Unit 2: Laws of Stoichiometry | Class 11 Chemistry Notes

Laws of Stoichiometry

Dalton's Atomic Theory:

In 1808 AD John Dalton developed atomic theory after several research. He used to write his views in a Series of article and titled as “A new system of chemical philosophy".

His views become successful to become basic principle about ultimate Particle of matter.

Postulates of Dalton's Atomic Theory based on Atom: 

1.       Every Element consist of individual Particle Called Atoms. 

2.       Atom of Same Elements are identical.

3.       Atom cannot be created or destroyed 

4.       Atom of different Element may Combine with each other to form a Compound atom. 

5.       The atom is the smallest unit of matter that can take Part in chemical reaction.

6.       Atom of Same elements can combine in more than one ratio to form two or more Compound.

7.       Since all the Postulates of Dalton are not perfectly correct so it was modified by other researchers and scientist.

Modification of Dalton Theory based on Atom

1.       An atom is not the smallest particle of matter but it can be further divided into further smallest particle called electron protons & neutrons.

2.       Atom of Same Element may not be identical in all respect.

3.       Atom Combine in fixed ratio but may not be simple whole number ratio.

4.       According to Einstein's mass Theory, mass of an atom can be converted into different form of energy by given relationship.

E=mc²

where:

e is released energy 

m is mass

v is velocity

Law of Stoichiometry:

The quantitative relation between the number of moles of various products and read ants in a chemical reaction is called Stoichiometry. All the chemical reaction are given on the basis of five basic laws which are called law of Stoichiometry or laws of chemical Combination.

1. Law of Conservation of Mass

Antonio Lavoisier enunciated Law of Conservation of mass in 1774.

It States that "mass of product is equal to mass of reactant" i.e., mass remains conserved in a chemical reaction.

Let us consider hypothetical reaction

A    +    B    →    C    +    D

(x) gm (y) gm (m)gm   (n) gm

According to law of conservation of mass:

Total mass of reactant= total mashup product

(x+y) gm=(m+n) gm

It is also called law of indestructibility of matter because mass and energy remain constant or conserved. Modern form of law conservation of mass and energy:

According to the Einstein mash energy relationship E=mc². Mass of an and atom can be converted into energy. So, this law states “Total mass and energy of a reactant before the reaction is equal to the mass and energy of the product after the reaction.”

2. Law of Definite proportion:

It is proposed by Joseph Lous Proust 1779 booster love definite proportion states that, “A given chemical compound always contains it’s components element in fixed ratio and does not depend on its source and methods of preparation.”

Examples:

(i)                  H₂O is formed by oxygen and hydrogen in ratio of 2:1 by their master regardless to source

(ii)                CO₂ contains carbon and oxygen in 3:8 this year by their mass regardless their source.

Exception of Law definite proportion:

Law of definite proportion is not obeyed by following two kinds of Compounds:

(i)                  Non- Stoichiometric Compounds

The compound in its constituent element do not have definite whole number ratio as in usual molecular formula are called Stoichiometric compound.

Example: ZnO_0.998

(ii)                Isotopic Compounds

The compound which contains different isotope with different atomic mass are called isotopic compound. Such compound does not have definite ratio of constituent element by their mass.

Example:

Water: H₂O d₂O T₃O

3.Law of Multiple Proportion:

It was proposed by John Dalton in 1803. It states,” whenever who are more different compounds are formed from the same set of elements the combining weight of the ratio of these elements in several compounds are related to each other by simple whole number ratio.”

Examples:

H and O combines to give two different compound H₂O, H₂O₂

In H₂O:

2 gram of hydrogen combines with 16 grams of oxygen.

1 gram of hydrogen combines with 8 gm of oxygen.

In H₂O₂:

2 grams of hydrogen combines with 32 grams of oxygen.

1 gram of hydrogen combines with 16 grams of oxygen.

Then the ratio of mass of oxygen which Combines with Constant mass (1 gram) of hydrogen is 8:16. This is the simple whole number and hence it verifies the law of multiple proportions.

4.Law of Reciprocal Proportions:

It is proposed by Richter. It states that,” the weight ratio of two different elements which combines with fixed weight of third elements in two different compounds is either same or simple multiple of the ratio in which they combined with each other.”

It is also called as law of equivalent proportion because element combined together in either proportion of their equivalent weight or simple whole number multiple of equivalent compound

Example:

The ratio of mass of hydrogen and oxygen in H₂O is 1:8 which is the ratio of equivalent weight of hydrogen and oxygen.

5.Gay-Lussac's Law of Gaseous Volume:

It was proposed by Gay-Lussac. It states that," under normal temperature and pressure the volume ratio of the gaseous reactant is always in simple whole number ratio."

Example:

Let us consider the formation of ammonia from nitrogen and hydrogen gas under favorable condition.

N₂ + 3H₂→ 2NH₃

Volume Ratio of N₂, H₂ and NH₃ is 1:3:2 which is simple whole number. Hence, this example illustrates the Gay-Lussac's law of gaseous volume.

Background of Avogadro Hypothesis:

From Dalton's atomic theory which says an atom is indivisible and Gay-Lusacs deals with volume of gases. It was concluded that there is some short of relationship between volume of gas and number of particles.

A Swedish scientist Berzelius first introduced a relationship between volume of a gas and the number of atom.

Berzelius hypothesis states that, “equal volume of all gases contain equal number of atom at similar condition of temperature and pressure.”

Since Berzelius hypothesis was in the opposition of Dalton Atomic theory due to the word half atom and it was rejected by most of the scientist.

Then a new scientist Avogadro put forward his hypothesis introducing a new term molecule instead of atom which was in Berzelius hypothesis. In this way the difference of single word molecule is theory was accepted widely all over the world.

Application of Avogadro’s Hypothesis:

1.       Deduction of atomicity of elementary gases:

Atomicity: The total number of moles of atom present in 1 mole of molecule is called atomicity.

Elementary Gas: The gas which consists of some elements is called elementary gas. Elementary gas are diatomic and can be proved as:

a.       Hydrogen is Diatomic:

Let us consider the formation of hydrogen gas from hydrogen chloride. It is experimentally found that one volume of hydrogen gas combines with one volume of chlorine gas and gives two volume of hydrogen chloride gas. Let 1 volume of each gas contains n-molecules. Then,

Hydrogen + Chlorine à Hydrogen Chloride

1Vol.              1Vol.                      2Vol.

n-molecule n-molecule         2n-molecule

Let n=1 molecule,

1 molecule  1 molecule          2 molecule

½molecule  ½molecule          1 Molecule

So, 1 molecule of hydrogen chloride is formed by ½ molecule of hydrogen and ½molecule of chlorine.

We Know: ½molecule = 1 atom

So, 1 molecule = 2 atom.

2.       Deduction of relationship between molecular mass and vapor density:

Vapor Density: The ratio of certain volume of any gas or vapor to the weight of same volume of hydrogen gas under similar condition of temperature and pressure. It is denoted by V.D.

Mathematically,

Vapor Density =

Let us consider certain volume of gas contains n-molecules. Then,

V.D= 

Let n =1 molecule

Or, V.D=

Since hydrogen is diatomic so 1 molecule of H₂ = 2 atom

Or, V.D=

Or, V.D=

Or, 2V.D=

Or 2V.D = Molecular Mass

1.       Deduction of molar volume of gas:

The relationship between molecular mass and vapor density is

2V.D = Molecular Mass………. (i)

But, Vapor Density =  …….. (ii)

Since, Molecular Mass =  ……. (iii)

From i, ii, iii

Molecular Mass = 

                                =

Let v=1 Liter

Molecular Mass = 

Wt. of 1 Liter of H₂ gas =0.089

Molecular Mass = 

Molecular Mass = 22.4 × Weight of 1 Liter of Gas

Molecular Mass = Weight of 22.4 Liter of Gas

Hence, Gram Molecular Volume of all gases at NTP occupies 22.4 Liter

2.       Deduction of molecular formula for volumetric composition:

Avogadro Hypothesis can be applied to deduce molecular formula from volumetric composition.

Example: Deduce the molecular formula of nitrogen oxide which contains its half volume of oxygen gas and its vapor density is 15.

Here:

Vapor density =15

Molecular Mass = 2 × Vapor Density = 2 × 15 =30

Also by Question, v- volume of nitrogen oxide = v/2 volume of oxygen gas

Let v volume = n molecules. Then

n molecules of nitrogen oxide = n/2 molecules of oxygen gas

Let n=1. Then,

1 molecules of nitrogen oxide = ½molecules of oxygen gas.

Since oxygen is diatomic gas. So, 1 molecules of nitrogen oxide = 1 atom of oxygen.

Let no. of nitrogen atom be x then

Expected formula = N_xO.

Molecular Mass = 14 × x + 16

i.e. 30 = 14x+16

Or, 14x= 14

Or, x=1 atom

Thus required molecular formula of nitrogen oxide is NO.

Practical Use of Avogadro’s Hypothesis:

1.       It helps to calculate the absolute weight of an atom or molecular compound.

2.       It else to calculate the percentage of particular element in chemical compound.

Mole Concept:

It is defined as the collection of particles which is numerically equal to the number of carbon atom present in one gram of C-12 isotopes. One mole is equals to Avogadro number.

Expression of moles in terms of:

1. Mass: Atomic weight expressed in gram is called gram atomic weight.

1 Mole atom = gram atomic weight = Avogadro number= 6.023×10²³ molecules

2. Molecular Mass: Molecular mass expressed in gram is called gram-mole or gram molecular weight.

1 mole of molecule = 1 gram molecule = gram molecular weight = Avogadro number = 6.023×10²³ molecules

3. Volume: Mole is also defined as the volume occupied by 22.4 liter of gas at NTP. It is called molar volume.

1 mole of molecule = gram molecular weight = Avogadro number = 6.023×10²³ molecules=22.4 liter at NTP

Limiting Reagent:

The reactant which is stoichiometrically supplied in less amount than required by the chemical reaction is called limiting reagent.

Features: It limits the formation of product.

Access reagent: The reagent in stoichiometrically supplied in more amount then required by the chemical reaction is called reagent.

Significance of limiting reagent:

1.       To determine mass of product

2.       To determine mass of reactant consumed.

3.       To determine number of moles of on unreacted reactant.

4.       To determine volume of gases product formed.

5.       To determine molecules of product formed.