BioPHYSICAL chemistry

The first law of thermodynamics provides a means to balance work and

heat changes and the parameters that are changed when work is done

or when heat flows into or out of a system. However, knowing what is

energetically allowable is not the same as knowing which processes will

actually occur. From a global perspective, energy within the universe is

a constant, yet it is constantly changing with solar systems forming and

stars evolving from red giants into white dwarfs. Clearly, these changes

are being driven by factors other than energy.

Rather than explore the changes in the universe, let’s consider why

objects change in simple processes. If a ball is dropped from a height, it

will drop until it hits the ground and then bounces back (Figure 3.1).

Why did this happen? The ball initially is stationary but has potential

energy due to gravity. As the ball falls the potential energy is converted

into kinetic energy and the ball speeds up. When it hits the ground the

direction of velocity is reversed and the ball slows down and the energy

is converted into potential energy. The ball

will stop when it reaches the original height

and all of the energy is in the form of

potential energy again.

The ball travels according to the first law,

with the energy converting between kinetic

and potential energy and the total energy

remaining the same throughout the travel.

Now consider using an egg instead of the ball.

The egg travels downward like the ball but

the egg does not bounce up after hitting the

ground. Is energy still conserved? For the egg,

the kinetic energy is converted upon impact with the ground into heat;

that is, random motion of the molecules. Unlike the ball, the egg lost its

ability to move upward against the gravitational force and instead became

a more disordered state with random motion.

3 Second law of thermodynamics

3 Second law of thermodynamics

Potential

energy

Ball Egg

Potential

energy

Kinetic

energy

Kinetic

energy

Heat

Figure 3.1
A comparison of
energy changes for
a bouncing ball and
an egg.