4.1 Introduction to History of Chemical Reactions
The first known chemical equation was written in 1615 in the first chemistry textbook called Tyrocinium Chymicum (meaning “Begin Chemistry”) by the French scientist Jean Beguin. The chemical equation was presented as a diagram rather than symbols:
Jean Beguin used antimony sulfide to react with chloride of mercury. The mercury became vapor, leaving behind a residue of antimony oxychloride.
In1747, William Cullen an English chemist and a professor at University of Glasgow had written lecture notes for chemical equations which were closer to what we have today. In addition, he had classified 4 types of chemical reactions:
In 1774, a French chemist Antoine Lavoisier had developed that the mass of the reactants equal the mass of the products which then was known as mass conservation. This was the first crucial step to understand how to balance the chemical equation.
Figure 4.1 Antoine Lavoisier
At same period of time,Lavosier, Joseph Proust a French scientist had developed Law of definite proportions which stated that in any given compound, the elements occur in fixed ratios, regardless of their source.
In 1803, English chemist John Dalton combined all these laws and concepts together and proposed that matter is made of atoms of unique substances that could not be created or destroyed.
Figure 4.2 John Dalton
Identifying Chemical Reactions
Chemical reactions can be broadly classified in five categories.
- Decomposition reaction: When one reactant breaks down to two more reactant, the reaction is called decomposition reaction
Figure 4.3 Electrolysis of water
Ref: commons.wikimedia.org/
- Combination reaction: When two or more reactants join together to form one single product, the reaction is called combination or synthesis reaction. Here are some example of combination reaction.
3) Single replacement reaction:
- An active element replaces another element in a compound.
Figure 4.4 Reaction of Mg with water
Ref: commons.wikimedia.org/
In the above example, Magnesium replaces Hydrogen from water to form Hydrogen gas( H2) and Magnesium Hydroxide is formed.(Mg(OH)2)
4) Double replacement Reactions:
Ref: commons.wikimedia.org/
Figure 4.5 Formation of yellow PbI2 precipitate
When the cations of two ionic compounds switch places to form products, the reaction is classified as double displacement reaction. When Potassium Iodide(KI) is added to colorless Lead Nitrate solution ( Pb(NO3)2) , yellow precipitate of Lead Iodide ( PbI2) is formed.
Below is the generic representation of all the different types of reactions discussed .
Figure 4.6 Classification of Different types of reactions
Ref: commons.wikimedia.org/
5) Combustion reaction: When a hydrocarbon burns to produce Carbon dioxide and gaseous water, the reaction is called combustion reaction. A combustion reaction always produces heat energy along with the products.
Figure 4.7 Combustion Reaction: molecular view
Ref: commons.wikimedia.org/
Figure 4.8 Combustion Reaction
Ref: commons.wikimedia.org/
There are many other categories of reactions like acid-base reactions, precipitation reactions, etc. These reactions will be described under respective chapters. Only redox reaction is described in this chapter.
Acidbase Precipitation
Figure 4.9Acid Base & Precipitation Reaction
Ref: commons.wikimedia.org/
Questions:
- Classify the following reactions:
- a) N2 + 3H2 —> 2NH3
- b) Na2SO3 + H3PO4 —> H2SO3 + Na3PO4
Ans: a) combination, b) double displacement