Consider the following data on constant volume heat capaciti

Consider the following data on constant volume heat capacities of two gases: CO2 (28.2 J/K) and CS2 (40.9 J/K). From a molecular perspective, explain the difference between these two, as well as their deviation from the ideal gas (linear molecule) case.

Solution

Ans) Both are given heat capacities at constant volume. That is if heat is given at constant volume it means no mechanical work is done in these processes and there is only change in internal energy due to this heat.

For one degree rise in Kelvin CO2 requires 28.2 J and CS2 requires 40.9 J. Now temperature is a measure of molecular motion or agitation. More the intermolecular force more it is difficult to increase the molecular motion and consequently the temperature. So the CO2 molecules have a lesser intermolecular force of attraction that is they are held by internointermo forces with lower magnitude as compared to CS2. Saying the other way round CS2 has a greater intermolecular force among its molecules hence it requires a greater energy to increase their temperature.

An ideal gas is assumed to have no intermolecular forces of attraction between its molecules. But real gases aren\'t the same. Intermolecular forces do exist for real gases. Greater the intermolecular forces greater is the deviation from the ideal behaviour.

So CS2 (carbon disulfide) deviates at a greater extent from ideal behaviour whereas CO2 (Carbon dioxide) deviates from ideal behaviour to a lower extent as compared to CS2 because CO2 has a lower intermolecular force of attraction between its molecules.