Heat capacity

[SophiaBarden 2E]

I have memorized that heat capacity increases with molar complexity, but could anyone explain the concepts or reasoning behind this statement? I know the final is going to be very conceptual so I have been reviewing the reasoning behind things that i had simply memorized before

[Mikayla Kwok 3K]

I think it's because the more complex a molecule is, the more energy it takes to break/rearrange its intermolecular bonds. If a lot energy is required to break/rearrange its bonds, you need even more energy to raise the kinetic energy of these particles and, therefore, its temperature. As a result, the substance's heat capacity becomes higher.

[Sami Siddiqui 1J]

First, let me say that the internal energy of an ideal gas can be contributed by the kinetic energy of the substances in our system at hand. This kinetic energy can be broken down into three different categories: translational motion (movement as a trajectory motion), rotational motion, and vibrational motion (not important for this explanation). You'll find that, as you increase the complexity of the gaseous molecule (from monatomic to linear to nonlinear molecules), you're slowly increasing the overall kinetic energy by increasing the contribution of rotational motion to the internal energy (while also observing the same contribution by translational motion). In short, this increase in the internal energy with an increasing kinetic energy by more complex molecules leads to higher heat capacities, especially for those at constant volume. At constant volume, given that there is no expansion work, w = 0 and delta U = q + w = q + 0 = q. This means that C = q / delta T and C = delta U / delta T are equivalent. With an increase in internal energy with increasing molecular complexity, we can notice an increase in heat capacities on a quantitative level. If you wanted to convert the C value at constant volume to that at constant pressure, you would only need to add R (8.3145) to that total. Hope that helps!

[Karen Elrayes 1L]

You could also think of it as it would take more energy to break it's bonds in terms of heat capacity like changing it's state. The more complex it is the more heat is required to change its state.

[Kevin Zhang 3K]

more molecular complexity results in more bonds to heat up/break