Classwork Series and Exercises {Chemistry – SS1}: Law Of Chemical Combination

Chemistry SS1 Week 4

Topic: Law of Chemical Combination

Introduction

Compounds are formed by chemical combination of reactants (atoms or molecules) which may be solid, liquid or gaseous. Chemical combination occurs in definite proportion by weight or by volume. Based on various experiments performed by different scientists, the laws of chemical combinations were formulated. These laws laid the foundation of stoichiometry, a branch of chemistry in which quantitative relationship between masses of reactants and products is established. The study of these laws led to the development of a theory concerning the nature of matter. These laws summarize the experimental results that are obtained about the masses of the reactants and products in a very large number of chemical reactions.

The various laws are:

1. Law of conservation of mass
2. Law of definite proportions
3. Law of multiple proportions

Law of Conservation of Mass 
This law states that the total mass of the reactants is equal to the total mass of the products in every chemical reaction. The total mass of material present after a chemical reaction is the same as before the reaction. This Law was discovered by Antoine Lavoisier in about 1789.
Whenever a chemical change occurs, the total mass of products is the same as the total mass of reactants. Alternatively the law can be stated as “the total mass of substances taking part in a chemical reaction remains the same throughout the change.”

Experimental verification of the law of conservation of mass

Experiment 1:

This law can be verified by the study of any chemical reaction. In the laboratory, it can easily be verified by the study of the following reaction.

Ag NO₃+ NaCl → AgCl ↓+ NaNO₃

When a solution of silver nitrate (AgNO₃) is treated with a solution of sodium chloride, a white precipitate of silver chloride (AgCl) is obtained along with a solution of sodium nitrate (NaNO₃). If the law is true, the total mass of AgNO₃ and NaCl should be the same as the total mass of AgCl precipitate and NaNO₃ solution. The experiment is done in a specially designed H shaped tube called Landolt’s tube. Sodium chloride solution is taken in one limb of the tube while silver nitrate solution is taken in the other limb. Both the limbs are now sealed and tube is weighed. Now the tube is inverted so that the solutions can mix up together and react chemically. The reaction takes place as mentioned above and a precipitate of silver chloride is obtained. The tube is again weighed. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube. This experiment clearly shows that the law of conservation of mass is true

Experiment 2

The law can also be verified by the below mentioned experimental setup. Take a conical flask and a test tube. Take 10ml barium chloride solution in the conical flask and 10ml copper sulphate solution in the test tube. Measure the initial mass of reactants and note it.
Now mix the solutions together. Copper sulphate reacts with barium chloride to give a white precipitate of barium sulphate. Then take the weight of the products formed. We can observe that the mass of reactants before the reaction and the products formed after the reaction will be equal.

As a result of numerous experiments conducted on systems in closed containers, so as to retain any gases present, Lavoisier was able to unambiguously verify his assumptions and formally state the Law of Conservation of Mass. The law of conservation of mass states that matter can neither be created nor destroyed in the course of chemical reaction.

Note: When an energy difference occurs during a reaction, minute amounts of mass are either gained or lost. Mass is either converted to energy or energy is converted to mass. The energy-mass equivalence was first postulated by Einstein in his famous formula; E = mc². While these mass differences are not detectable by the chemist, they are important in nuclear reactions. Therefore, law of conservation of mass is applicable in all chemical reactions it is not applicable in the case of nuclear reaction where a fraction of the mass is converted to energy.

Example 1: In an experiment 5.0g of CaCO₃ on heating gave 2.8 g of CaO and 2.2 g of CO₂. Show that these results are in accordance to the law of conservation of mass.

Solution:

CaCO₃ ——> CaO + CO₂

Weight of CaCO3 = 5.0 gms

Weight of CaO = 2.8 gms

Weight of CO2 = 2.2 gms

Total weight of reactant = Total weight of products.

5.0 = 2.8 + 2.2

5.0 = 5.0

Since, the mass of the reactants are equal to the mass of the product, these results are in accordance to the laws of conservation of mass.

Example 2: In an experiment, 48 gms of magnesium combines with 32 gms of oxygen to form 80 gms of magnesium oxide. Show that this reaction illustrates the Law of Conservation of Mass. [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][Hint: 2Mg + O2 → MgO, Atomic mass of Mg = 24 and O = 16].

Solution:

2 Mg + O₂ → 2MgO

Magnesium + oxygen →Magnesium oxide

Weight of Magnesium = 48 gms

Weight of oxygen = 32 gms

Weight of magnesium oxide = 80 gms

∴ Total weight of reactants = Total weight of products

48 + 32 = 80

80 gms = 80 gm.

So these results are in accordance to the laws of conservation of mass.

Law of Definite Proportions

A chemical compound, no matter what its origin or its method of preparation, always has the same composition; i.e., the same proportions by mass of constituent elements. This Law, sometimes known as the Law of Definite Composition, was first enunciated by Joseph Proust in 1799. Proust discovered this law while analyzing samples of Cupric Carbonate.

Experiment to Verify the Law

Prepare pure samples of cupric oxide by two different methods

1. By heating copper carbonate
2. By the decomposition of cupric nitrate

The cupric oxide prepared by both the methods always contains the same elements copper and oxygen combined together in the same fixed proportion of 4: 1 by weight. This illustrates the law of definite proportions. It can be verified by taking a known weight of a pure sample (W₁ gm) of cupric oxide in a porcelain boat.

2 Cu(NO₃)₂ → 2 CuO + 4 NO₂ ↑ + O₂ ↑

It is placed inside a hard glass tube kept horizontally. A current of pure dry hydrogen is sent inside the tube and the tube is heated. The cupric oxide is reduced to metallic copper.

CuO + H₂ → Cu + H₂O

The Weight of copper formed is found out W₂ gm.

Calculation:

Method 1:

Weight of cupric oxide = W₁ gm

Weight of Copper = W₂ gm

∴ Weight of oxygen = W₁ – W₂ gm

Ratio of copper: oxygen = W₂ : (W₁ – W₂)

The same experiment is repeated with a known weight W₃ gm of cupric oxide prepared by heating copper carbonate

CuCO3 ——> CuO + CO₂↑

The cupric oxide formed is reduced to metallic copper by passing a current of pure and dry hydrogen inside the tube as before. The weight of metallic copper was found to be W₄ gm. The ratio of the weight of copper to the weight of oxygen in both the samples is calculated as follows:

Method 2:

Weight of cupric oxide = W₃ gm.

Weight of copper = W₄ gm

∴ Weight of oxygen = W₃ – W₄ gm

Ratio of copper to oxygen = W₄ : (W₃ – W₄)

The two ratios [W₂ : W₁ – W₂] and [W₄ : W₃ – W₄] are found to be the same and is equal to 4: 1. Thus the law of definite proportions is verified experimentally

Example 1: 1.375 g of CuO were reduced by H and 1.098 g of Cu were obtained. In another experiment, 1.178 g of Cu were dissolved in nitric acid and the resulting copper nitrate was converted into CuO by ignition. The weight of CuO formed was 1.476 g. Show that these results prove the law of constant proportion.

Solution

Experiment 1:

Weight of CuO = 1.375 g

Weight of Cu = 1.098 g

Weight of oxygen = (1.375 – 1.098) g = 0.277 g

Ratio of copper oxygen = 1.098 : 0.277 = 3.96 : 1

Experiment 2:

Weight of CuO = 1.476

Weight of Cu = 1.178

Weight of oxygen = 1.476 – 1.178 = 0.298

Ratio of copper: oxygen = 1.178 : 0.298 = 3.96 : 1

In both experiments the ratio of Copper: oxygen is some (3.96: 1). Hence it illustrates the law of definite proportions.

Example 2: In an experiment 0.2430 gm of magnesium on burning with oxygen yielded 0.4030 gm of magnesium oxide. In another experiment 0.1820 gm of magnesium on burning with oxygen yielded 0.3020 gm of magnesium oxide. Show that the data explain the law of definite proportions.

Solution

Experiment 1:

Weight of Magnesium oxide = 0.4030 gm

Weight of Magnesium = 0.2430 gm

Weight of oxygen = 0.4030 – 0.2430 = 0.16 gm

Ratio of Magnesium: oxygen = 0.2430: 0.16 = 1.552: 1

Experiment 2:

Weight of Magnesium oxide = 0.3020

Weight of Magnesium = 0.1820

Weight of oxygen = 0.3020 – 0.1820 = 0.12

Ratio of magnesium: oxygen= 0.1820: 0.12 = 1.552: 1

In both experiments the ratio of magnesium: oxygen is same (1.518:1) Hence it illustrates the law of definite proportions.

The Law of Definite Proportion or Constant Composition states that all pure samples of the same chemical compound contain the same chemical compound contain the same elements combined in the same proportion by mass.

Law of Multiple Proportions
This Law of Multiple Proportions was enunciated by John Dalton at about the same time he postulated his Atomic Theory of Matter in 1803. This law states that when two elements A and B combine to form two or more compounds, then different weights of B which combine with a fixed weight of A bears a simple numerical ratio to one another.

Carbon combines with oxygen to form two different oxides, namely, carbon monoxide (CO) and carbon dioxide (CO₂). The proportions by weight of the two elements are:

Carbon monoxide – C: O :: 12 : 16

Carbon dioxide – C: O :: 12 : 32

There, the weights of oxygen that combine with a fixed weight of carbon (12g) are in the ratio 16g : 32g i.e. 1:2, a simple numerical ratio.

Experiment to Verify the Law

The law can easily be verified by the study of oxides of copper. Copper reacts with oxygen to form two oxides – the red cuprous oxide (Cu2O) and the black cupric oxide (CuO). In order to verify the law of multiple proportions, fixed amounts of these oxides (say 20g each) are separately reduced to metallic copper by heating them in a current of hydrogen and the masses of copper obtained from them are estimated. The difference in the mass of oxide taken and the mass of copper obtained from it gives the mass of oxygen present in it.

Now the masses of oxygen which combine with a definite mass of copper in the two oxides are calculated. These masses are found in a simple whole number ratio. This verifies the law of multiple proportions.

Note: The law is valid till an element is present in one particular isotopic form in all its compounds. When an element exists in the form of different isotopes in its compounds, the law does not hold good.

Example: In an experiment, 34.5 g oxide of a metal was heated so that O₂ was liberated and 32.1 g of metal was obtained. In another experiment 119.5 g of another oxide of the same metal was heated and 103.9 g metal was obtained and O₂ was liberated. Calculate the mass of O₂ liberated in each experiment. Show that the data explain the law of multiple proportions.

Solution

Experiment 1

Weight of the metal oxide = 34.5 g

Weight of the metal = 32.1 g

32.1 g metal combines with 2.4 g oxygen

1 g of the metal combine with 2.4/32.1 = 0.075 g

Experiment 2

Weight of the oxide taken = 119.5 g

Weight of the metal formed = 103.9 g

Weight of oxygen liberated = 15.6 g

103.9 g of metal combines with 55.6 g oxygen.

1 g of metal = ——– 15.6/103 x 1 = 0.15014 oxygen

Therefore different weights of oxygen that combine with the fixed weight of the metal (1 g) are in the ratio:

0.1501   :   0.075

2           :        1

The proportion by weight of oxygen is indicated by simple ratio. Thus law of multiple proportions is obeyed

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