22 



GENERAL BIOCHEMISTRY 



Low temperature 



High temperature 



Velocity *- 



FIGURE 2-1. The velocities of molecules in a gas. An increase in temperature in- 

 creases the average velocity by increasing the numbers of molecules having the 

 higher velocities. If the dashed line represents the threshold velocity required before 

 a molecule can undergo a process, a much larger fraction of the molecules present 

 can participate in that process when the temperature is raised. The area under a 

 curve is proportional to the total number of molecules. Both curves dehmit equal 

 areas when the amount of gas is kept constant. The areas under the curves and to 

 the right of the dashed line are proportional to the numbers of molecules sufficiently 

 energetic for the process. 



temperature increases the average velocity. However, in no case do 

 all the molecules in a material possess the same velocity. Rather 

 there is a relatively broad range of velocities with some moving slowly, 

 some rapidly, and many at intermediate speeds. The typical distribu- 

 tion curve of the molecular velocities is shown in Figure 2-1. 



The strong intermolecular forces in liquids tend to limit the paths 

 of individual molecules to the body of the liquid. Whenever a mole- 

 cule reaches the liquid surface, the field of force surrounding it is un- 

 symmetrical. On the liquid side of the surface there is a strong attrac- 

 tion restraining escape of the molecule. From beyond the surface there 

 is little attraction. Therefore, liquid molecules do not escape from 

 the surface into the gas phase above unless they have sufficient energy 

 or velocity to break away from the restraining attraction of their fellow 

 liquid molecules. Ordinarily some molecules do escape by virtue of 

 their high velocities. This process is called evaporation. The rate of 



