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Dr. Christopher S. Baird

Why is mass conserved in chemical reactions?

Category: Chemistry      Published: October 21, 2013

propane flame
Every chemical reaction involves conversion between energy and mass. Public Domain Image, source: Christopher S. Baird.

Mass is not conserved in chemical reactions. The fundamental conservation law of the universe is the conservation of mass-energy. This means that the total mass and energy before a reaction in a closed system equals the total mass and energy after the reaction. According to Einstein's famous equation, E = mc2, mass can be transformed into energy and energy can be transformed into mass. This is not some exotic process, but in fact happens every time there is a reaction. Mass is therefore never conserved because a little of it turns into energy (or a little energy turns into mass) in every reaction. But mass+energy is always conserved. Energy cannot be created out of nothing. It can only be created by destroying the appropriate amount of mass according to E = mc2. Between mass and energy, energy is the more fundamental property. In fact, modern physicists just consider mass an alternate form of energy. For this reason, they don't usually call it the "Law of Conservation of Mass/Energy" but rather call it the "Law of Conservation of Energy" with the implication that this statement includes mass.

In nuclear reactions (changes to the nucleus of atoms), there is enough energy released or absorbed that the change in mass is significant and must be accounted for. In contrast, chemical reactions (changes to only the electrons in atoms) release or absorb very little energy compared to nuclear reactions, so the change in mass of the system is often so small that it can be ignored. As a reasonable approximation only, therefore, chemists often speak of the conservation of mass and use it to balance equations. But strictly speaking, the change in mass of the system during a chemical reaction, though small, is never zero. If the change in mass were exactly zero, there would be no where for the energy to come from. Chemists like to speak of "chemical potential energy" and talk as if the energy released in a reaction comes from the potential energy. But "chemical potential energy" is just an old-fashioned term for what we now know is mass. Fundamentally, when chemists say "potential energy" they mean "mass". There is not some bucket of potential energy in an atom from which a reaction can draw. There is just mass.

The loss (or gain) of mass during all reactions, whether chemical or nuclear, is very well established and has been confirmed experimentally. There are four general types of basic reactions:

  1. The breaking of bonds, which always absorbs energy and increases mass.
  2. The formation of bonds, which always releases energy and decreases mass.
  3. The transformation of existing bonds which is really the excitation of the system to different states (always absorbs energy and increases mass) and de-excitation of the system to different states (always releases energy and decreases mass).
  4. The creation of particles (always absorbs energy and increases mass) and annihilation of particles (always releases energy and decreases mass).

Note that when a chemical reaction absorbs energy, and therefore gains mass, it's not like electrons are created. The extra mass is not caused by the appearance of new particles. Rather, the extra mass is held in the system as a whole. Depending on the position and state of particles in a system relative to each other, the system gains or loses mass in addition to the individual masses of the particles. This concept is very similar to the classical concept of potential energy, but we now know that the energy is actually stored as mass. If you measure with very sensitive equipment the sum of the mass of two million hydrogen atoms and one million oxygen atoms that are separated from each other, then measure the mass of one million water molecules, you will find theses masses to be different.

Topics: chemical reaction, conservation of energy, conservation of mass, energy, mass, reaction