Nomenclature & Units

Energy terms

Symbol	Quantity	Typical unit
\(E\)	Total energy	kJ
\(U\)	Internal energy	kJ, kJ/kg, or kJ/mol
\(E_k\)	Kinetic energy	kJ
\(E_p\)	Potential energy	kJ
\(H\)	Enthalpy	kJ, kJ/kg, or kJ/mol

Input / output terms

Symbol	Quantity	Typical unit
\(Q\)	Heat	kJ
\(\dot{Q}\)	Rate of heat transfer	kW
\(W\)	Work	kJ
\(\dot{W}\)	Rate of work (power)	kW
\(m\)	Mass	kg
\(\dot{m}\)	Mass flow rate	kg/s
\(n\)	Moles	mol
\(\dot{n}\)	Molar flow rate	mol/s

Process Variables & State Properties

Symbol	Quantity	Typical unit
\(T\)	Temperature	K or °C
\(P\)	Pressure	kPa (or bar)
\(V\)	Volume	m³
\(\rho\)	Density	kg/m³
\(v\)	Specific volume	m³/kg
\(x_i\)	Mole fraction of component \(i\)	— (dimensionless)
\(C_P\)	Heat capacity (constant pressure)	kJ/(kg·K) or kJ/(mol·K)
\(F\)	Degrees of freedom	—
\(C\)	Number of components	—
\(\pi\)	Number of phases	—

Reaction, Mixing & Phase-Change Terms

Symbol	Quantity	Typical unit
\(\Delta H_{vap}\)	Latent heat of vaporisation	kJ/kg or kJ/mol
\(\Delta H_{rxn}\)	Enthalpy of reaction	kJ/mol
\(\Delta H_f\) (or \(\Delta H_f^\circ\))	Enthalpy of formation (standard)	kJ/mol
\(\Delta H_{dissolution}\)	Enthalpy of dissolution / mixing	kJ/kg or kJ/mol
\(\xi\)	Extent of reaction / mixing	mol or kg

Subscripts & notation

• \(sys\) — the system · \(in\) / \(out\) — streams crossing the boundary · \(ref\) — reference state
• \(i\) — careful: used both as an initial-state subscript (\(H_{sys,i}\)) and a component index (\(x_i\)) — context decides
• \(\dot{(\ )}\) — rate, per unit time
• \(\Delta\) — change, “final − initial”

Sign convention

\(Q\) and \(W\) are always defined as energy transferred into the system. Energy leaving (heat rejected, work done by the system) is negative.