Chemical Reactions - Part I

In the chapter “Introduction to Chemistry - Part III” we had introduced ourselves to the concept of chemical reactions. In this chapter we will see in details about the various ways in which elements or compounds transform while combining with each other. Transformation of a substance into a new substance or substances is called a chemical reaction. The resultant of a reaction has properties completely different from its constituent elements.

Examples of chemical reactions in our every day lives are many. Dissolving of sugar in tea, cooking food, digestion of food, ripening of fruits, cleaning of surfaces with a solvent, etc. 

The number of atoms of each element have to be balanced, before and after the chemical reaction takes place. This is an important fact that atoms are neither created nor destroyed in a chemical reaction. The left hand side of the equation is called the reactants and the right hand side is called the products. Chemical reactions involve breaking of bonds of the reacting substances and creating new bonds in the atoms or molecules of the products.

What we will study in this chapter : 

1. Types of chemical reactions
2. How to balance a reaction
3. Energy changes in a chemical reaction
4. Reaction rates
5. Concepts of mole and calculations

1. Types of chemical reactions
There are many types of chemical reactions which can be categorized as follows : 

• Combination reactions

• Decomposition reactions

• Displacement reactions

• Oxidation-Reduction reactions

• Isomerisation reactions

Combination reactions : This type of chemical reaction is the simplest. Here two or more types of atoms or molecules (or compounds) react or combine to give products. Some examples below will illustrate combination reactions.

 2H₂ + O₂                  2H₂O

 2Na  +  Cl₂               2NaCl

Decomposition reactions : Chemical reactions where a compound splits or decomposes into simpler substances is called a decomposition reaction.

Some examples below will illustrate decomposition reactions.

 2KClO₃           2KCl + 3O₂  
                 heat

2H₂O              2H₂ + O₂  
             electricity

Displacement reactions : Chemical reactions where one element displaces another by virtue of it being more reactive, is called a displacement reaction. For knowing which element is more reactive than the other, one has to consider the reactivity of the elements. Mostly displacement reactions are seen when one metallic salt solution reacts with another metal. If the second metal is more reactive, then it replaces the first metal in its salt. An example below illustrates a typical displacement reaction.

 CuSO₄  (aq)  +  Zn (s)           ZnSO₄    + Cu (s)

Blue copper sulphate solution reacting with solid zinc will give rise to colorless zinc sulphate solution and solid copper. Thus Zn displaces Cu in the salt form. Zn is more reactive than Cu.

Oxidation-Reduction Reactions : Oxidation reaction means addition of oxygen to a reaction. Reduction reaction means addition of hydrogen to a reaction. In a broader perspective, an oxidation reaction is a reaction where an atom or ion looses electrons. Similarly a reduction reaction is a reaction where an atom or an ion gains extra electrons. Oxidizing and reducing agents are those substances or compounds that accept and donate electrons respectively.

Example below discusses these concepts in details.

 CuSO₄  (aq)  +  Zn (s)             ZnSO₄    + Cu (s)

Here Zn atom donates two electrons

 Zn          Zn²⁺  +  2e^-

Zn is donating electrons, hence it is a reducing agent. Zn itself is getting oxidized.

 Cu²⁺  SO₄²⁺                Cu²⁺   +   SO₄²⁺

 Cu²⁺      +     2e^-           Cu

Cu is accepting electrons, hence it is an oxidising agent. Cu itself is getting reduced.

Oxidation-reduction reactions are also known as redox reactions.

A typical redox reaction is as follows :

 CuO  +   H₂        Cu   + H₂ O

In the above reaction, copper oxide looses an oxygen atom, so it is getting reduced. H₂ is getting extra oxygen, hence it is getting oxidized. But CuO is an oxidizing agent and H₂ is a reducing agent.

The term used to describe the degree of oxidation of an element is oxidation number, or oxidation state. In electrovalent compounds the oxidation number of the element is the same as the electrovalence of each ion; but for covalent compounds the oxidation number is not the same as the covalence. For covalent compounds, the oxidation number is defined as the charge the atom would have in the molecule if it were there as an ion. To determine oxidation number of a given reaction, write down on top how the electronic configuration of the atom or ion is changing.  
For Example:

  Zn  +  2 HCl      ZnCl₂ + H₂

The oxidation state of Zn is changing from 0 to 2. Zn is suffering a loss of electrons, hence it is getting oxidized. Its oxidation number is changing from 0 to 2. In the same equation, the hydrogen is reduced. The oxidation number has changed from +1 to 0.

If everything is counted through the entire equation, oxidation and reduction are equal and balance to 0.

Isomerisation reactions : Chemical reactions where rearrangements of atoms occurs within a substance without any change in the molecular formula is called isomerisation reaction. Compounds having the same molecular formula but different arrangements of atoms are known as isomers (isomers are different from allotropes).  Isomers occur mostly in organic chemistry.
Example below shows one example of isomerisation reaction.

Even though ammonium cyanate and urea have the same chemical formula, their arrangements of atoms inside the molecules differ. They have different structures and hence  widely differing physical and chemical properties.

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