Home»Math Guides»First Law of Thermodynamics Example Problem – Reversible Expansion
Finding Q,W,U,H (Heat, work, internal energy, enthalpy) when Heat Capacity of a Gas Is a Function of T, Expansion (First Law of Thermodynamics)
Sometimes you will find thermodynamics questions with gas expansion, but the heat capacity of the gas is not a constant; instead, it is a function of temperature.
These questions aren’t too difficult, you just need to integrate the function of C\(_p\) or C\(_v\) with respect to T in your thermodynamics equations.
The molar heat capacity of a gas is approximated and given as:
\[{C_{p,m}} = 6.15\frac{{cal}}{{molK}} + \left( {0.0031\frac{{cal}}{{mol{K^2}}}} \right)(T)\]
Assuming ideal gas behaviour:
a.) Find Q, W, (Delta H), (Delta U) when 2 mol of this gas is reversibly heated from 27 C to 127 C, with P fixed at 1 atm. (isobaric)
b.) Find Q, W, (Delta H), (Delta U) when 2 mol of this gas initially at 1 atm is reversibly heated from 27 C to 127 C with volume being fixed. (isochloric)
Let’s start with a.), the isobaric case.
Remember the relation between C\(_v\)\(_m\) and C\(_p\)\(_m\), which holds true even if C is a function of T. Also use the Kelvin temperatures as the integral bounds.
\[{C_{v,m}} = {C_{p,m}} – R\]
\[\Delta U = n\int {{C_{v,m}}} dT\]
\[\Delta U = 2\int_{300}^{400} {(0.0031T + 4.1641)dT} \]
\[\Delta U = 1050cal\]
\[\Delta H = n\int {{C_{p,m}}} dT\]
\[\Delta H = 2\int_{300}^{400} {(0.0031T + 6.15)dT} \]
\[\Delta H = 1447cal\]
\[W = – nR\Delta T\]
\[W = – (2mol)\left( {8.314\frac{J}{{molK}}} \right)(400K – 300K)\]
\[W = – 1662.80J = – 397cal\]
For isobaric expansion:
\[Q = \Delta U – W = \Delta H\]
Q = 1447 cal
Now let’s do part b, which is constant volume or isochloric expansion.
Since V is fixed, W = 0.
\[\Delta U = n\int {{C_{v,m}}} dT\]
\[\Delta U = 2\int_{300}^{400} {(0.0031T + 4.1641)dT} \]
\[\Delta U = 1050cal\]
\[\Delta H = n\int {{C_{p,m}}} dT\]
\[\Delta H = 2\int_{300}^{400} {(0.0031T + 6.15)dT} \]
\[\Delta H = 1447cal\]
And since the V is fixed (isochloric):
\[Q = \Delta U = 1050cal\]