Nazarsir.blogspot.com: 4. Measurement of Matter

4. Measurement of Matter

Laws of chemical combination :

The composition of a substance changes during a chemical change.
Compounds are formed by chemical combination of elements.

1. Law of conservation of matter :

Mass of the original matter and the mass of the matter newly formed as a result of the chemical change are equal.
In 1785, the French Scientist Antoine Lavoisier inferred from his research that ‘there is no rise or drop in the mass of the matter during a chemical reaction.’
In a chemical reaction the total mass of the reactants is same as the total mass of the products formed due to the chemical reactions and this is called the law of conservation of matter.

2. Law of constant proportion :

1794 J. L. Proust stated the law of constant proportions as “The proportion by mass of the constituent elements in the different samples of a compound is fixed,”
e.g., the proportion by mass of hydrogen and oxygen in water is obtained from any source 1:8.
This means that 9 g water is formed by chemical combination of 1 g hydrogen and 8 g oxygen.
Similarly, the proportion by mass of carbon and oxygen in carbon dioxide obtained from any source is 3:8.
This means that in 44 g of carbon dioxide there is 12 g of carbon and 32 g of oxygen
Proportion by mass of carbon and oxygen is 3:8.

Antoine Lavoisier :

Nomenclature of oxygen and hydrogen.
Showed that matter combines with oxygen during combustion.
Was the first to use accurate weighing techniques to weigh mass of reactants and products in a chemical reaction.
Discovered that water is made up of hydrogen and oxygen.
First writer of the law of conservation of mass, in a chemical reaction.
Assigned systematic names to the compounds, e.g. sulphuric acid, copper sulphate, sulphite.
Authored the first book on modern chemistry- ‘Elementary Treatise on Chemistry in 1789.’ •
Studied the elements such as oxygen, hydrogen, nitrogen, phosphorus, mercury, zinc and sulphur.

Atom : size, mass and valency :

The centre of an atom is the nucleus and that there are moving electrons in the extra-nuclear part.
The electrons are negatively charged elementary particles while the elementary particles that make up the nucleus are positively charged protons and electrically neutral neutrons.
The size of an atom is determined by its radius.
The atomic radius of an isolated atom is the distance between the nucleus of an atom and its outermost orbit.
Atomic radius is expressed in nanometres.

The atomic size depends on the number of electron orbits in the atom.
The greater the number of orbits the larger the size. For example, an atom of K is bigger than an atom of Na.
If two atoms have the same outermost orbit, then the atom having the larger number of electrons in the outermost orbit is smaller than the one having fewer electrons in the same outermost orbit.
For example an atom of Mg is smaller than an atom of Na.

The mass of an atom :

The mass of an atom is concentrated in its nucleus and it is due to the protons (p) and neutrons (n) in it.
The total number (of protons and neutrons) in the atomic nucleus is called the atomic mass number.
Protons and neutrons are together called nucleons.
A reference atom was required for expressing the relative mass of an atom.
The hydrogen atom being the lightest was initially chosen as the reference atom.
The relative mass of a hydrogen atom which has only one proton in its nucleus was accepted as one (1).
Therefore, the magnitude of the relative atomic masses of various atoms became equal to their atomic mass number (p+n).
The mass of one nitrogen atom is fourteen (14) times that of a hydrogen atom.
Therefore, the relative mass of a nitrogen atom is 14. This is how the relative atomic masses of various elements were determined.
On this scale, the relative atomic masses of many elements came out to be fractional.
Therefore, in the course of time, some other atoms were chosen as reference atoms.
Finally in 1961, the carbon atom was selected as the reference atom. In this scale, the relative mass a carbon atom was accepted as 12.
The relative atomic mass of one hydrogen atom compared to the carbon atom becomes 12 x 1/12, that is 1.

The mass of one proton and of one neutron on the scale of relative atomic masses is approximately one

Unified Mass has now been accepted as the unit of atomic mass. It is called Dalton.
Its symbol is ‘u’. 1u = 1.66 x 10^-27 kg

Molecules of elements and compounds :

Atoms of some elements such as helium, neon have independent existence.
It means that these elements are in a mono-atomic molecular state.
Sometimes, two or more atoms of an element combine to form molecules of that element. Such elements are in a polyatomic molecular state.
For example, the elements oxygen, nitrogen are in a diatomic molecular state as O₂ , N₂ respectively.
When atoms of different elements combine with each other, the molecules of compounds are formed.
In other words, compounds are formed by chemical attraction between different elements.

Molecular mass :

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a single molecule of that substance.
Like atomic mass, molecular mass is also expressed in the unit Dalton (u).

Mole :

A mole is that quantity of a substance whose mass in grams is equal in magnitude to the molecular mass of that substance in Daltons.
Thus, the molecular mass of oxygen is 32u, and therefore 32g oxygen is 1mole of oxygen.
The molecular mass of water is 18u. Therefore, 18g of water make 1 mole of water
1 mole of a compound is the mass of that subtance in grams equal in magnitude to its molecular mass.
The SI unit is mol.
No Of Mole = mass / Molar mass
n = m/MM

Avogadro’s number:

The number of molecules in one mole of any substance is constant.
Therefore this number is called Avogadro’s number and is denoted by the symbol N_A .
Scientists demonstrated experimentally that the value of Avogadro’s number is 6.022 x 10²³.
A mole of any substance stands for 6.022 x 10²³ molecules.
For example, a mole of water, that is, 18g of water contains 6.022 x 10²³ molecules of water.
The number of molecules in a given quantity of a substance is determined by its molecular mass.
The number of molecules in equal masses of different substances are different.
One mole quantities of different substances have different masses measured in grams.

Valency :

The capacity of an element to combine is called its valency.
The valency of an element is indicated by a specific number. It is the number of chemical bonds formed by one atom of that element with other atoms.
“The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.”
The valency of both the elements sodium and chlorine is 1.
Positively charged ions are called cations while negatively charged ions are called anions.
For example, MgCl2 contains Mg++ and Cl- as cation and anion respectively.
The electrons present in the outermost orbit of an atom are called valence electrons.
The number of electrons that are given away or taken up is always a whole number. Therefore, valency is always a whole number.

NCL :

The National Chemical Laboratory (NCL), a unit of the CSIR, was established in 1950.
Its objectives are to conduct research in the various branches of chemistry, to aid industry and to develop new technology with a view to making profitable use of the country's natural resources.

Variable valency:

Under different conditions the atoms of some elements give away or take up different numbers of electrons.
In such cases those elements exhibit more than one valency. This property of elements is called variable valency.
Example : Iron (Fe) exhibits the variable valencies 2 and 3. Therefore iron forms two compounds with chlorine FeCl2 and FeCl3 .
FeCl2 - Ferrous Chloride
FeCl3 - Ferric Chloride

Radicals :

Compounds with ionic bonds have two constituents. These are cations (positively charged ions) and anions (negatively charged ion).
They take part independently in chemical reaction and are therefore, called radicals.
Different bases such as NaOH, KOH are formed when various cationic radicals are paired with the different anionic radical, hydroxide.
KOH ----> K+ + OH-
Hence the cationic radicals are also called basic radicals.
Different bases are distinguished from each other by the basic radicals in them.
On the other hand, different acids, such as HCl, HBr are formed when different anionic radicals are paired with the cationic radical H+.
Therefore, the anionic radicals are called acidic radicals.
The difference in the composition of various acids becomes clear from the acidic radicals present in them.
Generally, basic radicals are formed by removal of electrons from the atoms of metals, such as Na+, Cu2+.
But there are some exceptions, such as NH4 +.
Similarly, the acidic radicals are formed by adding electrons to the atoms of non-metals, such as Cl-, S2-.
But there are some exceptions like MnO4-.
Monoatomic radicals such as Na+, Cu+, Cl- are called simple radicals.
When a radical is a group of atoms carrying charge, such as SO4 2-, NH4 +, it is called composite radical.
The magnitude of charge on any radical is its valency.

Chemical formulae of compounds : A recapitulation :

Ions/Radicals :