Hello and welcome back guys.
I am Sumer Poddar and you are watching play chemistry. In this tutorial, we will discuss the laws of chemical combination. The combination of the elements to form a compound is governed by certain laws called laws of chemical combination.
The various laws of the chemical combination are: Law of conservation of mass, law of gaseous volumes, Avogadro’s law, law of constant composition, multiple proportions and reciprocal proportions. Laws of conservation of mass In all physical and chemical changes, the total mass of the reactants is equal to that of the products.
It means, there will be no increase or decrease in mass during any chemical or physical changes.
Physical change Let’s take a conical flask and put 250 grams of ice cubes in it.
Close the opening of the conical flask with a cork.
Now, heat it until it changes to water completely. Now, weigh it again.
You will find that it is still 250 grams.
There is no change in its weight. So, the physical change of ice into water is following the law of conservation of mass. Chemical change
In a chemical change, mass before and after reaction will remain same.
Mass of all the reactants will be equal to the mass of all the products.
So, let’s take an example.
Put copper sulfate in a conical flask and immerse a test tube filled with sodium hydroxide in it.
On mixing the the both reactants completely we will get blue precipitates of copper hydroxide and colorless liquid sodium sulfate. If the mass of reactants copper sulfate and sodium hydroxide was like 60 grams then
the mass of the product copper hydroxide and sodium sulfate will also be 60
But in some cases, the mass of the reactants is not equal to the mass of the products.
Uranium which has atomic mass mass of 235 changes to Thorium having
mass equal to 231. In this case, mass before and after is not same.
There’s a loss of 4 units of mass. What’s happening to this lost mass?
This lost mass changes to energy.
And the value of the energy will be equal to mc2.
where m is the mass lost and c is the speed of
light which is equal to 3X10^8 m/s Now the law of conservation of mass is
called law of conservation of mass and energy. According to it, the mass and energy are interconvertible but the total sum of mass and energy during any physical and chemical changes remains constant.
In the former example, mass is converting to energy and sum of mass and energy will be constant. GAY LUSSAC’S LAW OF GASEOUS VOLUME Consider the x volume of gas 1 reacts with y volume of gas 2 to form z volume of gas 3. Then according to gay Lussac’s law of gaseous volume this x : y : z will bear simple whole number ratio provided the condition of temperature
and pressure remains same. Now let’s see the statement
It says “When Gas reacts together, they always do so in volumes which bears a simple ratio to one another and to the volume of the products, if these are also gases, provided all measurements of volumes are done under similar conditions of temperature and pressure.”
Now, let’s see an example.
On reacting 1 volume of nitrogen with 3 volume of hydrogen we get 2 volume of ammonia These volumes bear simple whole number ratio to one another and to the product. That is 1:3:2.
Hence, this reaction follows laws of gaseous volume. Avogadro law
“Equal Volume of Gasses at same Temperature and Pressure contains equal number of molecules”
Let’s take two systems one with 1 volume of hydrogen and other with 1 volume of chlorine under the same condition of pressure and temperature conditions. According to Avogadro’s law both of them will have the same number of molecules.
If system 1 have n molecules, then the system 2 must have n molecules too.
Also, Volume is directly proportional to the number of molecules. The increase in the volume implies an increase in the number of molecules.
In system 1 we have 1 volume of gas and in system 2 we have double of it. So, system two will have twice the number of molecule that system 1 have. Applying it in hydrogen and chlorine reaction.
1 volume of hydrogen combines with 1 volume of chlorine to form two volume of hydrochloric acid.
Thus, n molecule of hydrogen combines with n molecule of chlorine to form 2n molecules of hydrochloric acid. LAW OF CONSTANT COMPOSITION
A chemical compound is always found to be made up of same elements combined together in the same fixed proportion by mass. Let’s take an example of carbon dioxide.
Carbon dioxide has 12 grams of carbon for every 32 grams of oxygen.
So, in carbon dioxide, the ratio of carbon to oxygen is 12:32 that gives 3:8.
So, carbon dioxide is following the law of constant composition, as it is always made up of same elements “carbon and oxygen” and always mixed in a fixed proportion of 3:8. Law of multiple proportions
“When two elements combine to form two or more chemical compounds, the masses of one of the elements which combines with a fixed mass of the other, bear a simple ratio to one another.”
Let’s take an example. Sulfur and Oxygen forms Sulfur dioxide as well as Sulfur trioxide.
Every 32 gram of sulfur combines with the 32 gram of oxygen in sulfur dioxide and 48 gram of oxygen in sulfur trioxide.
The ratio of oxygen in both the compounds will be 32: 48 which is 2:3. “A simple whole number ratio”
Hence, it’s following the law constant composition. law of reciprocal proportion let’s
understand it with a diagram. We have three elements “A, B and C” on the vertex of the triangle.
On the edge of the triangle, we have compounds made up of these elements.
On LHS, we have compound formed of A and C, on RHS compound of B and C. Let’s take a fixed mass of C in both the compounds.
Now, the ratio of the masses of A and B which combine separately with a fixed mass of the third element C is calculated. This is the step 1. At the bottom, we have a compound which is formed by direct combination of elements A and B.
Now, the ratio of the masses in which A and B combine directly with each other is calculated. This is the step 2. You can observe that the ratio we get from the step 1 and 2 is same or some simple multiple. So, According to law of reciprocal proportion
“The ratio of the masses of two elements A and B which combine separately with a fixed mass of the third element C is either the same or some simple multiple of the ratio of the masses in which A and B combine directly with each other.”
Carbon and oxygen combine separately with a third element hydrogen to form CH4 “methane” and H2O “water”. The carbon and oxygen combines directly with each other to form carbon dioxide.
In methane, 4 parts of hydrogen combines with 16 parts of carbon.
In water 2 parts of hydrogen combines with 16 parts of oxygen.
Thus, 4 parts of hydrogen combines with 32 parts of oxygen.
The ratio of of masses of carbon and oxygen is 12:32, that is 3:8. In CO2, 12 parts of carbon combines with 32 parts of oxygen. The ratio of carbon to oxygen in carbon dioxide will be 12:32 that is also, 3:8.
Hence, it illustrates the law of reciprocal proportions. Guys, now play chemistry have a discussion page. You can post your feedback, opinion and the question there. I’ll make sure I reply all of them as soon as possible. I post videos every weekend. So, don’t forget to subscribe.
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