The laws which deal with the composition of substances or compounds by mass or by volume are called laws of chemical combination or laws of stoichiometry. These laws are experimental laws which have been prepared by scientists based on performing many experiments involving various types of chemical reactions.
There are five basic laws of chemical combination by which elements combine to form compounds or substances. They are:
- Law of conservation of mass
- Law of constant proportions
- Law of reciprocal proportions
- Law of multiple proportions
- Gay-Lussac’s law of Gaseous Volumes
The first four laws deal with the mass relationship, while the last law concerns with volume of reacting gasses. Let’s understand each law with the help of important examples.
Law of Conservation of Mass or Matter
In 1789, a French chemist Antoine Lavoisier proposed this law. According to the law of conservation of mass, in all chemical changes or reactions, mass is neither created nor destroyed. In other words, matter is neither created nor destroyed in a chemical reaction. There is no change in mass or matter during a chemical change. That is, the total mass of the system remains constant during chemical reaction. All chemical reactions occurring in nature follow this law.
When a chemical reaction occurs, substances which react or combine with each other in a chemical reaction, is called reactants. The new substances formed as a result of chemical reaction are called products.
The law of conservation of mass means in a chemical reaction or physical change, the total mass of reactants is always equal to the total mass of products. It means that there is no change in the mass during chemical reaction because there is no gain or loss in the total mass of matter. Mass of matters remains conserved in the entire reaction. Thus, this law is also known as the law of indestructibility of matter.
Suppose we carry out a chemical reaction between A and B as reactants that form C and D as products. Then,
A + B ⟶ C + D
Here, mass of (A + B) = mass of (C + D). That is, the mass of reactants is equal to the mass of products.
Let’s understand this law with the help of a realtime example.
Take a piece of ice in a small conical flask and measure its weight as well. Now heat the flask gently to melt ice (solid) into water (liquid).
Again, measure the weight of flask when ice melts into water. You will observe that there is no change in the weight though a physical change has taken place.
Thus, according to the laws of conservation of mass, there is no increase or decrease in the total mass of matter during a physical or chemical change.
Example:
100 gm of calcium carbonate when heated produce 56 g of calcium oxide and 44 g of carbon dioxide. Prove that these results illustrate the law of conservation of mass.
Mass of calcium carbonate (reactant) = 100g
Total mass of CaO and CO2 (products) = 56g + 44g = 100g
Here, the total mass of products (100g) is equal to the total mass of reactant (100g). There is no change of mass during chemical reaction. Thus, mass remains conserved or the same. Hence, this example proved the law of conservation of mass.
Law of Constant Proportions or Definite Proportions
In 1799, the Frenchman, Joseph Proust, proposed this law. It states that a chemical compound always consists of the same elements combined together in a fixed ratio by mass or weight.
In other words, when two or more elements combine together to form a compound, they combine in a fixed ratio of their masses irrespective of the method of preparation or the source from where it has been obtained.
That is, a pure chemical compound has always a fixed composition, and it does not depend on the method of its preparation or the source from where it is taken.
For instance, water (H2O) is a compound that consists of the same two elements, hydrogen and oxygen, and joined together in a fixed ratio of 1 : 8 by mass. Let us understand it in some more detail.
Suppose we decompose 100g of pure water by passing electricity through it, 11g of hydrogen and 89g of oxygen will obtain. If we repeat this experiment by taking pure water from different sources like sea, pond, river, etc., the same masses of hydrogen and oxygen elements will obtain in every case.
Thus, this experiment concluded that a pure compound always contains the same elements combined together in the same proportion by mass, irrespective of the source. Scientists has performed various experiments to validate this law. Sometimes, the law of constant proportions is also known as law of definite proportions or law of constant composition.
Example:
When 3g of carbon is burnt in 8g of oxygen, then 11g of carbon dioxide is formed. What mass of carbon dioxide it will produce when 3g of carbon is burnt in 50g of oxygen? Prove that these results illustrate the law of conservation of mass.
Since carbon and oxygen combine in a fixed ratio of 3 : 8 by mass to produce 11g of carbon dioxide, therefore, the same mass of carbon dioxide (i.e. 11g) will be got even if we burn 3g of carbon in 50g of oxygen. The extra oxygen (50 – 8 = 42g) will not react. This show that it follows the law of constant proportions.
Law of Multiple Proportions
In 1808, an English man John Dalton proposed this law based of his atomic theory. According to this law, if two elements combine to form more than one compound, then the different masses of one element which combine with a fixed mass of the other element, are in the simple ratio of small integers (or whole numbers).
For example, when hydrogen combines with oxygen, it forms two compounds, namely, water and hydrogen peroxide.
From the above figure, it is clear that the masses of oxygen (i.e., 16 g and 32 g), which combine with a definite mass of hydrogen (2g) are in the simple ratio, i.e., 16:32 or 1: 2.
Law of Reciprocal Proportions
In 1794, Richter proposed this law. According to this law, when three elements A, B, and C mutually combine to form binary compounds AB, BC, and CA, the proportion of masses of B and C when they combine together, is a simple multiple of mass ratio of B and C in which they combine separately with a definite amount of A.
For instance, hydrogen combines with sodium and chlorine to form compounds NaH and HCl, respectively.
In NaH, Sodium 23 parts and Hydrogen one part.
In HCl, Chlorine 35.5 parts and Hydrogen one part.
Sodium and chlorine also combine to form NaCI which contains 23 parts of sodium and 35.5 parts of chlorine. These are the same parts which combine with one part of hydrogen in NaH and HCl respectively.
Gay Lussac’s Law of Gaseous Volumes
In 1808, Gay Lussac proposed this law. According to this law, when gases react with each other or are produced in a chemical reaction they are in a simple ratio of their volumes, provided all gases are at the same temperature and pressure.
If the product is also in gaseous state, the volume of the product are also in the simple ratio with the volumes of gaseous reactants provided that all volumes are measured at the same temperature and pressure.
For example, 100 mL of hydrogen reacts with 50 mL of oxygen to form 100 mL of water vapour.
From the above figure, it is clear that 100ml volume of hydrogen and 50ml volume of oxygen when combine are in the simple ratio of 2:1.
In this tutorial, you have known about laws of chemical combination with many examples. Hope that you will have understood all the basic points of all laws of stoichiometry or chemical combination.
Thanks for reading!!!