Hydrogen and oxygen are at the same temperature. Which molecules move faster, and does that mean they have more kinetic energy?
At the same absolute temperature both gases have the same average translational kinetic energy per molecule, 3k_BT/2.
At the same absolute temperature both gases have the same average translational kinetic energy per molecule, 3k_BT/2. But v_rms = sqrt(3RT/M), so speed varies as 1/sqrt(M). Hydrogen's much smaller molar mass therefore gives a larger rms speed. Faster here does not mean greater average kinetic energy because its molecular mass is smaller.
Key point
Same T means same average translational KE, while v_rms varies as 1/sqrt(M).
Common mistake
equating greater speed with greater energy without considering mass.
Memory tip
same energy; lighter runs faster.
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