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Laws of Chemical Combination

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Laws of Chemical Combination are one of the most fundamental building blocks of the subject of chemistry. As in our surrounding different matter reacts with each other and form various kind of different substances. Laws of Chemical Combination are the collection of laws that explains how these substances combine together to form anything at all. When matter reacts with another matter, a chemical reaction happens which changes the form, properties, or characteristics of the matter drastically. This interaction of matter with each other is governed by the Laws of Chemical Combination.

Laws of Chemical Combination

A collection of laws that governs the interaction; such as how they combined to form other matter, of matter with each other, is combinedly called the Laws of Chemical Combinations. This collection includes five laws, which are as follows:

  • Laws of Conservation of Mass
  • Laws of Definite Proportions
  • Laws of Multiple Proportions
  • Gay-Lussac’s Law of Gaseous Volumes
  • Avogadro’s Law
Laws of Chemical Combination

 

Law of Conservation of Mass

Law of Conservation of Mass states that “Mass can neither be created nor can be destroyed in a chemical reaction” but it can be transformed from one form to another. 

In other words, in an enclosed system whenever matter undergoes a chemical or physical change, the total mass of reacting matter (reactants or matter before the change) is exactly equal to the total mass of reaction products. As no products or reactants are allowed to exit the system in a closed system, there is no loss of the substance, and hence the mass is conserved.

For example, in automobiles, fuel is burned to create energy for vehicles to move forward and the fuel is converted into fumes such as carbon dioxide, carbon monoxide, or sulfur dioxide, etc. This law was first outlined by Mikhail Lomonosov in 1756 and then further refinements are proposed by Antoine Lavoisier in 1773 after a lot of experiments.

Example of Law of Conservation of Mass

For an example of conservation of mass, let’s consider the formation of a water molecule from oxygen and hydrogen molecules. Balance chemical Reaction of formation of water is:

2H    +     O → 2H2O

Example of Law of Conservation of Mass

 

In the above example, 2 molecules of Hydrogen combine with 1 molecule of Oxygen to form 2 molecules of water. 

  • Mass of H is 1 unit and O is 16 units. 
  • Mass of 2 molecules of Hydrogen (2H2) is 4 units.
  • Mass of 1 molecule of Oxygen (O2)  is 32 units.
  • Mass of 2 molecules of water (2H2O) is 36 units.

2H    +     O → 2H2O

                4 unit        32 unit       36 unit           

The total mass of the reactants equals the total mass of the products in this case. Furthermore, the number of hydrogen and oxygen atoms in the reactant and product sides are also equal.

Law of Definite Proportions

Law of Definite Proportions which is also known as the Law of Constant Composition, states that in any given chemical compound the composition of the element by mass is always remains the same.

In other words, this means that the ratio of the mass of the elements in the chemical compounds always remains the same i.e., in a water molecule there will always be two hydrogens and one oxygen molecule. The law of Definite proportion is first proposed by  Joseph Louis Proust in the late 18th century and many scholars have proved it since then. This law helps us identify between different chemical compounds as well.

Example of Law of Definite Proportions

 Consider the different molecules of Oxides of Nitrogen for examples of the law of definite proportions.

Example of Law of Definite Proportions

 

In the above example of oxides of Nitrogen if we take any amount of Oxygen and Nitrogen in the sample then the ratio of nitrogen to oxygen in the formed NO will always be 1:1. Similarly, the ratio of nitrogen to oxygen for NO2, N2O, and N2O2 will be 1:2, 2:1 and 2:2 respectively. This is guaranteed by the law of definite proportions.

Law of Multiple Proportions

According to the Law of Multiple Proportions, If two elements combined to form more than one compound under different circumstances, then the ratio of the masses for one element when second mass is fixed for all different compounds is always a small whole number.

The Law of Multiple Proportions is also known as Dalton’s Law, as it was first proposed by Dalton in the year 1804. This law doesn’t hold for non-stoichiometric compounds as well as heavy molecules such as polymers and oligomers.

Example of Law of Multiple Proportions

Carbon and oxygen combine to form two distinct compounds (under different circumstances). The first is the most common gas, CO2 (Carbon dioxide), and the second is CO (Carbon monoxide).

Example of Law of Multiple Proportions

 

Lets take 12 grams of carbon, and by the calculatin of moles we can find that it reacts with 16 grams of Oxegen to make Carbon Monooxide and with 32 grams of oxygen to form Carbon dioxide.

As a result, the ratio of mass of oxygen in the first and second compounds is 2:1= 32/16 =2, (whole number). 

Gay Lussac’s Law of Gaseous Volumes

Gay Lussac enacted this law based on his observations in 1808. This law states that “when gases are produced or combined in a chemical reaction, they do so in a simple volume ratio provided that all the gases are at the same temperature and pressure.”

This law is regarded as an of definite proportions for gases and the difference between these two chemical combination laws is that Gay Lussac’s Law is stated the ratio of volume, whereas the law of definite proportions is stated in terms of mass.

Example of Gay Lussac’s Law of Gaseous Volumes

Example of Gay Lussac’s Law of Gaseous Volumes

 

In the above example 2 volumes of H2 combines 1 volume of O2 to form 2 volumes of H2O. 

H (g) +  O (g)  →  2H2O  (g) 

Avogadro’s Law

According to Avogadro’s Law, an ‘equal volume of all gases contains the equal number of molecules under the same conditions of temperature and pressure.’ 

This law was proposed In 1811 by none other than Avogadro himself. In other words, this law states that the volume and number of moles of any gas are always directly proportional to each other. This means that two liters of hydrogen have the same number of molecules as two liters of oxygen at the same temperature and pressure.

Example of Avogadro’s Law

Example of Avogadro’s Law

 

Equivalent volumes of different gases contain the same number of molecules at the same temperature and pressure. In the above example CL2 and H₂ has 1 volume each combines to form 2 volume of HCL.

Here,

Mole is a unit of measurement for substance. 1 mole substance contains 6.02214076×10²³ particles.

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FAQs on Laws of Chemical Combination

Q1: What are the  Laws of Chemical Combination?

Answer:

The collection of laws which explains how the elements react with each other to form compounds is called the law of chemical combination and this collection includes Laws of Conservation of Mass, Laws of Definite Proportions, Laws of Multiple Proportions, Gay-Lussac’s Law of Gaseous Volumes, and Avogadro’s Law.

Q2: What is the Law of Conservation of Mass?

Answer:

Law of Conservation of Mass states that “mass can neither can created not be destryoed, it can be converted from one form to another.”

Q3:  What is the Law of Definite Proportions?

Answer:

According to the Law of Definite Proportions, mass of the constituents of any given compounds always remains the same.

Q4: What are the limitations of the Law of Definite Proportions?

Answer:

The Law of Definite Proportions doesn’t hold true for non-stoichiometric compounds such as Iron Oxide, whose chemical formula is generally written FeO but actually, it is Fe0.95O.

Q5: What is the Law of Multiple Proportions?

Answer:

According to the Law of Multiple Proportions when a combination of two or more elements make more then two compositions, then the ratio of the mass of the compound of one element when the other remains fixed for any two compositions is always a small whole number.

Q6: Why are the laws of chemical combination important?

Answer:

These laws provide an understanding of how the atoms of different elements combine to form compounds with various properties. Thus, these laws are important for the quantitative analysis of chemical systems.



Last Updated : 19 Dec, 2023
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