The law of conservation of matter or mass is one that establishes that in any chemical reaction, matter is neither created nor destroyed. This law is based on the fact that atoms are indivisible particles in this type of reaction; while in nuclear reactions the atoms are fragmented, which is why they are not considered chemical reactions.
If the atoms are not destroyed, then when an element or compound reacts, the number of atoms before and after the reaction must be kept constant; which translates into a constant amount of mass between the reactants and products involved.
This is always the case if there is no leak that causes material losses; but if the reactor is hermetically closed, no atom “disappears”, and therefore the charged mass must be equal to the mass after the reaction.
If the product is solid, on the other hand, its mass will be equal to the sum of the reactants involved for its formation. The same happens with liquid or gaseous products, but it is more prone to making mistakes when measuring their resulting masses.
This law was born from experiments in past centuries, strengthened by the contributions of various famous chemists, such as Antoine Lavoisier .
Consider the reaction between A and B 2 to form AB 2 (top image). According to the law of conservation of matter, the mass of AB 2 must equal the sum of the masses of A and B 2 , respectively. So if 37g of A reacts with 13g of B 2 , the product AB 2 must weigh 50g.
Therefore, in a chemical equation, the mass of the reactants (A and B 2 ) must always equal the mass of the products (AB 2 ).
An example very similar to the one just described is that of the formation of metallic oxides, such as rust or rust. Rust is heavier than iron (although it may not look like it) since the metal reacted with a mass of oxygen to generate the oxide.
What is the law of conservation of matter or mass?
This law states that in a chemical reaction the mass of the reactants is equal to the mass of the products. The law is expressed in the phrase “matter is neither created nor destroyed, everything is transformed”, as it was enunciated by Julius Von Mayer (1814-1878).
The law was developed independently by Mikhail Lamanosov, in 1745, and by Antoine Lavoisier in 1785. Although Lamanosov’s research work on the Law of the Conservation of Mass predates Lavoisier’s, they were not known in Europe for being written in Russian.
The experiments carried out in 1676 by Robert Boyle led them to point out that when a material was incinerated in an open container, the material increased in weight; perhaps due to a transformation experienced by the material itself.
Lavoiser’s experiments on incinerating materials in containers with limited air intake showed weight gain. This result was in agreement with that obtained by Boyle.
However, Lavoisier’s conclusion was different. He thought that during incineration a quantity of mass was extracted from the air, which would explain the increase in mass that was observed in materials subjected to incineration.
Lavoiser believed that the mass of metals remained constant during incineration, and that the decrease in incineration in closed containers was not caused by a decrease in a loose (disused concept), a supposed essence related to the production of heat.
Lavoiser pointed out that the observed decrease was caused, rather, by a decrease in the concentration of the gases in the closed containers.
How is this law applied in a chemical equation?
The law of conservation of mass is of transcendental importance in stoichiometry, the latter being defined as the calculation of the quantitative relationships between reactants and products present in a chemical reaction.
The principles of stoichiometry were enunciated in 1792 by Jeremías Benjamin Richter (1762-1807), who defined it as the science that measures the quantitative proportions or mass relationships of the chemical elements that are involved in a reaction.
In a chemical reaction there is a modification of the substances that take part in it. It is observed that the reactants or reactants are consumed to originate the products.
During the chemical reaction there are breaks of bonds between the atoms, as well as the formation of new bonds; but the number of atoms involved in the reaction remains unchanged. This is what is known as the law of conservation of matter.
This Law implies two basic principles:
-The total number of atoms of each type is the same in the reactants (before the reaction) and in the products (after the reaction).
-The sum total of the electric charges before and after the reaction remains constant.
This is because the number of subatomic particles remains constant. These particles are neutrons with no electrical charge, positively charged protons (+), and negatively charged electrons (-). So the electrical charge does not change during a reaction.
Having said the above, when representing a chemical reaction using an equation (like the one in the main image), the basic principles must be respected. The chemical equation uses symbols or representations of the different elements or atoms, and how they are grouped into molecules before or after the reaction.
The following equation will be used again as an example:
A + B 2 => AB 2
The subscript is a number that is placed on the right side of the elements (B 2 and AB 2 ) at the bottom, indicating the number of atoms of an element present in a molecule. This number cannot be changed without the production of a new molecule, different from the original.
The stoichiometric coefficient (1, in the case of A and the rest of the species) is a number that is placed in the left part of the atoms or molecules, indicative of the number of them that take part in a reaction.
In a chemical equation, if the reaction is irreversible, a single arrow is placed, which indicates the direction of the reaction. If the reaction is reversible, there are two arrows in the opposite direction. To the left of the arrows are the reactants or reactants (A and B 2 ), while to the right are the products (AB 2 ).
Balancing a chemical equation is a procedure that makes it possible to equal the number of atoms of the chemical elements present in the reactants with those of the products.
In other words, the number of atoms of each element has to be equal on the reactants side (before the arrow) and on the reaction products side (after the arrow).
It is said that when a reaction is balanced, the Law of Mass Action is being respected.
Therefore, it is essential to balance the number of atoms and the electric charges on both sides of the arrow in a chemical equation. Likewise, the sum of the masses of the reactants must be equal to the sum of the masses of the products.
For the case of the represented equation, it is already balanced (equal number of A and B on both sides of the arrow).
Experiments that prove the law
Lavoiser, observing the incineration of metals such as lead and tin in closed containers with a limited intake of air, noticed that the metals were covered with a calcination; and furthermore, that the weight of the metal at a given time of heating was equal to the initial one.
As a weight gain is observed when incinerating a metal, Lavoiser thought that the observed excess weight could be explained by a certain mass of something that is removed from the air during incineration. For this reason the mass remained constant.
This conclusion, which could be considered with an unsound scientific basis, is not such, taking into account the knowledge that Lavoiser had about the existence of oxygen at the time he enunciated his Law (1785).
Release of oxygen
Oxygen was discovered by Carl Willhelm Scheele in 1772. Later, Joseph Priesley discovered it independently, and published the results of his research, three years before Scheele published his results on this same gas.
Priesley heated mercury monoxide and collected a gas that increased the brightness of the flame. In addition, when the mice were placed in a container with the gas, they became more active. Priesley called this gas dephlogistized.
Priesley reported his observations to Antoine Lavoiser (1775), who repeated his experiments showing that gas was found in air and in water. Lavoiser recognized gas as a new element, naming it oxygen.
When Lavoisier used as an argument to state his law, that the excess mass observed in the incineration of metals was due to something that was extracted from the air, he was thinking of oxygen, an element that combines with metals during incineration.
Examples (practical exercises)
Mercury monoxide decomposition
If 232.6 of mercury monoxide (HgO) is heated, it decomposes into mercury (Hg) and molecular oxygen (O 2 ). Based on the law of conservation of mass and the atomic weights: (Hg = 206.6 g / mol) and (O = 16 g / mol), state the mass of Hg and O 2 that is formed.
HgO => Hg + O 2
232.6 g 206.6 g 32 g
The calculations are very straightforward, since exactly one mole of HgO is being decomposed.
Incineration of a magnesium belt
A 1.2 g magnesium ribbon was incinerated in a closed container containing 4 g of oxygen. After the reaction, 3.2 g of unreacted oxygen remained. How much magnesium oxide was formed?
The first thing to calculate is the mass of oxygen that reacted. This can be easily calculated, using a subtraction:
Mass of O 2 that reacted = initial mass of O 2 – final mass of O 2
(4 – 3.2) g O 2
0.8 g O 2
Based on the law of conservation of mass, the mass of MgO formed can be calculated.
Mass of MgO = mass of Mg + mass of O
1.2 g + 0.8 g
2.0 g MgO
A mass of 14 g of calcium oxide (CaO) reacted with 3.6 g of water (H 2 O), which was completely consumed in the reaction to form 14.8 g of calcium hydroxide , Ca (OH) 2 :
How much calcium oxide reacted to form calcium hydroxide?
How much calcium oxide was left over?
The reaction can be outlined by the following equation:
CaO + H 2 O => Ca (OH) 2
The equation is balanced. Therefore it complies with the law of conservation of mass.
Mass of CaO involved in the reaction = mass of Ca (OH) 2 – mass of H 2 O
14.8 g – 3.6 g
11.2 g CaO
Therefore, the CaO that did not react (the one that is left over) is calculated by doing a subtraction:
Mass of excess CaO = mass present in the reaction – mass that took part in the reaction.
14 g CaO – 11.2 g CaO
2.8 g CaO
How much copper oxide (CuO) will be formed when 11 g of copper (Cu) react completely with oxygen (O 2 )? How much oxygen is needed in the reaction?
The first step is to balance the equation. The balanced equation is as follows:
2Cu + O 2 => 2CuO
The equation is balanced, so it complies with the law of conservation of mass.
The atomic weight of Cu is 63.5 g / mol, and the molecular weight of CuO is 79.5 g / mol.
It is necessary to determine how much CuO is formed from the complete oxidation of the 11 g of Cu:
CuO mass = (11 g Cu) ∙ (1mol Cu / 63.5 g Cu) ∙ (2 mol CuO / 2mol Cu) ∙ (79.5 g CuO / mol CuO)
Mass of CuO formed = 13.77 g
Therefore, the difference in masses between CuO and Cu gives the amount of oxygen involved in the reaction:
Mass of oxygen = 13.77 g – 11 g
1.77 g O 2
Formation of sodium chloride
A mass of chlorine (Cl 2 ) of 2.47 g was reacted with sufficient sodium (Na) and 3.82 g of sodium chloride (NaCl) were formed. How much Na reacted?
2Na + Cl 2 => 2NaCl
According to the law of conservation of mass:
Mass of Na = mass of NaCl – mass Cl 2
3.82 g – 2.47 g
1.35 g Na
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