Hsslive.net provided Plus One Chemistry notes for students in their higher secondary years in two languages English Medium & Malayalam Medium. Topics- “Law of Chemical Combination” that are usually covered in the first year of chemistry at the higher secondary level
Laws of Chemical Combinations
The rules for how different elements can be put together to make compounds are called the laws of chemistry combinations. They are based on what experts who have studied the nature and behavior’s of matter have seen and done. These rules help us understand the composition and properties of compounds, as well as the stoichiometry and conservation of mass in chemical reactions. In this chapter, we will learn about four rules. These are:
Law of Conservation of Mass:
This law says that during a chemical reaction, matter can neither be made nor removed. In other words, in a chemical process, the total mass of the reactants is the same as the total mass of the products. In 1789, Antoine Lavoisier came up with this law and did a number of experiments to show it. He showed, for example, that when mercury oxide is burned, it breaks down into mercury and oxygen, and the amount of mercury and oxygen is the same as the amount of mercury oxide. !Mercury oxide decomposition)
We can do a simple experiment with a closed system, like a sealed flask or a balloon, to test this rule. We can burn a known amount of a fuel, like a strip of magnesium, in air or oxygen. Then, we can measure the mass of the result, like magnesium oxide, and compare it to the mass of the reactant at the beginning. As shown below, we will find that there is no change in mass. !Magnesium burning)
Law of Definite Proportions:
This law says that a given compound always has the same parts in the same fixed mass ratio, no matter where it came from or how it was made. For example, water always has 1 part hydrogen to 8 parts oxygen by mass. In 1799, Joseph Proust came up with this law and did several experiments to show it. For instance, he showed that the amount of copper, carbon, and oxygen in copper carbonate is always 5:2:3 by mass. !Copper carbonate)
To test this rule, we can do a simple experiment in which we use a known amount of a compound, like sodium chloride, and use electricity to separate its parts. Then we can measure how much sodium and chlorine were made and compare them to how much sodium chloride there was to start with. As shown below, we will find that they are in a set ratio. !Sodium chloride electrolysis)
Law of Multiple Proportions:
This law says that when two elements combine to make more than one compound, the masses of one element that combine with a set mass of the other element are in a simple ratio of whole numbers. Carbon and oxygen, for example, can join to make two different compounds: carbon monoxide (CO) and carbon dioxide (CO2). In each of these molecules, 12 g of carbon combines with either 16 g or 32 g of oxygen. 16:32, or 1:2, is the ratio of the amounts of oxygen that combine with a given amount of carbon. John Dalton came up with this law in 1803. He used it to back up his atomic theory. !Carbon and oxygen compounds)
To test this rule, we can do a simple experiment in which we mix a known amount of one element, like nitrogen gas, with different amounts of another element, like hydrogen gas. Then, we can compare the amounts of ammonia (NH3) and hydrazine (N2H4) that were made to the amounts of nitrogen and hydrogen that were used at the beginning. As shown below, we will find that they are in a simple ratio. !Nitrogen and hydrogen compounds)
Law of Reciprocal Proportions:
This law says that when two elements combine separately with a third element, the ratio of the masses in which they do so is either the same as or a simple multiple of the ratio in which they combine with each other. For example, hydrogen and oxygen can each join with nitrogen to make ammonia (NH3) and nitric oxide (NO), respectively. In these molecules, 14 grammes of nitrogen are mixed with 3 grammes of hydrogen and 8 grammes of oxygen. When hydrogen and oxygen combine with a set amount of nitrogen, the ratio of their masses is 3:8. This is the same number of hydrogen atoms and oxygen atoms that come together to make water. In 1792, Jeremias Richter came up with this rule. He used it to study how salts are made. !Hydrogen and oxygen compounds)
To test this rule, we can do a simple experiment with a known mass of one element, like carbon, and different masses of another element, like chlorine, that react with it separately. Then, we can compare the amounts of carbon tetrachloride (CCl4) and carbon monochloride (CCl) that were made to the amounts of carbon and chlorine that were used in the beginning. We will find that they are in a simple multiple of the ratio of how much carbon and chlorine are needed to make carbon dichloride (CCl2), as shown below. !Carbon and chlorine compounds)
In this chapter, we’ve learned the four rules of chemical combinations. They help us understand the parts and properties of compounds, as well as the stoichiometry and conservation of mass in chemical processes. I hope these notes were fun and that you learned something new.