Grade 11

Grade 11Basic concepts of chemistry


laws of chemical combination


The laws of chemical combination are fundamental rules that explain how elements and compounds combine to form new substances. These rules were formulated based on systematic experiments and are the cornerstone of modern chemistry. There are several major laws associated with chemical combinations, and each law gives information about how substances interact at the molecular level.

Law of conservation of mass

The law of conservation of mass, established by Antoine Lavoisier in the late 18th century, states that mass in an isolated system is neither created nor destroyed by chemical reactions. In simple terms, the mass of the reactants in a chemical reaction is equal to the mass of the products. This principle is fundamental to understanding chemical processes.

Consider the reaction of hydrogen gas H 2 and oxygen gas O 2 to form water H 2 O:

2H 2 + O 2 → 2H 2 O

If we start with 4 grams of hydrogen and 32 grams of oxygen, the total mass will be 36 grams. After the reaction, 36 grams of water are formed, respecting the law of conservation of mass.

Law of definite proportions

The law of definite proportions, also called the law of constant composition, was proposed by Joseph Proust in 1799. This law states that the constituent elements in a chemical compound are always present in a fixed proportion by mass, regardless of the source or method of formation.

For water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8. If you have a sample with 2 grams of hydrogen, it will combine with 16 grams of oxygen to form 18 grams of water.

Mass ratio in water = 1 part hydrogen : 8 parts oxygen

Law of multiple proportions

The law of multiple proportions was formulated by John Dalton in 1804. This law states that if two elements combine to form more than one compound, then the ratio of the weight of the element that combines with the fixed weight of the other element are simple whole numbers.

An example of this is found in the compounds carbon monoxide CO and carbon dioxide CO 2. Here, 12 grams of carbon can combine with 16 grams of oxygen to form carbon monoxide and 32 grams of oxygen can combine with 32 grams of oxygen to form carbon dioxide. The ratio of the mass of oxygen combined with a constant mass of carbon (12 grams) is 1:2.

Mass ratio of oxygen in CO : CO 2 = 16 : 32 = 1 : 2

Law of reciprocal proportions

The law of reciprocal proportions, sometimes called the law of equivalent proportions, was proposed by Jeremias Richter in 1792. This law states that the masses of elements that separately combine with a fixed mass of another element are the same as the masses of the elements that combine directly with each other or with simpler multiples of it.

Consider the elements hydrogen H, oxygen O, and sulfur S:

Hydrogen combines with oxygen to form water, and with sulfur to form hydrogen sulfide. If water and hydrogen sulfide are formed via combination steps by stabilizing the respective masses of oxygen and sulfur against the same mass of hydrogen, the ratio is consistent.

Mass ratio of oxygen in H 2 O : Mass ratio of sulphur in H 2 S = 8:16

Gay-Lussac's law of gaseous volumes

In 1808, Joseph Louis Gay-Lussac proposed that when gases react together at a constant temperature and pressure, the volumes of the gaseous reactants and products are in the ratio of small whole numbers. This law is particularly useful when dealing with gaseous substances.

For example, ammonia is made by the reaction of nitrogen and hydrogen gases:

N 2 + 3H 2 → 2NH 3

One volume of nitrogen reacts with three volumes of hydrogen to form two volumes of ammonia, which represents a simple whole number ratio.

Practical applications

These rules are applied in a variety of chemical calculations and reactions:

  • Stoichiometry: The quantitative relationship between reactants and products in a chemical reaction is derived from these rules.
  • Preparing chemical equations: Chemical equations are balanced using the law of conservation of mass to ensure that matter is neither created nor destroyed.
  • Formula determination: The law of definite proportions is used to determine the empirical and molecular formulas of compounds.
  • Understanding compounds: Rules such as multiple proportions help distinguish between compounds with the same elements.
  • Gaseous reactions: Using Gay-Lussac's law, chemists can predict the outcomes of reactions involving gases.

Historical influences

The laws of chemical combination were important in the development of chemistry as a scientific discipline. They helped establish the atomic theory, which holds that matter is composed of atoms. Together with experimental evidence, these laws provided a systematic approach to understanding chemical reactions, thus laying the foundations of modern chemistry.

Before these rules, chemistry, which was mainly based on mysticism and pseudoscience, dominated the understanding of chemical processes. By incorporating quantitative methods and these rules, chemistry was transformed into a rigorous and systematic science. These rules paved the way for researchers to predict and quantify reactions with greater accuracy, leading to discoveries and advances in various chemical industries and scientific investigations.


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