Class 9, Science, Chapter-3, Lecture-1, Laws of Chemical Combination (Notes)
Law of Conservation of Mass: (Law of indestructibility of matter)
“During any physical and chemical change, the total mass of the products is equal to the total mass of the reactants.”
Stated by A. Lavoisier (French) in 1774
Example:
12 g of carbon combine with 32 g of oxygen to form 44 g (12 + 32) of carbon dioxide.
$${\mathop {\rm{C}}\limits_{12g} {\rm{ }} + {\rm{ }}\mathop {{{\rm{O}}_2}{\rm{ }}}\limits_{32g} \to \mathop {{\rm{ C}}{{\rm{O}}_2}}\limits_{44g} }$$
Law of Constant Composition: (Law of definite proportion)
“A pure chemical compound always contains same elements combined together in the same proportion by weight.”
Stated by Joseph Louis Proust
Example:
Carbon dioxide obtained by heating coal contains carbon and oxygen in the ratio of 3:8 by weight.
Carbon dioxide obtained by heating limestone contains carbon and oxygen in the ratio of 3:8 by weight
Hence, a pure sample of CO2 always contains same elements (carbon and oxygen) combined together in the same
proportion by weight (3:8).
Law of Multiple Proportion:
“When two elements combine to form two or more compounds, the weights of one of those elements which combine with a fixed weight of the other, bear a simple ratio to one another.”
OR
“When two elements X and Y combine to form more than one compound, the masses of X which combines with fixed
mass of Y are in simple ratio.”
Stated by Joseph Louis Proust
Example:
Hydrogen and Oxygen combine to form Water $\left( {{{\rm{H}}_2}{\rm{O}}} \right)$ and Hydrogen Peroxide $\left( {{{\rm{H}}_2}{{\rm{O}}_2}} \right)$
In : $\left( {{{\rm{H}}_2}{\rm{O}}} \right)$
2 g of Hydrogen combines with 16 g of oxygen
$ \Rightarrow $1 g of Hydrogen combines with 8 g of oxygen
In : $\left( {{{\rm{H}}_2}{{\rm{O}}_2}} \right)$
2 g of Hydrogen combines with 32 g of oxygen
$ \Rightarrow $1 g of Hydrogen combines with 16g of oxygen
Hence, the weights of oxygen (8 g and 16 g) which combines with a fixed weight of Hydrogen (1g) bear a ratio 8:16 = 1:2 which is a simple ratio.
DALTON’S ATOMIC THEORY:
- All substances are made up of tiny, invisible particles.
- Each chemical element is composed of characteristics particles (called atoms) which cannot be further subdivided.
- The atoms of same element are alike and have the same mass and the atoms of different elements differ in mass.
- Atoms cannot be created or destroyed during physical or chemical changes.
- Compounds (Now called molecules) result from the combination of atoms in some simple numerical ratio.
Drawbacks of Dalton’s Atomic Theory:
- Dalton’s theory could not explain the reason for the difference in the physical properties of diamond, graphite and charcoal which are chemically identical.
- Atoms were found to be subdivided into smaller particles (electron, proton, neutron)
- Dalton’s theory could not explain the reason for difference in the atoms of different elements.
- Dalton’s theory could not explain why organic compounds could not be prepared in laboratory
- The ratio of C, H, O in sugar $\left( {{{\rm{C}}_{12}}{{\rm{H}}_{22}}{{\rm{O}}_{11}}} \right)$ is $72:11:88$ which is not a simple ratio.