gas molecules
Summary
Gas molecules are the particles described by kinetic molecular theory: far apart, in constant random motion, with negligible volume and no intermolecular forces in an ideal gas. Average kinetic energy depends only on temperature, so at the same T lighter molecules move faster on average.
Detail
Meaning: Kinetic molecular theory models a gas as particles that are tiny compared with the space between them, move randomly in straight lines, and collide elastically with each other and the container walls. An ideal gas has no intermolecular attractions or repulsions. Pressure comes from wall collisions, and average kinetic energy is proportional to absolute temperature. Real gases deviate from this at high pressure (particle volume matters) and at low temperature (attractions matter).
Equation: urms = √(3RT/M), where urms is the root-mean-square speed (m/s), R is 8.314 J/(mol·K), T is absolute temperature (K), and M is molar mass (kg/mol).
On the MCAT: Passages ask you to compare gases at the same T and P using Graham's law (effusion rate is inversely proportional to √M), or to read Maxwell-Boltzmann curves. Raising T moves the peak right and flattens the curve. A heavier gas at the same T has its peak at lower speed and a narrower curve.
Common confusion: Equal temperature means equal average kinetic energy, not equal speed. Since KE = ½mv², the lighter gas must move faster. Also, molecules in one sample have a range of speeds, so temperature describes the average, not every molecule.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.