chemistry class 9th notes chapter 2
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CHEMICAL COMBINATIONSAND CHEMICAL EQUATION
Laws of Chemical Combinations
There are four laws of chemical combinations these laws explained the general feature of
chemical change. These laws are:
1.Law of Conservation of Mass
2.Law of Definite Proportions
3.Law of Multiple Proportions
4.Law Reciprocal Proportions
Antoine Lavoiser has rejected the worn out ideas about the changes that take place during
a chemical reaction. He made careful quantitative measurements in chemical reactions and
established that mass is neither created nor nor destroyed in a chemical change. Law ofConservation of Mass
Statement
It is presented by Lavoiser. It is defined as:
"Mass is neither created nor destroyed during a chemical reaction but it only changes
from one form to another form."
In a chemical reaction, reactants are converted to products. But the total mass of the
reactants and products remains the same. The following experiment easily proves law of
conservation of mass.Practical Verification (Landolt Experiment)
German chemist H. Landolt, studied about fifteen different chemical reactions with a
great skill, to test the validity of the law of conservation of mass. For this, he took
H.shaped tube and filled the two limbs A and B, with silver nitrate (AgNO3) in limb A and
Hydrochloric Acid (HCl) in limb B. The tube was sealed so that material could not escape
outside. The tube was weighed initially in a vertical position so that the solution should not
intermix with each other. The reactant were mixed by inverting and shaking the tube. The
tube was weighed after mixing (on the formation of white precipitate of AgCl). He
observed that weight remains same.HCl + AgNO3 ----------> AgCl + NaNO3
Law of Definite Proportions
Statement
It is presented by Proust. It is defined as:
"When different elements combine to give a pure compound, the ratio between the
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masses of these elements will always remain the same."
Proust proved experimentally that compound obtained from difference source will always
contain same elements combined together in fixed proportions.
Example
Water can be obtained from different sources such as river, ocean, well, canal, tube well,rain or by the chemical combination of hydrogen and oxygen. If different samples of water
are analyzed, it will have two elements, hydrogen and oxygen and the ratio between their
mass is 1:8.
Law of Multiple Proportions
Statement
This law is defined as:
"When two elements combine to give more than one compounds, the different masses
of one element, which will combine with the fixed mass of other element, will be in
simple whole number ratio."
Two different elements can combine to form more than one compound. They can do so by
combining in different ratios to give different compounds.
Example
Hydrogen and oxygen combine with one another to form water (H2O) and hydrogen
peroxide (H2O2). In water and hydrogen oxide 2 g of hydrogen combine with 16g and 32g
of oxygen respectively. According to law of multiple proportions, the different masses of
oxygen (16g and 32g) which have reacted with fixed mass (2g) of hydrogen will have a
simple ratio between each other i.e. 16:32 or 1:2. It means that hydrogen peroxidecontains double the number of oxygen atoms than water. This law proves this point of
Dalton's Atomic Theory that atoms do not break in a chemical reaction.
Law of Reciprocal Proportions
Statement
This law is defined as:
"When two element A, B combine separately, with the mixed mass of the third
element E, the ratio in which these elements combine with E is either the same or
simple multiple of the ratio in which A and B combine with each other."
Example
Hydrogen and Nitrogen separately combine to form ammonia (NH3) and dinitrogen oxide
(N2O), in these compounds, fixed mass of nitrogen is 14g and combines with 8 g of oxygen
and 3 g of hydrogen. The ratio between the mass of oxygen and hydrogen is 8:3. Hydrogen
and oxygen also combine with one another to form water (H2O). The ratio between
hydrogen and oxygen in water is 16:2. These ratios are not same. Let us observe whether
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Examples of Molecules of the elements are Hydrogen (H2). Nitrogen (N2), Sulphur (S8)
etc.
Molecules of different elements are called compounds. For example HCl, H2O, CH4 etc.
Valency
The combining capacity of all elements with other elements is called valency.Example
H = 1
C = 4
Al = 3
Mg = 2
Na = 1