Lavoisier’s Law

What is Lavoisier’s law?

Lavoisier’s law or the law of conservation of mass explains that during a chemical reaction there is no change of mass, i.e. the sum of the mass of the reactants (before the reaction takes place) is equal to the mass of the products (after the reaction takes place). It can be stated as follows:

The total mass of the substances involved in a chemical reaction remains unchanged. Therefore, the total mass of the reacting substances (reactants) is equal to the mass of the originating substances (products).

Lavoisier’s law is one of the most important in chemistry because it marked the birth of modern chemistry and the abandonment of its predecessor, alchemy. The law implies that mass can neither be created nor destroyed, but can be transformed in space, or the entities associated with it can change form.

Lavoisier’s experiment

The law of conservation of mass was enunciated in 1789 by Antoine-Laurent Lavoisier (1743-1794) after meticulous and numerous experiments.

In 1679, preliminary experiments by Robert Boyle (1627-1691) seemed to indicate an imbalance in mass: by heating a metal, it gained mass when it was converted into a new substance. These experiments, however, were carried out in open vessels.

Lavoisier during his related work on public lighting in Paris, had observed that by heating metals such as tin and lead in closed vessels with a limited amount of air, these materials were coated with a calcined layer until a certain time of heating, in the result the mass was the same as before starting the process.

If the metal had gained mass as it calcined, it was apparent that something in the vessel must have lost the same amount of mass, as was the case with the air inside the vessel.

Thus, Lavoisier demonstrated that the calcination of a metal was not the result of the loss of the proposed phlogiston, an essence related to the decrease of gases in the vessel.

Lavoisier measured in a closed vessel the masses of the solid and the air before and after combustion and concluded that the mass gained by the metal was equal to the mass of air lost.

In addition, he observed that if he placed a burning candle in a closed vessel, the weight before and after the candle burned was the same.

The hydrocarbons (alkanes, CnH2n+2) present in the candle wax react with the oxygen (O2) in the hood air (combustion reaction), generating carbon dioxide (CO2) and water (H2O), but the total mass was the same before and after combustion.

mass of wax + mass of O2 → masa de CO2 + mass of H2O + mass of unburned wax.

Video about Lavoisier’s Law

Limitations on Lavoisier’s law

Today it is known that Lavoisier’s law is an approximate conservation law since it is not exactly fulfilled in nuclear fission reactions. This is what makes it possible to provide an energy ΔE = Δmc2, where c is the speed of light.

Similarly, energy is also not conserved in such reactions. The generalization of mass conservation for high energy reactions is known as the mass-energy equivalence.

In these cases there is a decrease in mass, which although in normal reactions is negligible, for such nuclear reactions is called “mass defect”.

The mass defect of a nucleus is present in the form of energy, called “binding energy”. The energy could be converted into mass if the nucleus were to break apart and vice versa. This energy is a measure of the strong nuclear interaction that holds the nucleons together.