分子速度と運動エネルギー

Kinetic energy is a function of a particle's mass in kilograms and speed in meters per second, expressed as KE = ½mu². All gas particles possess kinetic energy, but individual molecules have different speeds due to collisions. However, the average kinetic energy of a gas sample remains constant at a given temperature, depending on both the mass and velocity of the particles involved.

Root-mean-square (RMS) speed is the square root of the average of the squares of molecular velocities. It is directly proportional to absolute temperature and inversely proportional to molar mass. The equation urms = √(3RT/M) shows that lighter gases move faster at the same temperature, while all gases move faster at higher temperatures.

Gas molecules undergo frequent elastic collisions where momentum is conserved. These collisions deflect molecules at different speeds, creating a distribution of velocities. Although individual molecules have widely varying speeds, the vast number of collisions maintains a constant average kinetic energy and molecular speed distribution at a given temperature.

When temperature increases, the average kinetic energy increases, more molecules have higher speeds, and the speed distribution shifts toward higher velocities. Conversely, when temperature decreases, fewer molecules have high speeds and the distribution shifts toward lower velocities. This explains why aroma particles from hot food are detected faster than those from cold food.

The Maxwell-Boltzmann distribution depicts the relative numbers of molecules in a gas sample that possess a given speed. It shows that at any instant, some molecules move slower than others, creating a range of velocities and kinetic energies. The distribution shape depends on temperature and molar mass, with lighter gases showing broader distributions and higher peak speeds.

Since all gases have the same average kinetic energy at a given temperature, lighter molecules must move faster to achieve that energy. The RMS speed equation urms = √(3RT/M) shows inverse proportionality to molar mass. For example, helium has a higher RMS speed and broader speed distribution than argon at the same temperature because helium has lower molar mass.

The average kinetic energy of one mole of gas is directly proportional to absolute temperature, expressed as KEavg = (3/2)RT, where R is the gas constant and T is temperature in kelvins. This relationship shows that increasing temperature increases molecular motion and kinetic energy. The proportionality constant 3/2 R is derived from kinetic molecular theory principles.