1.1.2

Kinetic Molecular Theory

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Molecular Velocities and Kinetic Energy

In a gas sample, individual molecules have widely varying speeds. However, because of the vast number of molecules and collisions involved, the molecular speed distribution and average speed are constant.

Root mean squared

Root mean squared

  • The kinetic energy of a particle of mass mm and speed uu is given by:
    • KE=12mv2KE = \frac{1}{2}mv^2
  • To deal with a large number of gas molecules, we use averages for both speed and kinetic energy
  • The root mean square velocity of a particle, vrmsv_{rms}, is defined as the square root of the average of the squares of the velocities:
    • vrms=v12+v22+v32+...nv_{rms} = \sqrt{\frac{v{_1}^2+v{_2}^2+v{_3}^2+...}{n}}
  • Where nn is the number of particles.
Average kinetic energy

Average kinetic energy

  • The average kinetic energy for a mole of particles, KEavgKE_{avg}, is then equal to:
    • KEavg=32RTKE_{avg} = \frac{3}{2}RT
  • Where RR is the gas constant and TT is the kelvin temperature.
  • When used in this equation, the appropriate form of the gas constant is 8.314 J/mol⋅K.
Average speed

Average speed

  • Combining these equations we can write:
    • 12Murms2=32RT \frac{1}{2}M{u_{rms}}^2 = \frac{3}{2}RT
    • urms=3RTMu_{rms} = \sqrt{\frac{3RT}{M}}
  • For urmsu_{rms}, M is measured in kg/mol rather than g/mol.
Temperature dependence

Temperature dependence

  • If the temperature of a gas increases, KEavgKE_{avg} increases.
    • More molecules have higher speeds and fewer molecules have lower speeds, and the distribution shifts to the right.
  • If temperature decreases, KEavgKE_{avg} decreases.
    • More molecules have lower speeds and fewer molecules have higher speeds, so the distribution shifts to the left.
  • This behavior is shown here for nitrogen gas.
Diagram
Jump to other topics
1

Structure - Models of the Particulate of Matter

2

Structure - Models of Bonding & Structure

3

Structure - Classification of Matter

3.1

The Periodic Table: Classification of Elements

3.2

Periodic Trends

3.3

Group 1 Alkali Metals

3.4

Halogens

3.5

Noble gases, group 18

3.6

Functional Groups: Classification of Organic

3.7

Functional Group Chemistry

3.8

Alkanes

3.9

Alcohols

3.10

Halogenoalkanes

4

Reactivity - What Drives Chemical Reaction?

5

Reactivity - How Much, How Fast & How Far?

6

Reactivity - The Mechanisms of Chemical Change

7

Measurement, Data Processing & Analysis

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