10^{12} |
Tera |
T |

10^{9} |
Giga |
G |

10^{6} |
Mega |
M |

10^{3} |
Kilo |
k |

10^{2} |
Hecto |
h |

10^{-2} |
Centi |
c |

10^{-3} |
Milli |
m |

10^{-6} |
Micro |
\(\mu\) |

10^{-9} |
Nano |
n |

10^{-12} |
Pico |
p |