Heat Capacity of a Gas
The heat capacity of a substance tells us how much heat is required to raise a certain amount of the substance by one degree. For a gas we can define a molar heat capacity C - the heat required to increase the temperature of 1 mole of the gas by 1 K.
Q = nCΔT
The value of the heat capacity depends on whether the heat is added at constant volume, constant pressure, etc.
Heat Capacity at Constant Volume
Q = nCVΔT
For an ideal gas, applying the First Law of Thermodynamics tells us that heat is also equal to:
Q = ΔEint + W, although W = 0 at constant volume.
| For a monatomic ideal gas we showed that |
ΔEint |
= |
| 3
|  |
| 2
|
|
nR ΔT |
Comparing our two equations
| Q = nCV ΔT and Q = |
| 3
| |
| 2
|
|
nR ΔT |
we see that, for a monatomic ideal gas:
| CV |
= |
| 3
| |
| 2
|
|
R |
For diatomic and polyatomic ideal gases we get:
| diatomic: CV |
= |
| 5
| |
| 2
|
|
R |
polyatomic: CV = 3R
This is from the extra 2 or 3 contributions to the internal energy from rotations.
Because Q = ΔEint
when the volume is constant, the change in internal energy can always be written:
ΔEint = n CV ΔT
Heat Capacity at Constant Pressure
For an ideal gas at constant pressure, it takes more heat to achieve the same temperature change than it does at constant volume. At constant volume all the heat added goes into raising the temperature. At constant pressure some of the heat goes to doing work.
Q = nCP ΔT
For an ideal gas, applying the First Law of Thermodynamics tells us that heat is also equal to:
Q = ΔEint + W
At constant pressure W = PΔV = nR ΔT
| For a monatomic ideal gas, where |
ΔEint |
= |
| 3
| |
| 2
|
|
nR ΔT |
, we get: |
| Q |
= |
| 3
| |
| 2
|
|
nR ΔT |
+ nR ΔT |
= |
| 5
| |
| 2
|
|
nR ΔT |
| So, for a monatomic ideal gas: CP |
= |
| 5
| |
| 2
|
|
R |
For diatomic and polyatomic ideal gases we get:
| diatomic: CP |
= |
| 7
| |
| 2
|
|
R |
polyatomic: CP = 4R
The ratio CP / CV
It turns out that the ratio of the specific heats is an important number. The symbol we use for the ratio is γ. For a monatomic ideal gas we have:
| γ |
= |
| CP
| |
| CV
|
|
= |
| 5R
| |
| 2
|
|
* |
| 2
| |
| 3R
|
|
= |
| 5
| |
| 3
|
|