Chemical Combination & Chemical Equation

Chemical Combinations & Chemical Equation   
(Chapter-2)

Laws of Chemical Combinations
There are four laws of chemical combinations these laws explained the general feature of chemical change. These laws are:
1. Law of Conservation of Mass
2. Law of Definite Proportions
3. Law of Multiple Proportions
4. Law Reciprocal Proportions 


Antoine Lavoiser has rejected the worn out ideas about the changes that take place during a chemical reaction. He made careful quantitative measurements in chemical reactions and established that mass is neither created nor nor destroyed in a chemical change. 



Law of Conservation of Mass
Statement
It is presented by Lavoiser. It is defined as: 
"Mass is neither created nor destroyed during a chemical reaction but it only changes from one form to another form."
In a chemical reaction, reactants are converted to products. But the total mass of the reactants and products remains the same. The following experiment easily proves law of conservation of mass. 


Practical Verification (Landolt Experiment)
German chemist H. Landolt, studied about fifteen different chemical reactions with a great skill, to test the validity of the law of conservation of mass. For this, he took H.shaped tube and filled the two limbs A and B, with silver nitrate (AgNO3) in limb A and Hydrochloric Acid (HCl) in limb B. The tube was sealed so that material could not escape outside. The tube was weighed initially in a vertical position so that the solution should not intermix with each other. The reactant were mixed by inverting and shaking the tube. The tube was weighed after mixing (on the formation of white precipitate of AgCl). He observed that weight remains same. 
HCl + AgNO3 ----------> AgCl + NaNO3 


Law of Definite Proportions
Statement
It is presented by Proust. It is defined as: 
"When different elements combine to give a pure compound, the ratio between the masses of these elements will always remain the same." 
Proust proved experimentally that compound obtained from difference source will always contain same elements combined together in fixed proportions. 
Example
Water can be obtained from different sources such as river, ocean, well, canal, tube well, rain or by the chemical combination of hydrogen and oxygen. If different samples of water are analyzed, it will have two elements, hydrogen and oxygen and the ratio between their mass is 1:8. 


Law of Multiple Proportions
Statement
This law is defined as: 
"When two elements combine to give more than one compounds, the different masses of one element, which will combine with the fixed mass of other element, will be in simple whole number ratio."
Two different elements can combine to form more than one compound. They can do so by combining in different ratios to give different compounds. 


Example
Hydrogen and oxygen combine with one another to form water (H2O) and hydrogen peroxide (H2O2). In water and hydrogen oxide 2 g of hydrogen combine with 16g and 32g of oxygen respectively. According to law of multiple proportions, the different masses of oxygen (16g and 32g) which have reacted with fixed mass (2g) of hydrogen will have a simple ratio between each other i.e. 16:32 or 1:2. It means that hydrogen peroxide contains double the number of oxygen atoms than water. This law proves this point of Dalton's Atomic Theory that atoms do not break in a chemical reaction. 


Law of Reciprocal Proportions
Statement
This law is defined as: 
"When two element A, B combine separately, with the mixed mass of the third element E, the ratio in which these elements combine with E is either the same or simple multiple of the ratio in which A and B combine with each other." 
Example


Hydrogen and Nitrogen separately combine to form ammonia (NH3) and dinitrogen oxide (N2O), in these compounds, fixed mass of nitrogen is 14g and combines with 8 g of oxygen and 3 g of hydrogen. The ratio between the mass of oxygen and hydrogen is 8:3. Hydrogen and oxygen also combine with one another to form water (H2O). The ratio between hydrogen and oxygen in water is 16:2. These ratios are not same. Let us observe whether these ratios are simple multiple to each other or not following mathematical operation is carried out. 
8:3 ::16:2
8/3 : 16/2
or
8/3 x 2/16
or
1/3 => 1:3 


Definitions
Atomic Mass
The mass of an atom of the element relative to the mass of some reference or standard element is called atomic mass. Atoms are very small particles. They have very small mass. If the masses of atoms were to be expressed in gram. It is a very big unit for this very tiny object. Then it was decided by the chemists that masses of the atoms were to be found after comparing with mass to some standard form. 


Hydrogen being the lightest element is taken as standard. The mass of the hydrogen atom taken as one. 


The atomic mass could be defined as 
"Atomic mass of an element is the mass of an atom of that element as compared to the mass of an atom of hydrogen taken as one." 


Example
The atomic mass of sodium is 23. It means that an atom of sodium is 23 times heavier than hydrogen atom. Similarly atomic mass of oxygen is 16. It means that an atom of oxygen is 16 times heaviest than that of hydrogen. 


Atom
The smallest particle of an element which cannot exist independently and take part in a chemical reaction is known as Atom. 


Examples
Hexogen(H), Carbon (C), Sodium (Na), Gold (Au) etc. 


Molecule
The particle of a substance (Element or Compound) which can exist independently and show all the properties of that substance is called molecule.
Atoms of the same or different elements react with each other and form molecule.
Atoms of some elements can exist independently, since they have property of molecule so they are called mono atomic molecule. 


Examples
Examples of Molecules of the elements are Hydrogen (H2). Nitrogen (N2), Sulphur (S8) etc.
Molecules of different elements are called compounds. For example HCl, H2O, CH4 etc. 


Valency
The combining capacity of all elements with other elements is called valency. 
Example
H = 1
C = 4
Al = 3
Mg = 2
Na = 1


Chemical Formula
"A brief name used for full chemical name at a compound is called Chemical Formula."
A chemical formula is used to represent an element or a compound in terms of symbols. It also represents the number and type of atoms of elements present in the smallest unit of that substance.


Example
The chemical formula of hydrogen sulphide is H2S. It shows two types of elements (H and S) and number of atoms of element (2H and 1S). Similarly the formula of NaCl show number and type of different atoms present in its smallest unit.

Empirical Formula
"The formula which shows the minimum (simple) ratio between atoms present in a compound is known as Empirical Formula." 


Example
For example the empirical formula of hydrogen peroxide is HO that of water is H2O and benzene is CH. 
Molecular Formula
The formula of an element or a compound which represents the actual number of atoms present in the molecule of these substances is called molecular formula.


Example
Water, Hydrogen Peroxide, Ethylene Benzene and Sulphur have molecular formula H2O, H2O2, C2H4, C6H6 and S8 respectively. 


Molecular Mass
Molecular mass of an element or a compound is defined as the mass of its molecule relative to 1/12th of the mass of C-12. It is the sum of the atomic masses of all the atoms presents in its molecular formula.


Example
Molecular mass of water (H2O) = 2 + 16 = 18 a.m.u
Mass of hydrogen sulphide (H2S) = 2 + 32 = 34 a.m.u 


Formula Mass
Formula mass of a compound is the mass of its formula unit relative to 1/12th of the mass of C-12. 


Example
Formula mass of Sodium Chloride NaCl = 23 + 35.5 = 58.5 a.m.u
Formula mass of Calcium Chloride CaCl2 = 40 + 35.5x2 = 111a.m.u 


Molar Mass
The mass of one mole of a substance is called molar mass.


Example
1 mole of Hydrogen atom (H) = 1.008g
1 mole of Hydrogen molecule (H2) = 2.016g
Thus mass of substance is related to the particles by mole. 


Chemical Reaction
A chemical change in which reactants are converted to products is called chemical reaction. 
Zn + 2HCl --------> ZnCl2 + H2 
The fact that a chemical reaction is taking place can be inferred from the following observation.
1. Evolution of a gas
2. Change in colour
3. Change in temperature.
4. Emission of light. 


Types of Chemical Reaction
The chemical reaction is classified into following types: 


1. Displacement Reaction
The reaction in which an atom or group of atoms is displaced by another atom or group of atoms in a compound is called displacement reaction. 
Fe + CuO ---------> Cu + FeO


2. Double Displacement Reactions
The reactions in which reacting substances exchange their radicals or ions are double displacement reaction. Insoluble salts are formed by mixing soluble salts. 


3. Addition Reactions
When two different compounds or elements react together to give only one confound, the reaction will be called addition reaction.
2Mg + O2 --------> 2MgO 


4. Decomposition Reaction
The reaction in which some compounds may decompose into elements or simpler compounds on heating is called decomposition reaction. 
CaCO3 ---------> CaO + CO2 (Heat) 


Chemical Equation
Symbolic representation of chemical change in terms of symbols and formulae is called Chemical Equation.


Method of Equation Writing
A chemical equation can be written as follows:
1. Write the formulae and symbols of the reactants on the left hand side.
2. Write the formulae and sympols of the products on the right hand side.
3. Separate the reactants and products by an arrow which is directed towards the products. 


Characteristics of Chemical Equation
1. Chemical equation must be representative of a chemical reaction.
2. It should represent molar quantities.
3. It should be balanced in terms of atoms/molecules of reactants and products. 


Reactants
Those substances, which react together in a chemical reaction, are called reactants. 
Zn + 2HCl ------> ZnCl2 + H2 


In the above reaction Zn and HCl are the reactants. 


Products
Those substances, which are formed in a chemical reaction, are called products. 
Zn + 2HCl ------> ZnCl2 + H2 


In the above reaction, ZnCl2 and H2 are products. 


Information obtained from a Chemical Equation
1. A balanced equation indicates that which reactant undergo chemical change. It indicates that which products are formed.
2. It indicates that how many moles of reactants under go chemical change. It indicates that how many moles of products are formed. 


Why are Chemical Equations Balanced
A chemical equation must be balanced in order to satisfy the law of conservation of matter, which states that matter can neither be created nor be destroyed during a chemical reaction