Chemistry Laws of Chemical Combinations and Dalton's Atomic Theory

Topics Covered :

This lecture covers the following topics :

`●` Laws of Chemical Combinations
`●` Dalton's Atomic Theory

Laws of Chemical Combinations :

The combinations of elements to form compounds is given by five basic laws.

Law of Conservation of Mass :

Statement : Matter can neither be created nor destroyed.

Antoine Lavoisier (1789) gave this law based on his experimental studies for combustion reactions.

1.7 gram of silver nitrate dissolved in 100 gram of water is taken. 0.585 gram of sodium chloride
dissolved in I 00 gram of water is added to it and chemical reaction occurs. 1.435 gm of AgCI and 0.85
gm `NaN0_3` are formed. Show that these results illustrate the Jaw of conservation of mass.

Total mass before chemical change = mass of`AgN0_3` + Mass of NaCJ + Mass of water
= 1.70 + 0.585 + 200 = 202.285 gram
Total mass after the chemical reaction = mass ofAgCI + Mass of`NaN0_3` + Mass of water
= 1.435 + 0.85 + 200 = 202.285 gram
Thus in the given reaction
Total mass of reactants = Total mass of the products.

Law of Definite Proportions :

Statement : A given compound always contains exactly the same proportion of element by weight.

Joseph Proust gave this law. This law is sometimes called as law of definite composition.

Proust studied the samples of cupric carbonate obtained naturally and synthetically. And he found that both samples have same composition of elements.

So, a given compound always contains same element in the same proportion irrespective of the source.


For example : (1) For the formation of `NaCl , Na ` and `Cl` must combine in exact same proportion for the formation of salt.

(2). Vinegar `(C_2H_4O_2)` contains 2 atoms of `C , 4` atoms of `H` and `2` atoms of `O.` No other combination can be used for the formation of vinegar.

Law of Multiple Proportions :

Statement : If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. This law was given by Dalton in 1803.

e.g Hydrogen +oxygen → water

`\ \ \ \ \ \ \ 2g \ \ \ \ \ \ \ 16g \ \ \ \ \ \ \18g`


Hydrogen + Oxygen → Hydrogen peroxide

`2g \ \ \ \ \ \ \ \ \ 32 g \ \ \ \ \ \ \ \ \ \ \ \ 34g`


Here, the ratio of masses of oxygen (i.e. `16` g and `32` g) which combine with a fixed mass of hydrogen (`2` g) is `1 : 2`.
Q 1228434301

Weight of oxygen in `Fe_2O_3` and `FeO` in the simple ratio for the same amount of `Fe` is

(A)

`1:2`

(B)

`2 : 1`

(C)

`3 : 2`

(D)

`1 : 3`

Solution:

Ratios `112 : 48,56 : 16`

`therefore` Simple ratio for `O` for same amount of `Fe` is `48 : 32`

(i.e) `3 : 2`

Hence `3` is the correct answer.
Correct Answer is `=>` (C) `3 : 2`

Gay Lussac's Law of Gaseous Volumes :

This law was given by Gay Lussac in 1808.

Statement : When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at
same temperature and pressure.

Hydrogen + Oxygen `→` Water

`\ \ \ \ 100 \ mL \ \ \ \ \ \ \ 50 \ mL \ \ \ \ \ \ \100 \ mL`

Here, ratio of volumes of hydrogen and oxygen is `2:1`.

This law is also known as the law of definite proportions by volume.

(1) The Gay Lussac's law was explained properly by the work of ... A ... in 1811. Here, A refers to

Ans : Gay Lussac law states that when gases combine or are produced in a chemical reaction they do so in a simple ratio
by volume provided all gases are at same temperature and pressure. The Gay Lussac's law was explained properly by the work of Avogadro in 1811.

Avogadro’s Law :

This law was given by Avogadro in 1811.

Statement : Equal volumes of gases at the same temperature and pressure should contain equal number of molecules.

Note : Avogadro made distinction between atoms and molecules.


Avogadro's proposal was published in the French Journal de physidue.

In 1860, first international conference of chemistry was held in Karlsruhe, Germany to resolve various ideas

Dalton’s Atomic Theory :

John Dalton proposed the following

(i) Matter consists of indivisible atoms.

(ii) All the atoms of a givens elements have identical properties including identical mass.

(iii) When atoms of different elements combine in a fixed ratio, compounds are formed.

(iv) Chemical reactions involve reorganization of atoms. These are neither created nor destroyed in a chemical reaction.


(1) Matter is composed of small indivisible particles called .. . A ... (meaning indivisible), dates back to the time of .. . B... , a Greek philosopher
(460-370 BC) Here, A and B refer to

Ans : Matter is composed of small indivisible particles called 'a-tomio ' (meaning indivisible) dates back to the time of Democritus, a Greek philospher (460-370 BC).

(2) In ... A ... , Dalton published ... B ... in which he proposed the theory. Here, A and B refer to

Ans : In 1808, Dalton published 'A new system of chemical philosophy in which he proposed the theory.

 
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