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Chemical Reactions

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Chemical reactions are such a normal part of our lives that we hardly note when they take place. We take for granted that when we put gas in our cars. It will fuel them. We don't think about science when we bake cookies or cakes. And chemistry is the alst thing on our minds when we are warming ourselves by a wood burning fire, watching fireworks up in the sky, or admiring the changing colors of autumn leaves. But without chemical reactions, none of these things would happen. 

In the early days of chemistry, these substances were named in a variety of ways. Sometimes, the name referred to the use of the compound, such as baking soda. In others, it reflected a property, such as quicksilver. Or sometimes, it referred to the source of the substance as in wood alcohol. This system worked well when the number of compounds was less. It was realized that a standard system would not only provide better communication with Universities all over the world, but also allow a scientist to name and recognize a large number of substances without having to memorize it, by learning a few rules.

Chemical Equation

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A chemical equation is a "balanced account of a chemical transaction." In any chemical transaction or reaction, the number of atoms of all the participating elements will remain proportionately constant before and after the reaction (The Law of Conservation of Mass).

The substance(s) that undergo the chemical reaction are known as "reactants". The "products" are the substances produced during the chemical reaction. In a chemical equation, the formulae of the reactants and products are used. The chemical formula of a substance is the symbolic representation of the actual number of atoms present in one molecule of that substance. 

Chemical Reaction

A word-equation shows change of reactants to products through an arrow placed between them (image). The arrow may be read as "to yield" or "to form" or "to give" and shows the direction of the reaction. The reactants are placed on the left hand side (LHS) of the arrow and the products on the right hand side (RHS). The different reactants as well as products are connected by a plus sign (+).

Types of Chemical Reactions

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When a chemical reaction occurs how are new substances produced? Do atoms of one element transform themselves into atoms of another element? Do atoms withdraw from the mixture or turn up from elsewhere? Neither of these happens; what takes place is the breaking of old bonds between atoms and making of new bonds to form new substances. This can happen in a number of ways.
  1. Combination or Addition or Synthesis Reaction
  2. Decomposition Reaction
  3. Displacement or Substitution Reaction
  4. Double Displacement Reaction
  5. Oxidation - Reduction Reactions
Combination reaction:

The reactions in which two or more substances combine to form a single substance are called combination reaction.
eg : Magnesium and oxygen combine, when heated, to form magnesium oxide

2Mg(s) + O2 (g) $\rightarrow$ 2MgO (s)
     
Decomposition reaction: 

The reaction in which a compound splits up into two or more simpler substances are known as decomposition reaction. Decomposition reaction is opposite of combination reaction
eg : When calcium carbonate is heated, it decomposes to give calcium oxide and calcium dioxide

CaCO3 (s) $\rightarrow$ CaO(s) + CO2 (g)

Displacement reaction:  

The reaction in which one element takes the place of another element in a compound, is known as displacement reaction.
eg : When a strip of zinc metal is placed in copper sulfate solution, then zinc sulfate solution and copper are formed.

CuSO4(aq) + Zn (s) $\rightarrow$ ZnSO4 (aq) + Cu (s)

Double displacement reaction: 

The reaction in which two compounds react by an exchange of ions to form two new compounds are called double displacement reaction.
eg:  When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver chloride is formed along with sodium nitrate solution.

AgNO3(aq) + NaCl (aq) $\rightarrow$ AgCl (s) + NaNO3 (aq)

Oxidation and reduction reactions:
  • Oxidation: The addition of oxygen to a substance is called oxidation. The removal of hydrogen from a substance is also called oxidation
  • Reduction: The addition of hydrogen to a substance is called reduction. The removal of oxygen from a substance is also called reduction.
The oxidation and reduction reactions are also known as redox reactions.

eg:  When copper oxide is heated with hydrogen, then copper metal and water is formed.

CuO + H2 $\rightarrow$ Cu + H2O

Here, copper oxide is reduced to copper. So, oxygen is removed.  Hydrogen is added so that water is formed.

Chemical Reaction Examples

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Chemical reactions are an integral part part of life and technology. They determine the appearance and behavior of organisms and the process by which foods, plastics and other products are manufactured.Knowledge of the basic properties and mechanisms of chemical reactions also influences the way in which chemists group and categorize reactions. Some examples of chemical reactions are

Eg - 1

In the presence of heat iron (II) and sulfur is an example.

Fe + S $\rightarrow$ FeS 

Eg-2

One the reaction takes off in the presence of heat, it then continues exothermally. Formation of Aluminium Iodide is quite a spontaneous reaction and takes place in the presence of a few drops of water.
  
Al + 3I2 $\rightarrow$ 2 AlI3 

Eg -3

However, there is one synthesis reaction which is endothermic, which is photosynthesis. This is a type of photochemical reaction. Carbon dioxide and water react to form glucose in the presence of sunlight and chlorophyll. 
 
6CO2 + 6H2O $\rightarrow$ C6H12O6 + 6O2 
 
Eg - 4

Combustion is a type of synthesis reaction which is basically an exothermic reaction which takes place in the presence of oxygen as one of the reactants. Substances such as methane (natural gas) undergo combustion readily to give out a large amount of energy.

CH4 + 2O2 $\rightarrow$ CO2 + 2H2O