Q.Critical temperature for carbon dioxide and methane are 31.1 °C and –81.9 °C respectively. Which of these has stronger intermolecular forces and why?

Step 1 – List the given critical temperatures

Step 2 – Recall the relationship between $T_c$ and intermolecular forces

A HIGHER critical temperature means the gas can still be liquefied (i.e. its molecules can still be held together by intermolecular attraction) at a comparatively higher temperature — this directly indicates STRONGER intermolecular forces of attraction. A gas with weak intermolecular forces needs to be cooled to a much lower temperature before those weak attractions can overcome thermal (kinetic) energy and cause liquefaction.

Step 3 – Compare

Since $T_c(\text{CO}_2) = 304.25\ \text{K}$ is much higher than $T_c(\text{CH}_4) = 191.25\ \text{K}$:

Step 4 – Conclusion …

Compare the two critical temperatures directly: the gas with the higher $T_c$ has stronger intermolecular attractive forces (it resists the di …

• Assuming lower $T_c$ = stronger forces (backwards) — a LOW $T_c$ means the gas liquefies only at very low temperatures, indicating WEAK attractive forces easily overcome by thermal motion.
• Forgetting to convert both temperatures to the same scale (kelvin) before comparing. …