How can energy be changed from one form to another?

Explain how energy can be changed from one form to another, e.g. chemical energy to heat energy.

It's important to accept that energy is an abstraction. It's not some sort of invisible stuff that flows from one object to another. Energy is like money. We use it to keep track of transactions of various kinds. It's useful because it helps us predict how one sort of change will affect another, when we can associate each change with a particular "form" of energy. If one form of energy decreases, other forms must increase, because the total energy has to stay constant (if you're looking at an isolated object, that is).

Let's consider a concrete example- a pendulum. The energy of the pendulum comes in two forms:
Energy = kinetic energy+potential energy
1/2 mv2+mgh
where m is the mass, v is the velocity, and h is the height of the pendulum weight above some reference point. As the pendulum swings back and forth, the total energy stays constant but each swing converts potential energy into kinetic energy and vice versa. At the top of a swing, just for an instant the velocity is zero and all of the energy is potential energy. At the bottom of a swing, the pendulum has its highest velocity and the kinetic energy term has its maximum value. As the pendulum swings upward again, its velocity slows as it rises, so the kinetic energy term gets smaller while the potential energy term gets bigger. (Notice how the energy expression makes it easy to relate the velocity of the pendulum with its height!)

You can write a total energy expression of this sort for a chemical reaction, like the combustion of hydrogen. The chemical energy has a kinetic component (the motion of electrons and nuclei within the molecules) and a potential component (the attraction of the electrons for the nuclei, along with repulsions between electrons and repulsions between nuclei). The molecules are also moving, which gives them kinetic energy overall. If you decrease the potential energy of the reaction mixture (by jumbling the nuclei and electrons around to maximize attractions and minimize repulsions), you must increase the kinetic energy of the mix. That increased kinetic energy appears as heat.

A mousetrap is a metaphor for this sort of conversion of energy. Before the trap is sprung, the energy is all potential energy, "stored" in the spring. When the trap is sprung, the spring can stretch, and the bar slams down - potential energy becomes kinetic energy. Chemical bonds are similar to the spring. When the bond is snapped, the potential energy becomes kinetic. The repulsion between the nuclei of the freed atoms is no longer balanced by the attraction they have for the bonding electrons, and they fly away from each other.

Author: Fred Senese

General Chemistry Online! How can energy be changed from one form to another?

Copyright © 1997-2010 by Fred Senese
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