USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

pecific Heats, Internal Energy and Enthalpy of·an Ideal Gas

The ideal gas is having its specific heats constant. It follows the equation Pv = RT. But real gas is having different specific heats at different temperatures and pressures. The change is less in case of pressure variation.


We have from 1st law

dq  = du + dw

or                                             Tds = du + pdu

or                                              ds = du / T + pdu / T


The specific internal energy u is assumed to be a function of T and v, i.e.,

u = f (T , u)

Differentiating equation (9.8) w.r.t. T when v is const.

Thus, u does not change when v changes at constant temperature.

We can also show if u = f (T, P);

Thus u does not change with P at aP T

constant temperature. It means u does not change unless T changes.

So                                u = f(T)


The equation (9.14) is valid for ideal gas only. Equation (9.14) is known as Joule’s law.

If                                 u=f(T,v)

Du = cu dt


Equation (9.15) is valid for ideal gas and for any process of ideal gas. For other substance

equation (9.15) is valid for constant volume process only.

Now                h = u + Pv = u + RT

dh = du + RdT

dh / dt = cu dt / dt + R =cu + R

cp = cu + R =or cp – cu = R


To note dh = cP dT also holds good for ideal gas even when pressure changes. but it is true for other substance for constant pressure process.


The ratio cp / cu  is written as γ

Cp / cu = γ

valid for ideal gas and is used in many computation.

Equations (9.16) and (9.17) give rise to

Cp = γR / γ -1 , and cu = R / γ -1

The S.I. unit of c1, and C1 is KJ/ kg K.

Molar or molal specific heats:

cp= M cp and C v =M cu.


Where cp, and cu” are molar or molal specific heats at constant pressure and constant

volume respectively.

Value of γ :                 γ = 5 /3 for monoatomic gas

= 7 /   for diatomic gas

= 4 /3 for polyatomic (more than two atoms) gas.

Thus                             1 < γ < 4 /3


So γ depends on the molecular structure of the gas.

cp and c1, for ideal gas depend only on y and R. C v and cp are independent of temperature

and pressure of the gas.