Introduction to Chemistry - Part III

5. Chemical Reactions
Transformation of a substance into a new substance or substances is called a chemical reaction.  For example water (which is the compound formed when Hydrogen and Oxygen bind together), the chemical reaction can be written as : 
                                    2H₂ + O₂    2 H₂O   

Hydrogen atoms exists as a gas with 2 Hydrogen atoms sticking together and is written as H₂  and the same is true with Oxygen atoms. This means that two elements, Hydrogen and Oxygen combine in a reaction to give a water molecule. The numbers 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 are generally accompanied by heat changes. A reaction, in which heat is evolved, is called an exothermic chemical reaction. A reaction in which heat is absorbed is called an endothermic chemical reaction. Chemical reactions, whether exothermic or endothermic, are also categorized as reversible and irreversible reactions.

If you burn sugar in air, you get carbon and water. This is an irreversible reaction, That is if you take carbon and water and mix together, you will never get the original sugar back!! A chemical reaction that proceeds in one direction only is known as an irreversible chemical reaction.

A chemical reaction in which substances react together to produce resultants and the resultants in turn react with one another to produce the original substances is known as an reversible chemical reaction. For example, if calcium oxide is kept in close contact with carbon dioxide, the two substances slowly unite to make calcium carbonate.

  CaO  + CO₂ CaCO₃

If you heat CaCO₃  you will get back CaO and CO₂  .

The two equations can be combined as 

CaCO₃      CaO  + CO₂   
               

The horizontal arrow shows that the reaction proceeds in both the directions. The vertical arrow shows that the carbon dioxide gas escapes.

Each chemical reaction is characterized by a reaction rate, which is the rate at which the reactants are combining to produce the final product. The rate depends on various factors such as temperature, nature of the reacting substances, etc.

In a reversible chemical reaction, if the rate of forward reaction and the rate of backward reaction are same, we say that the reaction has reached equilibrium.

Summary   
In this chapter we have seen what is an atom and how it can be represented as a symbol. When compounds are formed, we have seen how to write their formula. The formula tells us what is the exact quantity of element that is present in the compound. We have seen how elements display valency, which is a measure of how reactive they are. When chemical elements react, we have seen how to write down the reactions with the help of symbols. The exact balanced number of atoms of each element before and after a chemical reaction has to be strictly followed. More discussions about the types of reaction, etc. will be discussed in later chapters.

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